@article{ChristlReichRoberts1971,
author = {Christl, Manfred and Reich, H. J. and Roberts, J. D.},
title = {Nuclear Magnetic Resonance Spectroscopy. Carbon-13 Chemical Shifts of Methylcyclopentanes, Cyclopentanols, and Cyclopentyl Acetates},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57862},
year = {1971},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlRoberts1972,
author = {Christl, Manfred and Roberts, J. D.},
title = {Nuclear Magnetic Resonance Spectroscopy : Carbon-13 Chemical Shifts of Small Peptides as a Function of pH},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57885},
year = {1972},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlRoberts1972,
author = {Christl, Manfred and Roberts, John D.},
title = {Carbon-13 Nuclear Magnetic Resonance Spectroscopy : Conformational Analysis of Methyl-Substituted Cycloheptanes, Cycloheptanols, and Cycloheptanones},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-41663},
year = {1972},
abstract = {The 130 chemical shifts were determined of the carbons in 12 cycloheptanes, 21 cycloheptanols, and 8 cycloheptanones. In some cyc1oheptanols and cyc1oheptanones, the assignments have been obtained unambiguously by the synthesis of deuterated derivatives and the use of paramagnetic-shift reagents. Substituent effects for the different types of groups have been calculated. The most informative data about the cyc10heptane conformations were provided by the relatively well understood I' effects. The results are generally in,good agreement with predictions based on the twist-chair form, which has been predicted by Hendrickson to be the most stable conformation. Pairs of cis-trans isomers are found to have rather characteristic differences in their 130 spectra. This fact was used to assign the resonances found for cis-trans mixtures of methyl-substituted cyc1oheptanols to specific isomers.},
language = {en}
}
@article{ChristlWarrenHawkinsetal.1973,
author = {Christl, Manfred and Warren, J. D. and Hawkins, B. L. and Roberts, J. D.},
title = {\(^{13}\)C and \(^{15}\)N Nuclear Magnetic Resonance Spectroscopy of Nitrile Oxides and Related Reaction Products : Unexpected \(^{13}\)C and \(^{15}\)N Nuclear Magnetic Resonance Parameters of 2,4,6-Trimethylbenzonitrile Oxide},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57894},
year = {1973},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{BruentrupChristl1973,
author = {Br{\"u}ntrup, Gisela and Christl, Manfred},
title = {Darstellung und Thermolyse der Hexamethyldewarbenzol-Cycloaddukte aromatischer Nitriloxide und des Diphenylnitrilimins},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30257},
year = {1973},
abstract = {No abstract available},
language = {en}
}
@article{ChristlBruentrup1974,
author = {Christl, Manfred and Br{\"u}ntrup, Gisela},
title = {Diimine Reduction and Ozonolysis of Benzvalene},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57953},
year = {1974},
abstract = {Via reduction of benzvalene (1) with diirnine tricyclo[3.1.0.02•6]hexane is obtained in good yield. The procedure renders 3, which has already been synthesized by Lemal and Shim, accessible much easier and in larger quantities. IH and 13C n.m.r. spectroscopic data are discussed. Both the thermal and the AgBF4-catalyzed rearrangernent of 3 yield 1,3-cyclohexadiene (8). - The ozonolysis of 1 with subsequent LiAIH4-reduction results in cis-I,3- bis(hydroxyrnethyl)cyclobutane (13a).},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlHeinemannKristof1975,
author = {Christl, Manfred and Heinemann, U. and Kristof, W.},
title = {Thermal Rearrangement of Some Endo-Endo'-Bridged Bicyclo[1.1.0]butanes},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-57967},
year = {1975},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{Christl1975,
author = {Christl, Manfred},
title = {Carbon-13 Chemical Shifts and 13C-15N Coupling Constants of 3,4-Dihydroisoquinoline-15N, its 15N-Oxide and their Conjugate Acids},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30071},
year = {1975},
abstract = {No abstract available},
language = {en}
}
@article{VolzShinPrinzbachetal.1978,
author = {Volz, H. and Shin, J.-H. and Prinzbach, H. and Babsch, H. and Christl, Manfred},
title = {Stability of Tricyclo[4.1.0.0\(^{2,7}\)]heptenyl-Cations},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58001},
year = {1978},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlHerbert1979,
author = {Christl, Manfred and Herbert, R.},
title = {Unusual Carbon Shielding Effects of Cyclopropanes and Double Bonds in Strained Bicyclo[3.1.0]hexanes and Cyclopentenes},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58038},
year = {1979},
abstract = {Carbon-13 shieldings and one-bond \(^{13}\)C-H coupling constants of bicydo[2.1.1]hexane, bicydo[2.l.l]hex- 2-ene, tricydo[3.1.1.0\(^{2.4}\)]heptane and benzvalene are presented and compared. to the data of related. compounds. H a bicydo[3.1.0]hexane system is part of a rigid skeleton, the cydopropane ring exerts spedfk: 'Y substituent eflects of two ldnds. In the case of the bicyclobexane boat form an upfield shift of the C-3 signal is observed and in tbe esse of the chair form a downfield shift of 15-20 ppm. Compared to the corresponding cydopentanes the double bond in strained cydopentenes causes downfield shifts of the C-4 absorption. 1bis eftect increases witb increasing strain, reaching 8 45.9 ppm maximum in benzvalene. Hence it is tbe only known bicydo[l.l.O]butane baving 8 reversed order of carbon shieldings. The downfield shifts are e:xplained by means of simple orbital interaction schemes.},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlLang1982,
author = {Christl, Manfred and Lang, R.},
title = {Tricyclo[5.1.0.0\(^{2,8}\)]octa-3,5-diene (Octavalene)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58124},
year = {1982},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{LeiningerLanzendoerferChristl1983,
author = {Leininger, H. and Lanzend{\"o}rfer, F. and Christl, Manfred},
title = {{\"U}ber die Reduktion des Benzvalenozonids zum cis-1,3-Cyclobutandimethanol mit LiAlH\(_4\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58130},
year = {1983},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlLeiningerMattauch1983,
author = {Christl, Manfred and Leininger, Hartmut and Mattauch, Brigitte},
title = {The 13C NMR Spectra of Some Tricyclo[2.2.0.02,6]hexane Derivatives : Unexpected High Field Absorptions Due to Additive gamma-anti Subsituent Effects},
isbn = {x},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-41620},
year = {1983},
abstract = {By means of the BC NMR spectra of tricyclo{2.2.0~rfJ6Jhexane and thirteen of its derivatives the effects of substituents in endo-3- and endo-5-positions on the HC chemical shifts have been determined. The y-anti effects are at least as Jarge as in monosubstituted cyc1obutanes, where the shielding values of second-row hetero substituents exceed those in unstrained systems by far, and higher-row and carbon substituents still cause substantial upfield shifts. In the title system the y-anti effect of a substituent in the endo-3- and endo-5-position are operative additively, and thus shift the absorption of C-J upfieJd by a maximum of 27 ppm with respect to the unsubstituted hydrocarbon.},
language = {en}
}
@article{LanzendoerferChristl1983,
author = {Lanzend{\"o}rfer, Franz and Christl, Manfred},
title = {3,4-Bismethylentricyclo[3.1.0.02,6]hexan - Synthese und Diels-Alder-Addition an Tetracyanethylen},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30263},
year = {1983},
abstract = {No abstract available},
language = {en}
}
@article{ChristlLeiningerBrueckner1983,
author = {Christl, Manfred and Leininger, Hartmut and Br{\"u}ckner, Dieter},
title = {On the Nature of the Bicyclo [3.2.1]octa-3,6-dien-2-yl Anion: A 13C NMR spectroscopic study},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30060},
year = {1983},
abstract = {No abstract available},
language = {en}
}
@article{ChristlBrunnLanzendorfer1984,
author = {Christl, Manfred and Brunn, Erich and Lanzendorfer, Franz},
title = {Reactions of Benzvalene with Tetracyanoethylene, 2,3-Dichloro-5,6-dicyano-rho-benzoquinone, Chlorosulfonyl Isocyanate, and Sulfur Dioxide. Evidence for Concerted 1,4-Cycloadditions to a Vinylcyclopropane System},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30052},
year = {1984},
abstract = {No abstract available},
language = {en}
}
@article{GleiterBischofGubernatoretal.1985,
author = {Gleiter, Rolf and Bischof, Peter and Gubernator, Klaus and Christl, Manfred and Schwager, Luis and Vogel, Pierre},
title = {2,3-Bis(methylene)bicyclo[2.1.1]hexane and 3,4-Bis(methylene)tricyclo[3.1.0.0\(^{2,6}\)]hexane : Interaction between a π System and a Cyclobutane or Bicyclobutane Moiety},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31845},
year = {1985},
abstract = {The He (I) photoelectron spectra of 2-bicyclo[2.1.l]hexene (1), 2,3-bis(methylene)bicyclo[2.1.l]hexane (3), and 3,4-bis(methylene)tricyclo[3.l.O.0\(^{2.6}\)]hexane (4) have been investigated. The assignment given is based on a ZDO model and semiempirical calculations. Tagether with the PE data of benzvalene (2), the reported data allow a comparison between 1-2 and 3-4. This yields a measure of the interactions between 8 cyclobutane or 8 bicyclobutane moiety and a double bond system within a ZDO model. The resonance integral found in the case of 1 and 3 amounts to -1.9 eV, that for 2 and 4, to -2.3 eV. The investigations furthermore reveal that the electronic factors which contribute to the higher reactivity of the bicyclobutane compounds amount to 5 kcal/mol.},
subject = {Chemie},
language = {en}
}
@article{ChristlLangHerzog1986,
author = {Christl, Manfred and Lang, R. and Herzog, C.},
title = {The Synthesis of Octavalene (Tricyclo[5.1.0.0\(^{2,8}\)]octa-3,5-diene) and Several Substituted Octavalenes},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58254},
year = {1986},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPeyerimhoff1986,
author = {Engels, Bernd and Peyerimhoff, S. D.},
title = {Theoretical study of the bridging in β-Halo Ethyl},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58779},
year = {1986},
abstract = {Large-acale multi-reference configuration interaction (MRD-CI) calculations in a quite flexible AO basis are employed to study the energy hypersurface for the reaction intermediates XC\(_3\)H\(_4\) with X = Cl, Br and F. Particular emphasis is therby placed on determining the equilibrium conformations, the CH\(_2\) rotation barrier and the energy surface for a possible bridging (shuttling motion (1a] of X between the two carbon centers). The absolute minimum in the potential energy surface is found in all three cases for the asymmetric ß-halo radical in general agreement with ESR data at an XCC angle of ca. 110°, a c-c separation somewhat shorter than a single bond and an approximate sp3 type hybridization (\(\alpha _2 \approx \) 135-140°). In FC\(_2\)H\(_4\) the energy difference between the minimum in the symmetric conformation and the absolute minimum is found to be more than 30 kcal so that shuttling seems impossible in agreement with experimental findings. In BrC\(_2\)H\(_4\) the difference between these two potential minima is only between 1-2 kcal, i.e., smaller than the barrier to CH\(_2\), rotation, so that· shuttling is favored, while ClC\(_2\)H\(_4\) takes an intermediate position between these extremes. The use of correlated wavefunctions is found to be quite important for such a study; the results are related to various kinetic studies of these radicals.},
subject = {Organische Chemie},
language = {en}
}
@article{GleiterBischofChristl1986,
author = {Gleiter, Rolf and Bischof, Peter and Christl, Manfred},
title = {Electronic Structure of Octavalene : Photoelectron Spectroscopic Investigations},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31588},
year = {1986},
abstract = {The He I photoelectron (PE) spectra of octavalene (5) as weil as its hydrogenated products 6-8 have been investigated. The assignment given is based on an empirical comparison of 5-8 with related compounds, a ZDO model, and semiempirical and ab initio calculations. Within the ZDO model the interaction between the buta.diene moiety and the bicyclobutane fragment of 5 is described by a resonance integral of -2.3 eV. The orbitalsequence of 5 is found tobe 2a\(_2\) (\(\pi\)-\(\sigma\)), 9a\(_1\) (\(\sigma\)), 3b1 (\(\pi\) - \(\sigma\)), 1a\(_2\) (\(\sigma\) + \(\pi\)), 2b\(_1\) (\(\sigma\) + \(\pi\)).},
subject = {Chemie},
language = {en}
}
@techreport{Christl1986,
author = {Christl, Manfred},
title = {Cycloadditions of 1,3,4-Oxadiazin-6-ones},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-41677},
year = {1986},
abstract = {No abstract available},
language = {en}
}
@article{EngelsPeyerimhoffDavidson1987,
author = {Engels, Bernd and Peyerimhoff, S.D. and Davidson, E.R.},
title = {Calculation of hyperfine coupling constants : An ab initio MRD-CI study for nitrogen to analyse the effects of the basis sets and CI parameter},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58784},
year = {1987},
abstract = {The hyperfine coupling constant for the nitrogen atom is evaluated by large-scale MRD-CI calculations. A detailed analysis of the charge density at the nucleus and the spin polarization in the ls and 2s shell as a function of various technical parameters is undertaken. Various (s, p) AO basis sets and the inftuence of correlation orbitals is investigated as weil as selection threshold and other properlies in CI calculations. The best value, obtained for the isotropic hyperfine coupling constant in an s, p, d basis, based on theoretical judgment of' best' quantities, is 9·9 MHz compared to 10·4509 MHz.},
subject = {Organische Chemie},
language = {en}
}
@article{SchreckChristl1987,
author = {Schreck, Michael and Christl, Manfred},
title = {Generation and Interception of 1-Oxa-3,4-cyclohexadiene},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31601},
year = {1987},
abstract = {No abstract available},
subject = {Chemie},
language = {en}
}
@article{ChristlSchreck1987,
author = {Christl, Manfred and Schreck, Michael},
title = {1,2,3,5,8,8a-Hexahydronaphthalin aus 1,2-Cyclohexadien},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31656},
year = {1987},
abstract = {Reaktionen von 1,3-Butadien und einigen seiner Methylderivate mit 1a und 1- Methyl-1,2-cyclohexadien 1b sowie den {\"U}bergang der [2 + 2]-Cycloaddukte 2 und 3 in das bisher unbekannte 1,2,3,5,8,8a-HexahydronaphthaJin 4a und einige seiner Methylderivate},
subject = {Chemie},
language = {en}
}
@article{KorthMuellerSustmannetal.1987,
author = {Korth, H.-G. and M{\"u}ller, W. and Sustmann, R. and Christl, Manfred},
title = {Rearrangement of Free Radicals, XII. ESR Spectroscopic Study of the Ring Opening of the Homobenzvalenyl Radical},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58376},
year = {1987},
abstract = {Abstraction of an allylic hydrogen atom in homobenzvalene (4) either in solurion by photolyticaßy generated tert-butoxyl radicals or in an adamantane matrix by X-rays produces the homobcnzvalenyl radical (5). which tbennally rearranps · to tbe tropylium ndical (1). In solution tbe activation cnergy for the rate determined step of the reaction sequence was detennined· to be 13.4 ± O.S kcal/mol.},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPeyerimhoff1988,
author = {Engels, Bernd and Peyerimhoff, S.D.},
title = {Study of the 1s and 2s shell contributions to the isotropic hyperfine coupling constant in nitrogen},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58793},
year = {1988},
abstract = {The istropic part of the hyperfine coupling constant is investigated by means of multireference configuration interaction calculations employing Gaussian basis sets. A detailed study of the 1s and 2s spin polarisation in the nitrogen atom and the NH molecule shows that the structure of the lower-energy space of the unoccupied orbitals is essential for the results. A contraction of the Gaussian basis is possible without loss of accuracy if enough flexibility is retained to describe the main features of the original space of unoccupied functions. Higher than double excitations are found to be non-negligible for the description of α\(_{iso}\).},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPeyerimhoffKarnaetal.1988,
author = {Engels, Bernd and Peyerimhoff, S.D. and Karna, S.P. and Grein, F.},
title = {The hyperfine coupling constants of the five lowest states of CH : An ab initio MRD-CI study},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58807},
year = {1988},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlKraft1988,
author = {Christl, Manfred and Kraft, A.},
title = {Tricyclo[3.1.1.0\(^{2,6}\)]hexandion (the Valen of o-Benzochinons), Bicyclo[2.1.1]hexan-2,3-dion and Valene of a Chinoxalins, of Phenazins and of a Benzophenazine},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58462},
year = {1988},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlFreundHennebergeretal.1988,
author = {Christl, Manfred and Freund, S. and Henneberger, H. and Kraft, A. and Hauck, J. and Irngartinger, H.},
title = {Several Polycyclic Valence Isomers of Dimethyl [14]Annulene-1,8-dicarboxylate. Reactivity of a "Nonconjugated" Bis(bicyclo[1.1.0]butane)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58413},
year = {1988},
abstract = {Diels-Alder reaction of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (5) with benzvalene (4), norbornene, and norbornadiene afforded the azo compounds 7 and 8. Theseare derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene as is azo compound 3, which had been obtained previously from 5 and 2 equiv of benzvalene (4). The photochemical extrusion of nitrogen from 3, 7, and 8 has been studied. Whereas 7 and 8 on direct irradiation in benzene gave rise exclusively to the bicyclo[2.2.0]hexane derivatives 9 and 10, respectively, from 3 in addition to the bicyclo[2.2.0]hexane 11, the diolefin 1l was formed. Diolefin 12 has cisdouble bonds in the nine-membered ring and is fixed in a boat conformation in a manner so that the two bicyclobutane systems approach each other very closely. This geometry suggests the unusual ring opening of the intermediate 1,4-cyclohexanediyl diradical from a boat conformation, which arises by inversion of the primarily generated boat conformation. Sensitized photolysis of 3 as weilasthat of ll produced the saturated isomer 13 of 11 and 12. The proximity of the bicyclobutane systems in 1l causes unprecedented reactions leading to cage compounds. When ll was heated at 90 °C, a rearrangement to the pentacyclic product 10 took place. Utilization of tetradeuteriated substrate ll-d4 supported a pathway with two diradical intermediates. Behaving in a convcntional manncr, bicyclobutane 9 and bis(bicyclobutane) 11 took up 1 and 2 equiv of thiophenol most probably in a radical-chain addition to give the thioethers 28 and 19, respectively. In contrast, bis(bicyclobutane) ll was converted by 1 equiv of thiophenol into cagc compound 30 in a process involving both the strained a systems. Heating at 80 °C subjected 30 to a reversible Copc rearrangement, resulting in a 6:1 mixture of 31 and 30. When it was treated with bromine, 11 was transformed to cage compound 38. This addition is believed to proceed via a cationic intermediate. The structure of cage compound 10 was established by a singlc-crystal X-ray analysis of dialcohol 11 prepared from 20 and methyllithium.},
subject = {Organische Chemie},
language = {en}
}
@article{HegmannChristlPetersetal.1988,
author = {Hegmann, Joachim and Christl, Manfred and Peters, Karl and Peters, Eva-Maria and Schnering, Hans Georg},
title = {Conjugated and Nonconjugated Cyclopentenones by a Reaction Cascade from Methyl 6-0xo-5-phenyl-1,3,4-oxadiazine-2-carboxylate and 1,3-Butadienes},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30207},
year = {1988},
abstract = {No abstract available},
language = {en}
}
@article{EngelsPeyerimhoff1989,
author = {Engels, Bernd and Peyerimhoff, S.D.},
title = {Theoretical study of FC\(_2\)H\(_4\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58824},
year = {1989},
abstract = {Large-scale multireference configuration interaction (MRD-CI) calculations in a quite flexible AO basis are employed to study the energy hypersurface for the reaction intermediate FC\(_2\)H\(_4\) • The reaction F + C\(_2\)H\(_4\) -> FC\(_2\)H\(_4\) as weil as the 1,2 migration of the fluorine atom in FC\(_2\)H\(_4\) is investigated. In addition the rotation around the CC bond in the optimum conformation is studied. The absolute minimum in the potential energy is found for the asymmetric structure but the symmetric structure is also found to be stable with respect to the dissociation, so that a shuttling of the fluorine atom is in principle possible but highly unlikely because ( l) the activation energy is high ( II 5-130 kJ fmol) and the saddle point lies only 4(}-50 kJ jmol below the dissociation Iimit of F + C\(_2\)H\(_4\) and (2) the competitive motion, i.e., rotation around the CC axis, is nearly free (I 1-17 kJ/mol).},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPeyerimhoff1989,
author = {Engels, Bernd and Peyerimhoff, S.D.},
title = {The hyperfine coupling constants of the X\(^3\)Σ\(^-\) state of NH : Influence of polarization functions and configuration space on the description of spin polarization},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58832},
year = {1989},
abstract = {The hyperfine coupling constants for the \(^3\)Σ\(-\) ground state of the NH molecule are determined by configuration interaction calculations whereby the infl.uence of polarization functions as weil as of the configuration space on the spin polarization mechanism is analysed. The dipolar part Au(N) and Au(H) can be obtained very reliably without much computational effort (A .. (N) == -45·3 MHz and A"(H) = -62·3 MHz). The value for the isotropic contribution a1.., in the best AO basis and MRD-CI treatment is - 64·5 MHz for H and 16·6 MHz for nitrogen compared to the corresponding experimental quantities of -66 MHz and 19 MHz respectively. Their determination depends on a subtle balance of the lu, 2u and 3u shell correlation description, whereby the dominant contribution to a1..,(H) results from the 2u shell. It is shown that the often good agreement of a110 values with experiment in a small basis singledouble configuration interaction treatment results from a cancellation of two errors.},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPeyerimhoff1989,
author = {Engels, Bernd and Peyerimhoff, S.D.},
title = {Study of orbital transformations in configurational interaction calculation of hyperfine coupling in nitrogen and CH molecule},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58847},
year = {1989},
abstract = {Multi-reference configuration interaction calculations employing various orbital transformations are undertaken to obtain the isotropic hyperfine coupling constant a\(_{iso\) in nitrogen and a\(_{iso\) (H) in the CH molecule. The natural orbital (NO) basis is found to be more effective than the simple RHF-MO basis; the most obvious is a basis of spin natural orbitals (SNO). It is found that a\(_{iso\) is approached from opposite sides in the NO and 2s shell SNO basis if the CI expansion is increased. Both results are within a few percent of the full CI Iimit for the nitrogen atorn (in the given AO basis) and the experimental value for Hin the CH radical. Various features ofthe SNO are discussed.},
subject = {Organische Chemie},
language = {en}
}
@article{KarnaGreinEngelsetal.1989,
author = {Karna, S. and Grein, F. and Engels, Bernd and Peyerimhoff, S.D.},
title = {The hyperfine coupling constants of \(^{19}\)F\(_2\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58817},
year = {1989},
abstract = {The isotropic (a\(_{iso}\)) and dipolar (A\(_{dip}\)) hyperfine coupling constants of 19F2 were obtained from MRD-CI wave functions using a variety of basis sets. In series I, increasing numbers of d functions were added to a 5s4p contracted Huzinaga!Dunning basis. In series II, the 5s3p basis set was uncontracted in several steps until 9s5p was reached, to which were added from one to three d-polarization functions. Cl parameters (selectioo threshoids and the number of reference coofiguratioos) were also varied. A study of the R dependence of aiso and Adip was perfonned. The best values obtained at R\(_e\) are 260 G for a\(_{iso}\) and 308 G for A\(_{dip}\)• compared with experimental values of about 280 G for a;10 and 320 G for A\(_{dip}\)·},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlBraun1989,
author = {Christl, Manfred and Braun, Martin},
title = {Generation and interception of 1-OXA-2,3-Cyclohexadiene and 1,2,4-Cyclohexatriene},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56566},
year = {1989},
abstract = {The cycloadducts 6 and 7 of tricyc1o[4.1.0.0 2 ,7)hepta- 3,4-diene (~) with styrene and 1,3-butadiene rearrange to unusual products on thermolysis, namely the cycloheptatriene derivatives ~ and 10. 1-0xa-3,4-cyclohexadiene (20) is generated smoothly from 6,6-dichloro-3-oxabicyclo[3.1.0]hexane (22) and n-butyllithium. 1-0xa-2,3-cyclohexadiene (11) is formed from 6-exo-bromo-6-endo-fluoro-2-oxabicyclo[ 3.1.0]hexane (30) and methyllithium. In the presence of activated olefins, this reaction provides an efficient route to 28 and 33 - 38, the trapping products of 21. Interestingly, [2+2]-cycloadditions do not take place at the same double bond of 21 as [4+2]-cycloadditions. The reactions of 1,3-cyclopentadiene and indene with bromofluorocarbene afford 6-exo-bromo-6-endo-fluorobicyclo[3.1.0]hex-2-ene (50) and its benzo derivative ~, respectively. On treatment of these compounds with methyl lithium in the presence of styrene, the interception products 53 and 47 of 1,2,4-cyc10- hexatriene (44) and its benzo derivative 43, respectively, are formed in good yields.},
subject = {Chemie},
language = {en}
}
@article{ChristlBrunnRothetal.1989,
author = {Christl, Manfred and Brunn, E. and Roth, W. R. and Lennartz, H.-W.},
title = {7-Methyl- and 7-Phenylcyclohepta-1,3,5-trienes from Benzvalene Via 3,3a,4,5,6,6a-Hexahydro-4,5,6-methenocyclopentapyrazoles and Tetracyclo[4.1.0.0\(^{2,4}\).0\(^{3,5}\)]heptanes},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58471},
year = {1989},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlBraun1989,
author = {Christl, Manfred and Braun, Max},
title = {Photocycloadditions of Benzvalene},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58506},
year = {1989},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPeyerimhoffSkell1990,
author = {Engels, Bernd and Peyerimhoff, S.D. and Skell, P.S.},
title = {Theoretical study of the potential energy surface governing the stereochemistry in ClC\(_2\)H\(_4\) reactions},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58851},
year = {1990},
abstract = {Large-scale multireference configuration interaction calculations in a double·t·type AO basis including polarization functions are carried out for the potential surface of the ClC\(_2\)H\(_4\9 system. The charge distribution for various extreme points of the surface is discussed. The absolute minimum is found for an asymmetric ClC2H4 structure. The symmetrical bridged nuclear conformation is also found to be stable with respect to dissociation into Cl + C\(_2\)H\(_4\)• The activation energy for rotation about the C-C axis is calculated tobe around 18 kJ/mol, which is comparable tothat for the 1,2 migration {around 26 kJ/mol). The stereochemistry is governed by the fact that addition of CI to C\(_2\)H\(_4\) (or dissociation) is a two-step reaction proceeding through a symmetrica1 intermediate. The direct addition pathway possesses a small barrier of about 8 kJ jmol.},
subject = {Organische Chemie},
language = {en}
}
@article{KimChristlKochl1990,
author = {Kim, E. and Christl, Manfred and Kochl, J. K.},
title = {Charge-Transfer Cycloaddition of Homobenzvalene with Tetracyanoethylene},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58537},
year = {1990},
abstract = {The transient yellow color observed in the cycloaddition of homobenzvalene (HB) with tetracyanoethylene (TCNE) is associated with the charge-transfer complex [HB, TCNE). The deliberate photoexcitation of [HB,TCNE) affords a mixture of charge-transfer cycloadducts (1, 2, and 3) that differs from that obtained in thermal cycloaddition. The relationship of {HB t TCNE•) radical-ion pair (as the critical reactive intermediate in charge-transfer cycloaddition) to the activation process for thermal cycloaddition is discussed.},
subject = {Organische Chemie},
language = {en}
}
@article{KarnaGreinEngelsetal.1990,
author = {Karna, S.P. and Grein, F. and Engels, Bernd and Peyerimhoff, S.D.},
title = {Ab initio configuration-interaction studies of the ground state potential energy and hyperfine coupling constants of \(^{35}\)Cl\(_2^-\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58869},
year = {1990},
abstract = {Potential energy and spectroscopic constants for the X\(^2 \sum^+ _\mu\) ground state of a;, were calculated by configuration-interaction (Cl) methods, using large basis sets with polarization and diffuse functions. From these CI wavefunctions, the isotropic (a\(_{iso}\)) and dipolar (A\(_{dip}\)) components of the hyperfine coupling constant were obtained. The effects of various s, p basis sets, polarization and diffuse functions, as well as the influence of reference configurations and configuration selection thresholds were investigated. The best values obtained are 35·31 G for a\(_{iso}\) and 29·440 for A\(_{dip}\)• tobe compared with experimental values of 37 ± 1 G and 32 ± 1 G, respectively. It is shown that the contributions to a1so of the K and L shells are opposite in sign, differing by about 4 G. Upon vibrational averaging, both a\(_{iso}\) and A\(_{dip}\) move towards smaller values as v increases. An adiabatic electron affinity of 2·46eV was obtained for CL\(_2\) , and a vertical electron detachment energy of 3·71 eV for Cl;.},
subject = {Organische Chemie},
language = {en}
}
@article{FunkenEngelsPeyerimhoffetal.1990,
author = {Funken, K. and Engels, Bernd and Peyerimhoff, S.D. and Grein, F.},
title = {Study of the hyperfine coupling constants of the moleculs NH2, NHD and ND2},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58875},
year = {1990},
abstract = {In the present paper we c:alculate tbe magnetic hyperfine couplina constants (hfcc) ai.ID and A11 of the ground states of the isotopes NH2, NHD and ND2 using truncated MR..CI methods. Differences from other theoretical methocls and shortoominp of the truncated Cl approach in calculating tlj10 are studied. Polarization effects wbich detennirae ailo. as weU as a simple model to describe the dipolar hfcc's, are discussed. All results are in. excellent aareement with experimental data. lt is shown that ab initio methods are able to obtain reliable values for otf-diaaonal values of A41 which are difficult to measure experimentaDy.},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlReuchlein1990,
author = {Christl, Manfred and Reuchlein, H.},
title = {Synthesis and NMR Spectra of 2,3-Dihydro-1,3-methanoindene Derivatives and 1,2,3,5-Tetrahydro-1,3-methanopentalene},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58557},
year = {1990},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{IrngartingerReimannLangetal.1990,
author = {Irngartinger, H. and Reimann, W. and Lang, R. and Christl, Manfred},
title = {Electron Density Distribution in a Bicyclo[l.l.0]butane},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31576},
year = {1990},
abstract = {No abstract available},
language = {en}
}
@article{ChristlKrimmKraft1990,
author = {Christl, Manfred and Krimm, Stefan and Kraft, Arno},
title = {Some Valenes of Benzannelated Five-Membered Heteroarenes - Synthesis and NMR Spectra},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30026},
year = {1990},
abstract = {No abstract available},
language = {en}
}
@article{BuendgenEngelsPeyerimhoff1991,
author = {B{\"u}ndgen, P. and Engels, Bernd and Peyerimhoff, S.D.},
title = {An MRD-CI study of low-lying electronic states in CaF},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58880},
year = {1991},
abstract = {Dipole moments and various spectroscopic constants of some low-lying electronic states of the CaF molecule have been calculated using the multireference single· and double-excitation configuration-interaction (MRD-CI) method. The electronic structure of the highly ionic molecule in various excited states can be explained in tenns of different polarisations of the mainly Cacentered valence electron in the field of the F\(^-\) anion. Plots of natural orbitals occupied by the valence electron in the different states give a qualitative picture of the charge distribution and provide a visualisation of the different polarisations of the valence electron in the various states. Comparisons with the electrostatic polarisation model ofT{\"o}rring, Ernstand K{\"a}ndler (TEK model) are made. The unknown A' \(^2 \Delta\) state is predicted to lie about 21200 cm\(^{-1}\) above the ground state.},
subject = {Organische Chemie},
language = {en}
}
@article{Engels1991,
author = {Engels, Bernd},
title = {Estimation of the influence of the configurations neglected within truncated MR-CI wavefunctions on molecular properties},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58890},
year = {1991},
abstract = {Reliable prediction of the isotropic hyperfine coupling constant, a\(_{iso}\), is still a difficult task for ab initio calculations. Strong dependence on the method used for its calculation is found. Within a truncated multi-referencc ansatz a\(_{iso}\) is strongly affected by the size ofthe reference space and the nurober of terms in the truncated Cl expansion. In the present paperdifferent effects of the neglected Cl space are discussed. Modified B\(_K\) and A\(_K\) methods are used to estimate the contributions ofthe neglected configurations. lt can be shown that a combination of both methods is able to recover about 90-9 S\% of the total error in a\(_{iso}\)· Furthermore, it was found that to obtain about 90\% of the B\(_K\) correction only about I 0-20\% ofthe configurations within H0 have to be corrected.},
subject = {Organische Chemie},
language = {en}
}
@article{PericEngelsPeyerimhoff1991,
author = {Peric, M. and Engels, Bernd and Peyerimhoff, S.D.},
title = {Ab initio investigation of the vibronic structure of the C\(_2\)H spectrum Calculation of the hyperfine coupling constants for the three lowest lying electronic states},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58901},
year = {1991},
abstract = {The hyperfine coupling constants (isotropic hfcc and four Cartesian components of the ani~ tropic tensor) are calculated for all three atoms of C\(_2\)H in its three lowest-lying electronic states at various molecu)ar geometries by means of the ab initio configuration interaction ( MRO.CI) method. The off-diagonal electronic matrix elements involving the two species ofthe A' symmetry are also computed. A diabatic transforrnation is perforrned Jeading to simple geometrical depen· dences of the hyperline coupling constants.},
subject = {Organische Chemie},
language = {en}
}
@article{PericEngelsPeyerimhoff1991,
author = {Peric, M. and Engels, Bernd and Peyerimhoff, S.D.},
title = {Ab initio investigation of the vibronic structure of the C\(_2\)H spectrum Computation of the vibronically-averaged values for the Hyperfine Coupling Constants},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58915},
year = {1991},
abstract = {The vibronically averaged values for tbe hyperfine coupling constants in the X\(^2 \sum\)-A\(^2 \Pi\) system of the ethynyl radical are computed by means of tbe ab initio metbod calculations. The results point at tbe importance of taking into account the coupling of a1l tbree electronic states in question ( I\(^2\)A', 2\(^2\)A', and 1\(^2\)A") for a reliable explanation of the available experimental findings. The mean values of the hfcc's for K = 0 and 1 levels in \(^{13}\)C\(_2\)H and \(^{13}\)C\(_2\)D in the energy range up to 6000 cm\(^{-1}\) are predicted.},
subject = {Organische Chemie},
language = {en}
}
@article{MarkgrafCortDavisetal.1991,
author = {Markgraf, J. H. and Cort, J. R. and Davis, H. A. and Lindeman, N. I. and Myers, C. R. and Kraft, A. and Christl, Manfred},
title = {Strained Heterocyclic Systems. 20. Basicities of Bicyclic Quinoxalines},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58584},
year = {1991},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{BentleyChristlNorman1991,
author = {Bentley, T. W. and Christl, Manfred and Norman, S. J.},
title = {Methanesulfonate/p-Nitrobenzoate and p-Toluenesulfonate/p-Nitrobenzoate Rate Ratios. Solvolyses of 1-Adamantyl and Benzhydryl Substrates},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58599},
year = {1991},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{GigstadRicciJRMarkgrafetal.1991,
author = {Gigstad, Kenneth M. and Ricci JR., John S. and Markgraf, J. Hodge and Christl, Manfred and Kraft, Arno},
title = {Strained Heterocyclic Systems : 18. Structure of 1,2,3-Methylidyne-2,3-dihydro-1H-cyclopenta[b]quinoxaline},
isbn = {0108-2701},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-41645},
year = {1991},
abstract = {No abstract available},
language = {en}
}
@article{ReuterEngelsPeyerimhoff1992,
author = {Reuter, W. and Engels, Bernd and Peyerimhoff, S.D.},
title = {The Reaction of Singlet and Triplet Methylen with Ethene - A Multi-Reference CI Study},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58923},
year = {1992},
abstract = {Large-scale multireference configuration interaction (MRD-CI) calculations in a flexible atomic orbital (AO) basis are employed to study the reaction of C\(_2\)H\(_4\) with CH\(_2\) in its firSt triplet and singlet state. The minimum energy path (MEP) of both reactions is calculated, and different mechanisms are discussed. To examine the possible participation of the singlet state in the overall reaction starting from the triplet channel and terminating in the singlet-state c-C\(_3\)H\(_6\), various cuts through both hypersurfaces are calculated. lt is found that favorable interconversion from the trip1et to the singlet surface can only occur at !arge separations of the two fragments of CH2 and C\(_2\)H\(_4\). Experimental data considering the vibrational motion of CH\(_2\) in connection with the relative position of both surfaces are used to obtain an estimate for the overall barrier of the reaction. The height of the barrier is about 6 kcal/mol, while the barrier of the pure triplet reaction is calculated to be 7-9 kcal/mol.},
subject = {Organische Chemie},
language = {en}
}
@article{EngelsPericReuteretal.1992,
author = {Engels, Bernd and Peric, M. and Reuter, W. and Peyerimhoff, S.D. and Grein, F.},
title = {Study of the hyperfine coupling constants \(^{14}\)N and \(^1\)H) of the NH\(_2\) molecule in the X\(^2\)B\(_1\) ground state and the A\(^2\)A\(_1\) excited state},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58937},
year = {1992},
abstract = {The hyperfine coupling constants (hfcc) A\(_{iso}\) and A\(_{ij}\) are calculated for the atoms of NH\(_2\) in its, two lowest-lying electronk states at various molecular geometries by means of the ab initio multireference configuration interaction .method. The vibronically averaged values of the hfccs for the K = 0 and 1 levels in \(^{14}\)N \(^1\)H\(_2\) in the energy range up to 20 000 cm\(^{-1}\) are computed. Polarization elfects which determine A\(_{iso}\) as well as a simple model to describe the dipolar hfccs are discussed. All resrilts are in excellent agreement with experimental data.},
subject = {Organische Chemie},
language = {en}
}
@article{PericEngels1992,
author = {Peric, M. and Engels, Bernd},
title = {Ab initio calculations of the vibronically averaged values for the hyperfine coupling constants in NH\(_2\), NHD and ND\(_2\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58941},
year = {1992},
abstract = {Vibronically averaged values for K =0 and K = 1 bending levels in the energy range between 0 and 25 000 cm\(^{-1}\) are computed for the \(^{14}\)N, H, and D atoms in NH\(_2\), NHD, and ND\(_2\) The pure ab initio electronic potentials, as well as those derived by fitting of experimentally observed band positions are employed. Effects of vibronic coupling and local perturbations of close-lying levels belanging to different electronic states are discussed.},
subject = {Organische Chemie},
language = {en}
}
@article{PericEngels1992,
author = {Peric, M. and Engels, Bernd},
title = {Ab initio calculation of the vibronically averaged hyperfine coupling constants in the 1\(^2\)Π\(_u\) electronic state of CH\(_2 ^+\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58951},
year = {1992},
abstract = {The results of pure ab initio calculations of the hyperfine coupling constants for the 1 \(^2 \pi _u\) electronic state for various isotopomers of CHi in the energy range between 0 and 20 000 cm\(^{-1}\) are presented. Effects of vibronic and spin-orbit coupling are discussed.},
subject = {Organische Chemie},
language = {en}
}
@article{KauppSchleyer1992,
author = {Kaupp, Martin and Schleyer, Paul von Rague},
title = {Do Low-Coordinated Group 1-3 cations M\(^n\) \(^+\)L\(_m\) (Mn\(^+\) = K\(^+\), Rb\(^+\), Cs\(^+\), Ca\(^{2+}\), Sr\(^{2+}\), Ba\(^{2+}\), Sc\(^{3+}\), Y\(^{3+}\), La\(^{3+}\); L = NH\(_3\), H\(_2\)0, HF; m = 1-3) with a Formal Noble-Gas Electron Configuration Favor Regular or "Abnormal" Shapes?},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-60111},
year = {1992},
abstract = {No abstract available},
subject = {Anorganische Chemie},
language = {en}
}
@article{ChristlBraunMueller1992,
author = {Christl, Manfred and Braun, Martin and M{\"u}ller, Germar},
title = {1,2,4-Cyclohexatriene, an Isobenzene, and Bicyclo[4.4.0)deca-1,3,5,7,8-pentaene, an Isonaphthalene: Generation and Trapping Reactions},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58639},
year = {1992},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{MetropoulosEngelsPeyerimhoff1993,
author = {Metropoulos, B. and Engels, Bernd and Peyerimhoff, S.D.},
title = {On the chemi-ionization reaction O + CH ----> HCO\(^+\)+ e\(^-\). Coollinear O-CH Approach},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58963},
year = {1993},
abstract = {We have investigated theoretically the importance of the O(\(^3\)P)+CH(a\(^4\sum^-\)) and the O(\(^3\)P)+CH(X\(^2\Pi\)) channels in the collinear chemi-ionization reaction O+CH->HCO\(^+\) +e\(^-\). We have found that both channels may lead to chemi-ionization via favorable Franck-Condon overlaps with the states ofthe ionic species.},
subject = {Organische Chemie},
language = {en}
}
@article{Engels1993,
author = {Engels, Bernd},
title = {Study of influences of various excitation classes on ab initio calculated isotropic hyperfine coupling constants},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58973},
year = {1993},
abstract = {Reliable prediction of the isotropic hyperfine coupling constant A\(_{iso}\) is still a difficult task for ab initio calculations. Strang dependence on the method employed for its ca1culation has been found. Within a CI ansatz A\(_{iso}\) is considerably affected by the excitation classes taken into account within the CI calculation. In the present work the influence of various excitation classes on A\(_{iso}\) is examined. Calculations including all single, double, triple and a large part of the quadruple excitations are performed and the individual effects of the excitation classes are studied. It is found that the surprisingly good agreement found for S-CI treatments is due to large error cancellations. The importance of higher than double excitations arises from their indirect influence on the single excitations.},
subject = {Organische Chemie},
language = {en}
}
@article{GittinsHarrisFieldetal.1993,
author = {Gittins, C. M. and Harris, N. A. and Field, R. W. and Verges, J. and Ernst, W. E. and B{\"u}ndgen, P. and Engels, Bernd},
title = {Analysis and Depertubation of the C\(^2\)Π and D\(^2\)Σ\(^+\) states of CaF},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58980},
year = {1993},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{StaikovaEngelsPericetal.1993,
author = {Staikova, M. and Engels, Bernd and Peric, M. and Peyerimhoff, S. D.},
title = {Ab initio calculation of the vibronically averaged hyperfine coupling constants in the two lowest electronic states of H\(_2\)O\(^+\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58998},
year = {1993},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{BentleyNormanGerstneretal.1993,
author = {Bentley, T. W. and Norman, S. J. and Gerstner, E. and Kemmer, R. and Christl, Manfred},
title = {Solvolysis of Tricyclo[3.1.0.0\(^{2,6}\)]hex-3-yl and Bicyclo[2.1.1]hex-2-yl Sulfonates},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58689},
year = {1993},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{EpeHaeringRamaiahetal.1993,
author = {Epe, Bernd and H{\"a}ring, Martin and Ramaiah, Danaboyina and Stopper, Helga and Abou-Elzahab, Mohamed M. and Adam, Waldemar and Saha-M{\"o}ller, Chantu R.},
title = {DNA damage induced by furocoumarin hydroperoxides plus UV (360 nm)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-86870},
year = {1993},
abstract = {Wben irradiated at 360 nm, furocoumarins with a hydroperoxide group in a side chain effciently give rise to a type of DNA damage that can best be explained by a photoinduced generation of hydroxyl radicals from the excited pbotosensitizers. The observed DNA damage profiles, i.e. the ratios of single-strand breaks, sites of base loss (AP sites) and base modifications sensitive to fonnamidopyrimidine-DNA glycosylase (FPG protein) and endonuclease m, are similar to the DNA damage profile produced by hydroxyl radicals generated by lonizing radiation or by xanthine and xanthine oxidase in the presence of Fe(III)-EDTA. No such damage is observed with the corresponding furocoumarin alcohols or in the absence of near-UV radiation. The damage caused by the photo-excited hydroperoxides is not influenced by superoxide dismutase (SOD) or catalase or by D2O as solvent. The presence of t-butanol, however, reduces both the formation of single-strand breaks and of base odifications sensitive to FPG protein. The cytotoxicity caused by one of the hydroperoxides in L5178Y mome lymphoma cells is found to be dependent on the near-UV irradiation and to be much higher than that of the corresponding alcohol. Therefore the new type of photoinduced damage occurs inside cells. Intercalating photosensitizers with an attached hydroperoxide group might represent a novel and versatile class of DNA damaging agents, e.g. for phototherapy.},
subject = {DNS-Sch{\"a}digung},
language = {en}
}
@article{ChristlTuerkPetersetal.1994,
author = {Christl, Manfred and T{\"u}rk, M. and Peters, K. and Peters, E.-M. and Schnering, H. G. von},
title = {Octahydro-1,2,3:4,5,6-dimethenopentalene-2-carbonitrile, the First Derivative of a Yet-Unknown (CH)\(_{10}\)-Hydrocarbon},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58731},
year = {1994},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{BentleyChristlKemmeretal.1994,
author = {Bentley, T. W. and Christl, Manfred and Kemmer, R. and Llewellyn, G. and Oakley, J. E.},
title = {Kinetic and Spectroscopic Characterisation of Highly Reactive Methanesulfonates. Leaving Group Effects for Solvolyses and Comments on Geminal Electronic Effects Influencing S\(_N\)1 Reactivity},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-58748},
year = {1994},
abstract = {Highly reactive methanesulfonates (mesylates, ROMs) have been prepared from 1-phenylethanol. cyclohex-2-en-1-ol, diphenylmethanol and p-methoxybenzyl alcohol by treatment with methanesulfonyl chloride and triethylamine in dichloro- or trichloro-methane at - 20 to 0 °C. The mesylates. characterised in solution by \(^1\)H and \(^{13}\)C NMR at -20 °C, were obtained in satisfactory purity (ca. 95\%) in cold solutions but they decomposed by reaction with chloride, triethylamine or the parent alcohol. Rate constants for solvolyses in aqueous acetone and aqueous ethanol have been determined by a fast response conductimetric method. Product selectivities for solvolyses of pmethoxybenzyl mesylate in aqueous ethanol and methanol at 0 °C have been determined by HPLC. From additional new or Iiterature kinetic data for solvolyses of corresponding bromides. chlorides and p-nitrobenzoates (OPNB). Br/CI. OMs/Br and OMs/OPNB rate ratios were calculated; the results are consistent with electronic effects stabilising the carbocationic transition states and increasing OMs/Br rate ratios for these SN 1 solvolyses; none of the evidence supports a geminal electronic effect on Br/CI rate ratios (e.g. caused by stabilisation of the initial state in pmethoxybenzyl chloride). Steric effects on ester /halide rate ratios for solvolyses of tertiary substrates are confirmed. Relative rates over a 10\(^{16}\) range for ester and halide leaving groups are evaluated for solvolyses of 1-phenylethyl substrates in 80\% ethanol-water. updating previous work by Noyce et al. (1972).},
subject = {Organische Chemie},
language = {en}
}
@article{WissingKauppBoersmaetal.1994,
author = {Wissing, Elmo and Kaupp, Martin and Boersma, Jaap and Spek, Anthony L. and Koten, Gerard van},
title = {Alkylation Reactions of Dialkylzinc Compounds with 1,4- Diaza- 1,3-butadienes: Cationic and radical Anionic Organozinc Intermediates. Molecular Structure of the Cationic Organozinc Species [MeZn(t-BuN=CHCH=N-t-Bu)]O\(_3\)SCF\(_3\) and Me\(_2\)Zn(bpy)(bpy = 2,2' -Bipyridine)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-60008},
year = {1994},
abstract = {No abstract available},
subject = {Anorganische Chemie},
language = {en}
}
@article{SuterPlessErnzerhofetal.1994,
author = {Suter, H. U. and Pleß, V. and Ernzerhof, M. and Engels, Bernd},
title = {Difficulties in the Calculation of Electron Spin Resonance Parameters using Density Functional Methods},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59113},
year = {1994},
abstract = {Density functional theory is applied to the calculation ofthe isotropic byperfine coupJing constants in some small molecules. Various functionals are tested. The agreement of the calculated values to experimental data and values obtained from sophisticated ab initio methods depends on the functionals used and the system under consideration. With respect to spin density calculations the functional of Lee, Yang and Parr with Becke's excbange functional (BLYP) is found to give good results for tbe heavier center of the CH and the NH molecule, while the spin densities of other molecules such as OH, H\(_2\)CN, H\(_2\)CO\(^+\), NO and O\(_2\) deviate considerably from experimental and/or other theoretical results (30\%-60\%). In cases where the singly occupied orbital can contribute to the isotropic hyperfine coupling constants, accurate results are obtained. The reason fortbis is analyzed.},
subject = {Organische Chemie},
language = {en}
}
@article{MuehlhaeuserFroudakisZdetsisetal.1994,
author = {M{\"u}hlh{\"a}user, M. and Froudakis, G. and Zdetsis, A. and Engels, Bernd and Flytzanis, N. and Peyerimhoff, S. D.},
title = {Ab initio investigation of the stability of Si\(_3\)C<\(_3\) clusters and their structural and bonding features},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59060},
year = {1994},
abstract = {Various structural possibilities for Si\(_3\)C\(_3\) clusters are investigated by ab initio calculations employing basis sets of double- and triple-zeta quality augmented by d polarization functions. Correlation effects are included by a second-order Moeller Piesset perturbation treatment. For the two lowest-lying structures higher-order correlation corrections and multi-reference effects are also included. Bonding features are investigated by two different types of population analyses to obtain insight into the nature of chemical bonding. A total of 17 stationary points were investigated, 14 of which correspond to local minima and three being transition states. The energetically lowest-lying structures are: A "pyramidlike" structure with various multicenter bonds, followed by a es symmetric isomer closely related to the ground state Si6 structure. Planar structures, favoured in small carbon clusters, lie higher in energy and are transition states. The lowest-lying triplet system is found to be the linear nonsymmetric Si - C-C-C-Si -Si structure, which is calculated to lie about 38 kcalfmole above the singlet ground state. A building-up principle based on bonding criteria is suggested for the occurence of the various structural possibilities.},
subject = {Organische Chemie},
language = {en}
}
@article{WortmannSalehEngelsPeyerimhoff1994,
author = {Wortmann-Saleh, D. and Engels, Bernd and Peyerimhoff, S. D.},
title = {Theoretical Study of the Reaction O(\(^3\)P) + C\(_2\)H\(_4\) and comparison with the \(^3\)CH\(_3\) + C\(_2\)H\(_4\) Reaction},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59076},
year = {1994},
abstract = {The minimum energy path for the reaction O(\(^3\)P\(_g\)) + C\(_2\)H\(_4\)(\(^1\)A\(_g\)) has been calculated by optimizing all relevant geometrical parameters along the approach of oxygen to ethene. A barrier of 4.7 kcal/mol in the \(^3\)A"( ... 9a'\(^2\)- 10a'3a") potential energy surface and an energy difference of 14.4 kcal/mol between the product and the fragments is found at the multireference-configuration interaction level. The corresponding values at the lower-level treatment CASSCF are 9 kcal/mol for the barrier and 9 kcal/mol for the depth of the potential; this shows the importance of inclusion of electron correlation. The barrier for CH\(_2\) rotation for the lowestenergy structure (asymmetric OC\(_2\)H\(_4\)) is around 5 kcal/mol. The energy gap to the first excited state \(^3\)A'( ... 9a'l0a'3a'12) is found tobe 3.6 kcal/mol in MRD-CI calculations at the ground-state minimum. Comparison with \(^3\)CH\(_2\) + C\(_2\)H\(_4\) shows that in this system the lowest-energy surface is \(^3\)A', i.e., the state which is the excited state in 0 + C\(_2\)H\(_4\). This difference in energy ordering of \(^3\)A' and \(^3\)A" states results from the fact that the p\(_x\), p\(_y\), p\(_z\) degeneracy of oxygen orbitals is lifted in \(^3\)CH\(_2\)leading to b\(_1\), b\(_2\). and a\(_1\) MOs whereby the lowest b\(_2\) (a") remains doubly occupied; as a consequence, the reaction pattem between the oxygen and \(^3\)CH\(_2\) approach is different, which is also quite apparent in the calculated charge transfer.},
subject = {Organische Chemie},
language = {en}
}
@article{StaikovaPericEngelsetal.1994,
author = {Staikova, M. and Peric, M. and Engels, Bernd and Peyerimhoff, S. D.},
title = {Ab initio Investigation of the Structure of the X\(^2\)A', A\(^2\)A'' (1\(^2\)Π) Spectral System of HCO: Investigation of the Magnetic Hyperfine Effects},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59089},
year = {1994},
abstract = {Results ofan ab initio study ofthe hyperfine structure of the X\(^2\)A', A\(^2\) A" ( 1\(^2 \Pi\)) system ofthe formyl radical are presented. Special attention is paid to the analysis of the interplay between the vibronic and magnetic hyperfine etfects. The results of computations are in very good agreement with the available experimental findings. The values for the hyperfine coupling constants in lower bending Ievels of both electronic species are predicted.},
subject = {Organische Chemie},
language = {en}
}
@article{FroudakisZdetsisMuehlhaeuseretal.1994,
author = {Froudakis, G. and Zdetsis, A. and M{\"u}hlh{\"a}user, M. and Engels, Bernd and Peyerimhoff, S. D.},
title = {A comparative ab initio study of the Si\(_2\)C\(_4\), Si\(_3\)C\(_3\), Si\(_4\)C\(_2\) clusters},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59097},
year = {1994},
abstract = {Various structural possibilities for the Si\(_2\)C\(_4\) and Si\(_4\)C\(_2\) clusters are investigated by employing a basis set of triple-zeta plus polarization quality; electron correlation is generally accounted for by second-order M0ller-Plesset and, in certain instances, by higher-order perturbation (CASPT2) approaches. The building-up principle recently suggested from an analysis of Si\(_3\)C\(_3\) clusters is found to be fully operative for Si\(_2\)C\(_4\) and Si\(_4\)C\(_2\) clusters. A comparison of the structure and stability of various geometrical arrangements in the series C\(_6\) , Si\(_2\)C\(_4\) , Si\(_3\)C\(_3\) , Si\(_4\)C\(_2\), and Si\(_6\) shows that linear and planar structures become rapidly less stable if carbons are replaced by silicons and that the three-dimensional bipyramidal forms become less favorable as soon as silicons are exchanged by carbons in the parent Si\(_6\) structure. The effects can be rationalized in qualitative terms based on differences in silicon and carbon bonding.},
subject = {Organische Chemie},
language = {en}
}
@article{SuterHuangEngels1994,
author = {Suter, H. U. and Huang, M.-B. and Engels, Bernd},
title = {A Multireference Configuration Interaction Study of the Hyperfine Structure of the Molecules CCO, CNN and NCN in their triplet ground states},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59108},
year = {1994},
abstract = {The hyperfine structures of the isoelectronic molecules CCO. CNN, and NCN in their triplet ground states (X\(^3 \sum ^-\)) are investigated by means of ab initio methods. The infrared frequencies and geometries are detennined and compared with experiment. Configuration selected multireference configuration interaction calculations in combination with perturbation theory to correct the wave function (MRD-CI/B\(_K\)) employing extended atomic orbital (AO) basis sets yielded very accurate hyperfine properties. The theoretical values for CCO are in excellent agreement with the experimental values determined by Smith and Weltner [J. Chem. Phys. 62,4592 (1975)]. For CNN, the first assignment of Smith and Weltner for the two nitrogen atoms has to be changed. A qualitative discussion of the electronic structure discloses no simple relation between the structure of the singly occupied orbitals and the measured hyperfine coupling constants. Vibrational effects were found to be of little importance.},
subject = {Organische Chemie},
language = {en}
}
@article{StaikovaEngelsPeric1994,
author = {Staikova, M. and Engels, Bernd and Peric, M.},
title = {Ab initio investigation of the hyperfine structure in the 1\(^2\)Π\(_u\)(X\(^2\)A\(_1\), A\(^2\)B\(_1\) system of BH\(_2\))},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59000},
year = {1994},
abstract = {No abstract available},
subject = {Organische Chemie},
language = {en}
}
@article{Engels1994,
author = {Engels, Bernd},
title = {Detailed study of the configuration selected multi-reference configuration interaction method combined with perturbation theory to correct the wave function},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59019},
year = {1994},
abstract = {A reliable prediction of the isotropic hyperfine coupling constant A\(_{iso}\) is still a difficult task for ab initio calculations. In previous studies, the configuration selected multireference configuration interaction method in combination with perturbation theory to correct the wave function (MRCI/ B\(_K\)) yielded accurate isotropic hyperfine coupling constants very economically. The present study gives a detailed analysis of the MRCI/ B\(_K\) method based on the X\(^2 \pi\) state of CH as a test case. Furthermore, a comparison to various other methods such as Maller-Ptesset perturbation theory and the coupled cluster approach is made. The success of the MRCI/ B\(_K\) method in predicting isotropic hyperfine coupling constants is explained in terms of the inßuence of higher than double excitations.},
subject = {Organische Chemie},
language = {en}
}
@article{SuterEngels1994,
author = {Suter, H. U. and Engels, Bernd},
title = {Theoretical investigation of ESR parameters: H\(_2\)CN and H\(_2\)CO\(^+\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59029},
year = {1994},
abstract = {The hyperfine structure of the two isoelectronic molecules H\(_2\)CN and H\(_2\)CO\(^+\) in their electronic ground state (X\(^2\)B\(_2\)) is studied. The influence of the atomic orbital (AO), basis sets, of the correlation treatment, and of the. equilibrium geometry on the obtained hyperfine propertles 1s - investigated. It is found that the multireference double excitation-configuration interaction (MRD-CI)/ BK treatment in which an MRD-CI wave function is corrected by a modified B\(_K\) method yields equivalent results to quadratic CI [QCISD(T)], coupled cluster single doubles [CCSD(T)), or Brueckner doubled [BD(T)]. Uncertainties in the equilibrium geometries are found to be the major source for discrepancies between theoretically and experimentally determined isotropic hyperfine coupling constants (hfccs). For the heavier centers, the calculated values of the isotropic hfccs agrees nearly perfectly with experimental values (\(\approx\) 1\%-2\%). The calculated values for the hydrogens are too low, but using the equilibrium structure suggested by Yamamoto and Sato [J. Chem. Phys. 96, 4157 ( 1992)], the best estimate deviates by less than 3\%.},
subject = {Organische Chemie},
language = {en}
}
@article{HuangSuterEngels1994,
author = {Huang, M.-B. and Suter, H. U. and Engels, Bernd},
title = {Theoretical Study of the Dimethylamino Radical (CH\(_3\))\(_2\)N and its protonated cation (CH\(_3\))\(_2\)NH\(^+\)},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59047},
year = {1994},
abstract = {In the present work the dimethylamino radical ( ( CH\(_3\)) \(_2\)N) and its protonated cation ( ( CH\(_3\))\(_2\)NH\(^+\)) are investigated by means of ab initio methods. The geometries of various conformations of both compounds are obtained with UMP2/6·31 G** calculations, while the hyperfine structure and its dependence on the geometry is studied using the MRD-Cl/B\(_K\) method. The two molecules are compared to study the inftuence of the protonation on geometry and hyperfine structure. The effects of the rotational barriers on the hyperfine structures of (CH\(_3\))\(_2\)N, (CH\(_3\)CH\(_2\))\(_2\)N and ( (CH\(_3\))\(_2\)CH)\(_2\)N will be discussed.},
subject = {Organische Chemie},
language = {en}
}
@article{PlessSuterEngels1994,
author = {Pleß, V. and Suter, H. U. and Engels, Bernd},
title = {Ab initio study of the energy difference between the benzene and the cumulene form of the C\(_6\) molecule},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59059},
year = {1994},
abstract = {The energy difference between the three lowest-lying isomers of C\(_6\) the linear \(^3 \sum ^-\) state and the two ring forms,the benzene structure (\(^1\)A\(_{18}\)) possessing D\(_{6h}\) symmetry and a distorted cyclic form ( \(^1\)A'\(_1\), D\(_{3h}\) symmetry) have been calculated using various ab initio methods. Variational methods such as multireference configuration interaction (MR-CI) and complete active space second order perturbatiOn treatment (CASPT2) have been applied, as weil as perturbational treatments and coupled cluster calculations (CCD). The correlation of all valence shell electrons is found to be important for a balanced description of the isomers of C\(_6\) . Methods which do not account for higher-order effects appropriately proved to be unsuitable for calculating the energy difference correctly. The results from multireference configuration interaction methods show that the isomers are close in energy with the cyclic forms somewhat lower than the linear form. The ring form possessing D\(_{3h}\) symmetry (\(^1\)A'\(_1\)} is found tobe the lowest-lying structure.},
subject = {Organische Chemie},
language = {en}
}
@article{ChristlLessMueller1994,
author = {Christl, Manfred and Leß, Roland and M{\"u}ller, Heinrich},
title = {6,7-Dimethylene-2,4-diphenylbicyclo[3.2.l]oct-3-en-2-yl Anion : A Test for the Origin of the Unusual Properties of the Bicyclo[3.2.l]octa-3,6-dien-2-yl Anion},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31547},
year = {1994},
abstract = {No abstract available},
language = {en}
}
@phdthesis{Musch2003,
author = {Musch, Patrick},
title = {Large-Scale Applications of Multi-Reference Methods in Chemistry and Development of a Multi-Reference Moller-Plesset Perturbation Theory Program},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-7741},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2003},
abstract = {The first part of this work focuses on the characterization of systems which complex electronic structures require the application of multi-reference methods. The anti-tumor efficacy of the natural product Neocarzinostatin is based on the formation of diradicals and causes DNA cleavage and finally cytolysis. Computations on model systems performed in the present work show the influence of structural features on the mode of action and the efficacy of this antitumor-antibiotic. The cyclization of systems related to the enyne-cumulene framework like the enyne-allenes was investigated earlier and relations to the more unusual class of enyne-ketenes are analyzed. The class of enyne-ketenes (and also the enyne-allenes) show a broad spectrum of possible intermediates (diradicals, zwitterions, allenes). The electronic structures of these intermediates are also possible for the (heteroatom substituted) 1,2,4-cyclohexatriene and a model for their energetic sequence based on high-level multi-reference computations is proposed. In all three projects the application of multi-reference approaches is necessary to obtain a comprehensive picture of the reactivity and electronic structure but also shows up the limits inherently existing in the currently available programs with respect to the size of the molecules. In the second part, algorithms for a multi-reference Moller-Plesset perturbation theory (MR-MP2) program, designed to perform large-scale computations, were developed and implemented. The MR-MP2 approach represents the most cost-effective multireference ansatz and requires an efficient evaluation of the Hamilton matrix for which an algorithm is designed to instantly recognize only non-vanishing matrix elements and to employ the recurring interaction patterns of the Hamilton matrix. The direct construction of the Hamilton matrix is additionally parallelized to work on cluster environments.},
language = {en}
}
@phdthesis{Hupp2003,
author = {Hupp, Thomas},
title = {Ab Initio Treatment of Complex Systems Kohn-Sham Orbitals for Multi Reference Methods and the Base Pairing Properties of Xanthine},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-8244},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2003},
abstract = {Die vorliegende Arbeit besteht aus zwei Teilen. Der erste untersucht die Eignung von LHF-Orbitalen f{\"u}r Multireferenzverfahren. Das Ziel dieses Teils ist eine effizientere Berechnung angeregter Zust{\"a}nde, was zur spektroskopischen Charakterisierung vieler organischer und bioorganischer Molek{\"u}le wichtig ist. Der zweite Teil befasst sich mit bioorganischen Fragestellungen und untersucht die Paarungseigenschaften der Purinbase Xanthin. Es wird unter anderem die unerwartet hohe Stabilit{\"a}t der Xanthin Selbstpaarung in Alanyl-PNA erkl{\"a}rt und es wird untersucht, auf welche Weise Xanthin in der DNA mutagene Fehlpaarungen mit Thymin eingehen kann. Teil1: Im Unterschied zu HF- und Standard-DFT-Methoden f{\"u}hrt der LHF-Ansatz zu einem vollst{\"a}ndig gebundenen Orbitalspektrum, da Coulomb-Selbstwechselwirkungen im LHF-Ansatz exakt korrigiert werden. Durch die Korrektur der Coulomb-Selbstwechselwirkungen sind im LHF-Ansatz auch die Energien der besetzten Orbitale nicht wie in Standard-DFT-Methoden zu h{\"o}heren Werten verschoben, so dass das Koopmans' Theorem g{\"u}ltig bleibt und die besetzten LHF-Orbitale etwas kompakter als Standard-DFT-Orbitale sind. Die vorliegende Arbeit zeigt, dass beide Eigenschaften deutliche Vorteile f{\"u}r MR-Verfahren darstellen. Die virtuellen LHF-Orbitale sind gut optimiert und erlauben eine effizienteBeschreibung sowohl angeregter Zust{\"a}nde als auch statischer Korrelationseffekte in MRCI und MRPT2-Ans{\"a}tzen. Weiterhin f{\"u}hrt die kompaktere Struktur der besetzten LHF-Orbitale zu einer besseren Beschreibung des kationischen Rumpfes von Rydbergzust{\"a}nden. Andererseits wurden zu beiden genannten Vorteilen auch jeweils ein Beispielmolek{\"u}l gefunden, in dem die Vorteile nicht zum Tragen kommen, und zu deren Beschreibung Orbitale aus HF- oder Standard-DFT-Methoden besser geeignet sind. Diese Beispiele zeigen, dass jeder Einzelfall f{\"u}r sich getestet werden muss, auch wenn die angeregten Zust{\"a}nde der meisten Molek{\"u}le sehr gut mit LHF-Orbitalen beschrieben werden k{\"o}nnen. Teil 2: Im zweiten Teil der vorliegenden Arbeit wurden die Paarungseigenschaften von Xanthin und Xanthinderivaten untersucht. Ziel dieses Teils war es, eine Erkl{\"a}rung f{\"u}r die unerwartet hohe Stabilit{\"a}t des Xanthin Alanyl-PNA Selbstpaarung zu finden. Weiterhin wurde untersucht, weshalb Xanthin, das in der DNA u.a. unter chemischem Stress gebildet wird, mutagene Fehlpaarungen mit der Pyrimidinbase Thymin eingehen kann. Stabilit{\"a}t der Xanthin Alanyl PNA: Zun{\"a}chst wurde durch den Vergleich experimenteller und berechneter 13C-NMR-Spektra das Regiosomer von Xanthin bestimmt, welches zu der ungew{\"o}hnlich hohen Stabilit{\"a}t der Xanthin-Xanthin-Selbstpaarung in Alanyl-PNA verantwortlich ist. Zur Untersuchung der Stabilit{\"a}t der Xanthin-Selbstpaarung wurde ein stark vereinfachendes Modell aufgestellt,in dem die Stabilit� at der PNA-Duplexe nur {\"u}ber die Energiebeitr{\"a}ge aus den Wasserstoffbr{\"u}cken (EDim) und der Basenstapelung (EStap) bestimmt wird. Die Dimerisierungs- und Stapelungsenergien unterschiedlicher Paarungen wurden mit DFT- und MP2-Methoden bestimmt. Solvenseffekte wurden {\"u}ber ein Kontinuummodell erfasst und der Einfluss des peptidischen R{\"u}ckgrats auf die Stapelungsgeometrie wurde durch Kraftfeldmethoden ber{\"u}cksichtigt. W{\"a}hrend die einzelnen Energiekomponenten aus den H-Br� ucken und der Basenstapelung keinen eindeutigen Zusammenhang zu den Schmelztemperaturen erkennen lassen, korreliert die Summe aus beiden linear mit den experimentell ermittelten Tm-Werten. Dies bedeutet, dass die Beitr{\"a}ge aus der Entropie, der molekularen Wasserumgebung und der R{\"u}ckgratspannung sich entweder aufheben oder f� ur alle behandelten Systeme sehr {\"a}hnlich sind. Die Stabilit{\"a}t der Xanthin-Xanthin- und die der 2,6-Diaminopurin-Xanthin-Paarung, ergibt sich durch einen erh{\"o}hten Stapelungsbeitrag der Purinpaarungen, w{\"a}hrend die Wasserstoffbr{\"u}cken der Xanthin Selbstpaarung nur wenig zur Stabilisierung des Xanthin-Xanthin und des Xanthin-Diaminopurin-Alanyl-PNA-Doppelstrangs beitragen. Paarungseigenschaften von N9-Xanthin: Zur Untersuchung der Paarungseigenschaften von N9-Xanthin wurden zun� achst H-verbr{\"u}ckte Homodimere von Xanthin untersucht. Hierbei wurden extreme Variationen in den Bindungsst{\"a}rken der einzelnen H-Br{\"u}cken gefunden, die sich zwischen -4 bis -11 kcal/mol in der Gasphase und -2.5 bis -5 kcal/mol im Solvens betragen. Durch Vergleich mit Modellsystemen konnte die starke Varianz der H-Br{\"u}ckenst{\"a}rke auf anziehende bzw. abstoßende sekund{\"a}re elektrostatische Wechselwirkungen zur{\"u}ckgef{\"u}hrt werden. Weiterhin wurde das Homodimer von Hypoxanthin untersucht, bei dem die H-Br{\"u}cken durch eine Erh{\"o}hung der Aromatizit{\"a}t im Pyrimidinring zus{\"a}tzlich verst{\"a}rkt werden, was zu einer deutlichen Stabilisierung des Dimers f{\"u}hrt. Elektronische Effekte m{\"u}ssen vor allem deshalb ber{\"u}cksichtigt werden, da sie im Unterschied zu rein elektrostatischen Effekten deutlich weniger von der Solvensumgebung beein usst werden. Mutagenit{\"a}t von Hypoxanthin und Xanthin: Zur Erkl{\"a}rung der Mutagenit{\"a}t von Hypoxanthin und Xanthin wurden verschiedene neutrale und anionische Watson-Crick Basenpaarungen von Hypoxanthin und Xanthin mit Pyrimidinbasen berechnet. Hierbei wurden u. a. auch tautomere und anionische Formen von Xanthin ber{\"u}cksichtigt. Zur Bewertung der erhaltenen Dimerisierungsenergien wurden die Paarungen danach klassifiziert, ob ihre Geometrien mit denen der kanonischen Basenpaarungen deckungsgleich sind, oder ob sie in einer verzerrten Watson-Crick Geometrie vorliegen, was die Einbaurate in die DNA aufgrund des r{\"a}umlichen Anspruchs der DNA-Polymerase vermindert. Die Rechnungen zeigen, dass Xanthin nur mit Cytosin Watson-Crick-Paarungen eingehen kann, welche jedoch nur sehr schwach gebunden sind. In der neutralen Form scheint eine dreiz{\"a}hnige Basenpaarung unter Beteiligung einer tautomeren Form des Xanthins etwas stabiler zu sein als die zweiz{\"a}hnige Paarung von Diketoxanthin mit Cytosin. Da die Dimerisierungsenergie sowohl der neutralen als auch der anionischen Basenpaarung nur wenig unter 0 kcal/mol liegt, ist der Einbau der Xanthin-Cytosin-Paarung in die DNA zwar aufgrund der g{\"u}nstigen Geometrie m{\"o}glich, wird aber nicht durch einen Energiebeitrag aus den H-Br{\"u}cken verst{\"a}rkt. Die im Vergleich zur Guanin-Cytosin Paarung deutlich geringere Aromatizit{\"a}t von Xanthin zu Cytosin ist im Einklang mit dem experimentellen Befund, dass die Cytosin-Xanthin Paarung deulich langsamer als die Guanin-Cytosin Paarungen in die DNA eingebaut werden. W{\"a}hrend die Rechnungen nur eine geringe Aromatizit{\"a}t von Xanthin zu Cytosin vorhersagen, scheint das Anion von Xanthin in der Lage zu sein, eine sehr stabile Basenpaarung mit Thymin einzugehen. Allerdings muss die Dimerisierungsenergie die schlechtere Anpassung in die Bindungstasche der DNA-Polymerase ausgleichen, da die Paarung in einer etwas verzerrten Watson-Crick Geometrie vorliegt. Insgesamt wird die Paarung daher nicht schneller in die DNA eingebaut, wie erwartet aufgrund der H-Br{\"u}ckenst{\"a}rken, stattdessen besitzt sie eine {\"a}hnliche Einbaurate wie die geometrisch g{\"u}nstigere aber weniger stabile Xanthin-Cytosin Paarung.},
language = {en}
}
@phdthesis{Geiger2004,
author = {Geiger, Lars},
title = {The versatile use of Guanidiniocarbonylpyrroles : from self-assembly to peptide recognition},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9272},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2004},
abstract = {Die vorliegende Arbeit gliedert sich in zwei Themenschwerpunkte. Ein supramolekulares Projekt beinhaltete die Entwicklung von neuen flexiblen, selbst-aggregierenden Zwitterionen als Bausteine f{\"u}r supramolekulare Polymere. In einem zweiten bioorganischem Teil bestand das Ziel darin, Rezeptoren f{\"u}r Aminos{\"a}uren und Dipeptide in Wasser zu entwickeln. Beide Projekte basieren auf dem Guanidiniocarbonylpyrrol als effizientes Bindungsmotiv f{\"u}r die Komplexierung von Carboxylaten in w{\"a}ssrigen L{\"o}sungen. Eine notwendige Voraussetzung f{\"u}r die Realisierung dieser Projekte war jedoch zun{\"a}chst die Entwicklung einer allgemeinen, effizienten und milden Synthese f{\"u}r Guanidiniocarbonylpyrrole. Die bei der zuvor verwendeten Methode aggressiven Reaktionsbedingungen und die problematische Aufreinigung verhinderten eine gr{\"o}ßere Anwendung dieses Bindungsmotivs in bioorganischen und supramolekularen Projekten. Im Rahmen dieser Arbeit gelang es mir erfolgreich eine neue Syntheseroute zu entwickeln. Hierbei wurde mono-tBoc-Guanidine mit dem Benzylester mittels PyBOP gekuppelt und nach Entsch{\"u}tzung der Benzylschutzgruppe wurde die zentrale Zwischenstufe f{\"u}r die weiteren Synthesen, die tBoc-gesch{\"u}tzte Guanidinocarbonylpyrrol-S{\"a}ure erhalten. Durch diese neuartige Synthese war es m{\"o}glich, eine Reihe von flexiblen Zwitterionen 3-6 herzustellen und deren Selbst-Aggregation und den Einfluß der Kettenl{\"a}nge und somit Flexibilit{\"a}t der Alkylkette auf Struktur und Stabilit{\"a}t der gebildeten Aggregate in L{\"o}sung sowie auch in der Gasphase zu untersuchen. In DMSO deuten NMR-Verd{\"u}nnungsreihen darauf hin, dass die flexiblen Zwitterionen mit n = 1, 3 und 5 oligomere Strukturen ausbilden. Im Falle von n = 1 werden hoch stabile helicale und Nanometer große Aggregate in der gebildet. In den Gasphasen-Studien wurde die Stabilit{\"a}t und Zerfallskinetik einer Reihe von Natriumaddukten der Dimere von n = 2, 3 und 5 untersucht. Dieses gelang durch die Methode der „infrared multiphoton dissociation Fourier transform ion cyclotron resonance mass spectrometry" (IRMPD-FT-ICR MS). Solche Studien erm{\"o}glichen m{\"o}glicherweise in Zukunft das gezielte Design von supramolekularen Bausteinen. Der bioorganische Teil meiner Arbeit setzte sich aus drei Einzelprojekten zusammen. So synthetisierte ich durch eine f{\"u}nfstufige Synthesesequenz vier neue Arginin-Analoga, die in Zukunft als Ersatz f{\"u}r Arginin in Peptide eingebaut werden k{\"o}nnen. Als Testreaktion f{\"u}r die Eignung dieser Verbindungen in einer Festphasenpeptidsynthese, wurde ein Tripetid Ala-AA1-Val (AA: Arginin-Analogon) mit einem eingebauten Arginin-Analogon erfolgreich hergestellt. In einem zweiten Projekt habe ich den Einfluß einer zus{\"a}tzlichen ionischen Wechselwirkung in unserem Bindungsmotiv untersucht. Dazu wurde ein zweifach-kationischer Rezeptor und der dreifach-geladenen Rezeptor synthetisiert und physikalisch-organisch ihre Bindungseigenschaften mit Hilfe von NMR-Titrationsexperimenten gegen eine Reihe von Aminos{\"a}uren untersucht. Der dreifach-kationische Rezeptor 11 zeigte hierbei herausragende Bindungseigenschaften und war um ca. den Faktor 100 besser als f{\"u}r die bisher bekannten Guanidiniocarbonylpyrrole. Die Assoziationskonstanten waren auch fast reinem Wasser mit bis zu Kass = 2000 noch bemerkenswert hoch. Im dritten Projekt habe ich einen de-novo entwickelten Rezeptor f{\"u}r C-terminale Dipeptide in einer beta-Faltblatt Struktur entwickelt.Dieser Rezeptor wurde mittels NMR and UV-Titrationen untersucht. In 40 \% Wasser/ 60 \% DMSO waren die Bindungskonstanten zu hoch um {\"u}berhaupt quantifiziert zu werden. Deshalb wurden die Bindungseigenschaften des Rezeptors mittels UV Titrationen in einer Mischung aus 90 \% Wasser mit 10 \% DMSO gegen eine Reihe von Dipeptiden und Aminos{\"a}uren getestet. Die Bindungsdaten zeigen, dass Rezeptor Dipeptide mit ausgezeichneten Bindungskonstanten (Kass > 10000 M-1) komplexiert. Im Gegensatz dazu bindet der Rezeptor 12 Aminos{\"a}uren um den Faktor zehn schlechter (Kass > 1000 M-1). Die Komplexstabilit{\"a}t nimmt hierbei in Abh{\"a}ngigkeit von der Seitenkette des Dipeptids in der Reihe Gly < Ala < Val zu, was sich mit der abnehmenden Flexibilit{\"a}t und zunehmenden Hydrophobizit{\"a}t der Seitenkette erkl{\"a}ren l{\"a}sst. Diese Eigenschaften machen den Rezeptor 12 zu dem besten bisher bekannten Dipeptidrezeptor in w{\"a}ssrigen L{\"o}sungen. Innerhalb meiner Arbeit gelang es mir somit, nicht nur eine essentiell wichtige, milde und effiziente Synthese f{\"u}r Guanidinocarbonylpyrrole zu entwickeln, sondern es gelang mir ebenso ein neues Bindungsmotiv f{\"u}r die Komplexierung von Aminos{\"a}uren in Wasser zu entwickeln. Zus{\"a}tzlich konnte noch der Dipeptidrezeptor erfolgreich synthetisiert und untersucht werden. Mit Bindungskonstanten f{\"u}r von Kass > 10000 M-1 ist er der derzeit beste Dipeptidrezeptor in w{\"a}ssriger L{\"o}sung.},
subject = {Guanidinderivate},
language = {en}
}
@phdthesis{Henn2004,
author = {Henn, Julian},
title = {The electron density : a bridge between exact quantum mechanics and fuzzy chemical concepts},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9003},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2004},
abstract = {Summary The nature of the chemical bond is a topic under constant debate. What is known about individual molecular properties and functional groups is often taught and rationalized by explaining Lewis structures, which, in turn, make extensive use of the valence concept. The valence concept distinguishes between electrons, which do not participate in chemical interactions (core electrons) and those, which do (single, double, triple bonds, lone-pair electrons, etc.). Additionally, individual electrons are assigned to atomic centers. The valence concept is of paramount success: It allows the successful planning of chemical syntheses and analyses, it explains the behavior of individual functional groups, and, moreover, it provides the "language" to think of and talk about molecular structure and chemical interactions. The resounding success of the valence concept may be misleading to forget its approximative character. On the other hand, quantum mechanics provide in principle a quantitative description of all chemical phenomena, but there is no discrimination between electrons in quantum mechanics. From the quantum mechanical point of view there are only indistinguishable electrons in the field of the nuclei, i.e., it is impossible to assign a given electron to a particular center or to ascribe a particular purpose to individual electrons. The concept of indistinguishability of micro particles is founded on the Heisenberg uncertainty relation, which states, that wavepackets diverge in the 6N dimensional phase space, such that individual trajectories can not be identified. Hence it is a deep-rooted and approved physical concept. As an introduction to the present work density partitioning schemes were discussed, which divide the total molecular density into chemically meaningful areas. These partitioning schemes are intimately related to either the concepts of bound atoms in a molecule (as in the Atoms In Molecules theory (AIM) according to Bader or as in the Hirshfeld partitioning scheme) or to the concept of chemical structure in the sense of Lewis structures, which divide the total molecular density into core and valence density, where the valence density is split up again into bonding and non-bonding electron densities. Examples are early and recent loge theories, the topological analysis by means of the Electron Localization Function (ELF), and the Natural Bond Orbital (NBO) approach. Of these partitioning schemes, the theories according to Bader (AIM), to Becke and Edgecomb (ELF) and according to Weinhold (NBO and Natural Resonance Theory, NRT), respectively, were reviewed in detail critically. Points of criticism were explicated for each of the mentioned theories. Since theoretically derived electron densities are to be compared to experimentally derived densities, a brief introduction into the theory of X-ray di®raction experiments was given and the multipole formalism was introduced. The procedure of density refinement was briefly discussed. Various suggestions for improvements were developed: One strategy would be the employment of model parameters, which are to a maximum degree mutually orthogonal, with the object of minimizing correlations among the model parameters, e.g., to introduce nodal planes into the radial functions of the multipole model. A further suggestion involves the guidance of the iterative refinement procedure by an extremum principle, which states, that when di®erent solutions to the least squares minimization problem are available with about the same statistical measures of quality and with about the same residual density, then the solution is to prefer, which yields a minimum density at the bond critical point (BCP) and a maximum polarity in terms of the ratio of distances between the BCP and the nuclei. This suggestion is based on the well known fact, that the bond polarity (in terms of the ratio of distances between the BCP and the respective nuclei) is underestimated in the experiment. Another suggestion for including physical constraints is the explicit consideration of the virial theorem, e.g., by evaluating the integration of the Laplacian over the entire atomic basins and comparing this value to zero and to the value obtained from the integration of the electron gradient field over the atomic surface. The next suggestion was to explicitly use the electrostatic theorem of Feynman (often also denoted as Hellmann-Feynman theorem), which states, that the forces onto the nuclei can be calculated from the purely classical electrostatic forces of the electron distribution and the nuclei distribution. For a stationary system, these forces must add to zero. This also provides an internal quality criterion of the density model. This can be performed in an iterative way during the refinement procedure or as a test of the final result. The use of the electrostatic theorem is expected to reduce significantly correlations among static density parameters and parameters describing vibrations, since it is a valuable tool to discriminate between physically reasonable and artificial static electron densities. All of these mentioned suggestions can be applied as internal quality criteria. The last suggestion is based on the idea to initiate the experimental refinement with a set of model parameters, which is, as much as possible close to the final solution. This can be achieved by performing periodic boundary conditions calculations, from which theoretically created files are obtained, which contain the Miller indices (h, k, l) and the respective intensity I. This file is used for a model parameter estimation (refinement), which excludes vibrations. The resulting parameters can be used for the experimental refinement, where, in a first step, the density parameters are fixed to determine the parameters describing vibrations. For a fine tuning, again the electrostatic theorem and the other above mentioned suggestions could be applied. Theoretical predictions should not be biased by the method of computation. Therefore the dependence of the density analyzing tools on the level of calculation (method of calculation/basis set) and on the substituents in complex chemical bonding situations were evaluated in the second part of the present work. A number of compounds containing formal single and double sulfur nitrogen bonds was investigated. For these compounds, experimental data were also available. The calculated data were compared internally and with the experimental results. The internal comparison was drawn with regard to questions of convergency as well as with regard to questions of consistency: The resulting molecular properties from NBO/NRT analyses were found to be very stable, when the geometries were optimized at the respective level of theory. This stability is valid for variations in the methods of calculation as well as for variations in the basis set. Only the individual resonance weights of the contributing Natural Lewis Structures differed considerably depending on the level of calculation and depending on the substituents. However, the deviations were in both cases to a large extent within a limit which preserves the descending order of the leading resonance structure weights. The resulting bond orders, i.e., the total, covalent and ionic bond order from NRT calculations, were not affected by the shift in the resonance weights. The analysis of the bond topological parameters resulted in a discrimination between insensitive parameters and sensitive parameters. The stable parameters do neither depend strongly on the method of calculation nor on the basis set. Only minor variation occurs in the numerical values of these parameters, when the level of calculation is changed or even when other functional groups (H, Me, or tBu) are employed, as long as the methods of calculation do not drop considerably below a standard level. The bond descriptors of the sulfur nitrogen bonds were found to be also stable with respect to the functional groups R = H, R = Me, and R = tBu. Stable parameters are the bond distance, the density at the bond critical point (BCP) and the ratio of distances between the BCP and the nuclei A and B, which varies clearly when considering the formal bond type. For very small basis sets like the 3-21G basis set, this characteristic stability collapses. The sensitive parameters are based on the second derivatives of the density with respect to the coordinates. This is in accordance with the well known fact, that the total second derivative of the density with respect to the coordinates is a strongly oscillating function with positive as well as negative values. A profound deviation has to be anticipated as a consequence of strong oscillations. lambda3, which describes the local charge depletion in the direction of the interaction line, is the most varying parameter. A detailed analysis revealed that the position of the BCP in the rampant edge of the Laplacian distribution is responsible for the sensitivity of the numerical value of lambda3 in formal double bonds. Since the slope of the Laplacian assumes very high values in its rampant edge, a tiny displacement of the BCP leads already to a considerable change in lambda3. This instability is not a failure of the underlying theory, but it yields de facto to a considerable dependence of sensitive bond topological properties on the method of calculation and on the applied basis sets. Since the total second derivative is important to judge on the nature of the bond in the AIM theory (closed shell interactions versus shared interactions), the changes in lambda3 can lead to differing chemical interpretations. The comparison of theoretically derived bond topological properties of various sulfur nitrogen bonds provides the possibility to measure the self consistency of this data set. All data sets clearly exhibit a linear correlation between the bond distances and the density at the BCP on one hand and between the bond distances and the Laplacian values at the BCP on the other hand. These correlations were almost independent of the basis set size. In this context, the linear regression has to be regarded exclusively as a descriptive statistics tool. There is no correlation anticipated a priori. The formal bond type was found to be readily deducible from the theoretically obtained bond topological descriptors of the model systems. In this sense, the bond topological properties are self consistent despite of the numerical sensitivity of the derivatives, as exemplified above. Often, calculations are performed with the experimentally derived equilibrium geometries and not with optimized ones. Applying this approach, the computationally costly geometry optimizations are saved. Following this approach the bond topological properties were calculated using very flexible basis sets and employing the fixed experimental geometry (which, of course, includes the application of tBu groups). Regression coe±cients similar to those from optimized geometries were obtained for correlations between bond distances and the densities at the BCP as well as for the correlation between bond distances and the Laplacian at the BCP, i.e. the approach is valid. However, the data points scattered less and the coe±cient of correlation was clearly increased when geometry optimizations were performed beforehand. The comparison between data obtained from theory and experiment revealed fundamental discrepancies: In the data set of bond topological parameters from the experiment, the behavior of only 2 out of 3 insensitive parameters was comparable to the behavior of the theoretically obtained values, i.e. theoretical and experimental bond distances as well as theoretical and experimental densities at the BCP correlate. From the theoretically obtained data it was easy to deduce the formal bond type from the position of the BCP, since it changed in a systematic manner. The respective experimentally obtained values were almost constant and did not change systematically. For the SN bonds containing compounds, the total second derivative assumes exclusively negative values in the experiment. Due to the different internal behavior, experimentally and theoretically sensitive bond topological values could not be compared directly. The qualitative agreement in the Laplacian distribution, however, was excellent. In the third and last part of this work, the application to chemical systems follows. Formal hypervalent molecules, i.e. molecules where some atoms are considered to hold more than 8 electrons in their valence shell, were investigated. These were compounds containing sulfur nitrogen bonds (H(NtBu)2SMe, H2C{S(NtBu)2(NHtBu)}2, S(NtBu)2 and S(NtBu)3) and a highly coordinated silicon compound. The set of sulfur nitrogen compounds also contained a textbook example for valence expansion, the sulfur triimide. For these molecules, experimental reference values were available from high resolution X-ray experiments. The experimental results were in the case of the sulfur triimide not unique. Furthermore, from the experimental bond topological data no definite conclusion about the formal bonding type could be drawn. The situation of sulfur nitrogen bonds in the above mentioned set of molecules was analyzed in terms of a geometry discussion and by means of a topological analysis. The methyl-substituted isolated molecules served as model compounds. For the interpretation of the bonding situation additional NBO/NRT calculations were preformed for the sulfur nitrogen compounds and an ELF calculation and analysis was performed for the silicon compound. The ELF analysis included not only the presentation and discussion of the ELF-isosurfaces (eta = 0.85), but also the investigation of populations of disynaptic valence basins and the percentage contributions to these populations of the individual atoms when the disynaptic valence basins are split into atomic contributions according to Bader's partitioning scheme. The question of chemical interest was whether hypervalency is present in the set of molecules or not. In the first case the octet rule would be violated, in the second case Pauling's verdict would be violated. While the concept of hypervalency is well established in chemistry, the violation of Pauling's verdict is not. The quantitative numbers of the sensitive bond topological values from theory and experiment were not comparable, since no systematic relationship between the experimentally and theoretically determined sensitive bond descriptors was found. However, the insensitive parameters are in good agreement and the qualitative Laplacian distribution is, with few exceptions, in excellent agreement. The formal bonding type was deduced from experimental and theoretical topological data by considering the number and shape of valence shell charge concentrations in proximity to the sulfur and nitrogen centers. The results from NBO/NRT calculations confirmed the findings. All employed density analyzing tools AIM, ELF and NBO/NRT coincided in describing the bonding situation in the formally hypervalent molecules as highly polar. A comparison and analysis of experimentally and theoretically derived electron densities led consistently to the result, that regarding this set of molecules, hypervalency has to be excluded unequivocally.},
subject = {Elektronendichtebestimmung},
language = {en}
}
@phdthesis{Dobrawa2004,
author = {Dobrawa, Rainer Anton},
title = {Synthesis and characterization of terpyridine-based fluorescent coordination polymers},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10367},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2004},
abstract = {Complexation properties of 2,2':6',2''-terpyridine (tpy) have been studied with a series of first row transition metal ions by UV-vis, 1H NMR and isothermal titration calorimetry and ƒ´H values for the tpy complexation processes have been determined. These studies reveal that Zn2+ is the best suited metal ion for the reversible coordination of the terpyridine ligand. Thus, supramolecular coordination polymerization of perylene bisimide fluorophores containing terpyridine functionalities have been investigated by using Zn2+ as metal ion. The formation of the dimeric complexes in the case of monotopic model comounds and coordination polymerization of ditopic functional building blocks have been confirmed by 1H NMR studies. The optical properties of dimeric and polymeric complexes have been investigated by UV-vis and fluorescence spectroscopy. The Zn2+ coordination to the terpyridine unit does not effect the advantageous fluorescence properties of perylene bisimide moieties. The reversibility of the formation of coordination polymers has been established by 1H NMR and additionally by DOSY NMR and fluorescence anisotropy measurements. Coordination polymer strands can be visualized by atomic force microscopy (AFM), which also reveals the formation of an ordered monolayer film at higher concentration. The average polymer length has been determined by AFM to 15 repeat units, which correlates well with the value estimated by 1H NMR to >10 repeat units.},
subject = {Terpyridinderivate <2},
language = {en}
}
@phdthesis{Kriegisch2005,
author = {Kriegisch, Volker},
title = {Electron transfer processes between organic redox centres and electrodes via active bridges in self-assembled monolayers},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-15892},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2005},
abstract = {Cyclovoltammetrische Messungen der Ferrocenalkylthiole 1 - 3 belegen, dass homogene, gemischte Monolagen aus redoxaktiven Verbindungen und redoxinaktiven Alkylthiolen gebildet werden. Die von Creager et al. bestimmten ET Raten der Ferrocenalkylthiole 1 - 3 konnten hierbei verifiziert werden. Wie erwartet erfolgt eine Abnahme der ET Geschwindigkeit bei einer Kettenverl{\"a}ngerung des Alkylspacers von 2 nach 3. Eine unterschiedliche Konnektivit{\"a}t zwischen Redoxzentrum und Alkylspacer, z. B. die Einf{\"u}hrung einer Carbonyl-Funktion im Falle von 1, unter Beibehaltung der Kettenl{\"a}nge zeigt keinen bemerkbaren Einfluß auf den ET. Trotzt vergleichbaren Abstands der aromatischen Ferrocenthiole 4 und 5 zu der C8-Alkyl-Verbindung 2 zwischen Redoxzentrum und Elektrode, weisen diese aufgrund ihrer starken Konjugation sehr hohe ET Geschwindigkeiten auf. Die elektronischen Kopplungsfaktoren selbst deuten auf einen nichtadiabtischen ET zwischen Redoxzentrum und Elektrode hin. Wie erwartet kommt es zu einem Anwachsen der Kopplungsfaktoren bei sich verk{\"u}rzender Kettenl{\"a}nge oder bei Einf{\"u}hrung konjugierter Spacersysteme. Zusammenfassend kann gesagt werden, dass Erfahrungen hinsichtlich der Pr{\"a}paration der Monolagen gesammelt, die gemessenen ET Raten f{\"u}r der literaturbekannten Verbindungen 1 - 3 best{\"a}tigt und diese Informationen auf die konjugierten Verbindungen 4 und 5 angewandt werden konnten. Im zweiten Teil wurden die Triarylamin- (29, 32) und Phenothiazinalkylthiole (35) bez{\"u}glich ihres ET Verhaltens in gemischten Monolagen untersucht. Mittels Cyclovoltammetrie konnte gezeigt werden, daß einheitlich geformte, verd{\"u}nnte Monolagen vorliegen. Die ET Raten der Triarylamin- (29, 32) und Phenithiazinalkylthiole (35) sind jedoch um den Faktor 10 bis 100 h{\"o}her als vergleichbare Ferrocenalkylthiole gleicher Kettenl{\"a}nge [1, 2], wohingegen f{\"u}r Monolagen, welche [Ru(bpy)2(pp)]+-Alkythiole enthalten, {\"a}quivalente Werte gefunden wurden [3]. Die ET Geschwindigkeit wird von zwei Parametern beeinflusst: dem elektronischen Kopplungsmatrixelement und der Regorganisationsenergie \&\#61548;\&\#61472; [4]. Die ET Geschwindigkeit in Donor-substituierten Alkylthiolen wird haupts{\"a}chlich durch \&\#61548; beeinflusst und sogar kleine {\"A}nderungen dieser zeigen eine große Auswirkung auf die zu untersuchenden Prozesse. Aus diesem Grund wird eine Zunahme der ET Geschwindigkeit von Ferrocen (hohe Reorganisationsenergie) {\"u}ber die Phenothiazinverbindung 35 und [Ru(bpy)2(pp)]+ zu den Triarylaminchromophoren 29 und 32 (niedrige Reorganisationsenergie) beobachtet. Weiterhin spielt, im Gegensatz zu Beobachtung von Creager et al. an {\"a}quivalenten Ferrocenverbingungen, die Anbindung des Redoxzentrums an den Alkylspacer eine bedeutende Rolle. Im Falle der elektronenreichen Ether-verbr{\"u}ckten Verbindung 29 wird der ET nicht alleine durch \&\#61548;, sondern ebenso durch mesomere Effekte bestimmt. Bei 29 kommt es durch Lokalisation der positiven Ladung nahe der Ether Funktion formal zu einer Kettenverk{\"u}rzung um eine „Methyleneinheit", welche schließlich in h{\"o}heren ET Geschwindigkeiten resultiert. Im dritten Teil dieser Dissertation wurde ein Serie „molekularer Dr{\"a}hte" bestehend aus Methoxy- oder Chlorid-substituierten Triarylamin- und Phenothiazinverbindungen mit unterschiedlichen Br{\"u}ckeneinheiten und Br{\"u}ckenl{\"a}ngen zwischen Redoxzentrum und Ankerfunktion dargestellt und im Hinblick auf ihr ET Verhalten untersucht. Durch cyclovoltammetrische und UV/Vis-spektroskopische Untersuchungen konnte gezeigt werden, dass sowohl die Oxidationspotentiale als auch die energetischen Zust{\"a}nde der Chromophore recht gut durch Einf{\"u}hrung unterschiedlicher Redoxzentren und Br{\"u}ckeneinheiten beeinflusst werden k{\"o}nnen. Trotz erfolgreicher Kontrolle der Dichte der Chromophoreinheiten in den gemischten Monolagen konnte nur f{\"u}r die Verbindungen 49, 52 und 87 mit Nitril-substituierten Br{\"u}ckeneinheiten verl{\"a}ssliche ET Geschwindigkeiten erhalten werden. Bei diesen Chromphoren ist ein Absinken der ET Geschwindigkeit bei zunehmender Dichte der redoxaktiven Molek{\"u}le in den gemischten Monolagen zu beobachten, welche auf eine {\"A}nderung der Adsorptionsgeometrie hindeutet. Bei zunehmender Packungsdichte der Chromophore f{\"u}hrt dies zu einer aufrechteren Stellung der redoxaktiven Spezies. F{\"u}r alle anderen Verbindungen konnten keine Werte aufgrund der zu schnellen ET Geschwindigkeiten ermittelt werden. Konformelle, wie auch die sehr geringe Abstandsabh{\"a}ngigkeit des ET, resultieren in hohen ET Geschwindigkeiten oder auch ung{\"u}nstige HOMO-LUMO Energien bez{\"u}glich des Donors, der Br{\"u}cke und der Elektrode sind Gr{\"u}nde f{\"u}r dieses Verhalten. Die Tatsache, dass Verbindung 49 und 52 beinahe die gleichen Geschwindigkeitskonstanten des ETs unabh{\"a}ngig von der Anzahl der Br{\"u}ckeneinheiten (n = 2, n = 3) besitzen, deutet darauf hin, dass ein Hopping-Prozess stattfindet, bei welchem eine geringere L{\"a}ngenabh{\"a}ngigkeit des ETs als bei eine Superexchange-Mechanismus zu erwarten ist.},
subject = {Monoschicht},
language = {en}
}
@phdthesis{Amthor2005,
author = {Amthor, Stephan},
title = {Redox properties of Bis-Triarylamines and ligand properties of Thianthrenophane},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-15916},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2005},
abstract = {The one electron oxidation potential of ten TAAs with all permutations of Cl , OMe- and Me-substituents in the three p-positions were determined by CV. The half wave potential of the first oxidation wave correlates linearly with the number of Cl- and OMe-substituents. AM1-CISD derived values of the absorption energies are in good agreement with the experiments but differ strongly for the oscillator strengths as well as for neutral compounds and their corresponding mono radical cations. The small solvent dependence of the experimental UV/Vis spectra in CH2Cl2 and MeCN reflects a minor charge transfer character of the electronic transitions. The UV/Vis/NIR spectra of the series of TAAs and their corresponding radical cations and the AM1 computations reveal that even small substituents may lead to strong symmetry breaking and to a modified electronic structure. The spectroscopic properties of a series of four bis-TAA donor-bridge-donor X-B-X dimers, composed of two asymmetric TAA chromophores (monomers) were investigated. UV/vis-, fluorescence and transient absorption spectra were recorded and compared with those of the corresponding X-B monomers. The excited states of the dimers are described as MV states which show, depending on the chemical nature of the bridge, a varying amount of interactions. It was found that superradiant emission only proceeds in the case of weak and medium coupling. Whether the first excited state potential energy surface of the dimers is a single minimum or a double minimum potential depends on the solvent polarity and the electronic coupling. In the latter case, the dimer relaxes in a symmetry broken CT state. The [2.2]paracyclophane bridged dimer is an example for a weakly coupled system, because the spectroscopic behavior is very similar to the corresponding p xylene monomer. In contrast, anthracene as well as p-xylene bridges mediate a stronger coupling and reveal a significant cooperative influence on the optical properties. A series of [2.2]paracylophane bridged bis-TAA MV radical cations X-B-X+ were analyzed by a GMH three-level model which takes two transitions into account: the IV-CT band and the bridge band. From the GMH analysis, one can conclude that the [2.2]paracyclophane moiety is not the limiting factor which governs the intramolecular charge transfer. The electronic interactions are of course smaller than direct conjugation but from the order of magnitude of the couplings of the [2.2]paracyclophane MV species it can be assumed that this bridge is able to mediate significant through-space and through-bond interactions. From the exponential dependence of the electronic coupling V between the two TAA localized states on the distance r between the two redox centers, it was inferred that the HT proceeds via superexchange mechanism. The analysis reveals that even significantly longer conjugated bridges should still mediate significant electronic interactions, because the decay constant of a series of conjugated MV species is small. The absorption properties of a series of bis-TAA-[2.2]paracyclophane dications X+-B-X+ were presented. The localized and the CT transitions of these dications are explained and analyzed by an exciton coupling model which also considers the photophysical properties of the monomeric TAA radical cations. Together with AM1-CISD calculated transition moments, experimental transition moments and transition energies of the bis-TAA dications were used to calculate electronic couplings by a GMH approach. These couplings are a measure for interactions of the excited MV CT states. The modification of the diabatic states reveals similarities of the GMH three-level model and the exciton coupling model. Comparison of the two models shows that the transition moment between the excited mixed-valence states of the dimer equals the dipole moment difference of the ground and the excited bridge state of the corresponding monomer. Thianthrenophane (1) has a cavity which offers enough room to potentially enable endohedral coordination to small ions or molecules. For the complexation of silver(I) perchlorate, the complex stability constants of thianthrenophane logK1=5.45 and of thianthrene logK2=9.16 were determined by UV/Vis titration. Single competition transport experiments with ten metal salts demonstrate a very high selectivity of thianthrenophane as a carrier for silver(I) and a distinctly higher transport rate compared to carriers such as thianthrene and 14-ane-S4. Although the X-ray crystal structure analysis of the polymeric [Ag(1)]ClO4 shows an exohedral coordination to silver(I), the formation of an endohedral [Ag(1)]+ complex is suggested to be the explanation for the unusual carrier selectivity of silver(I) by 1 in bulk liquid membrane.},
subject = {Triarylamine},
language = {en}
}
@phdthesis{Stahl2005,
author = {Stahl, Rainer},
title = {Electroactive Conjugated Polymers as Charge-Transport Materials for Optoelectronic Thin-Film Devices},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-16980},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2005},
abstract = {In this work the electrochemical and spectroelectrochemical properties of a series of pi-conjugated organic polymers were studied. The polymers were deposited on platinum electrodes or ITO-coated glass substrates by potentiodynamic electro-polymerisation of the corresponding monomeric precursor molecules. The electro-chemical and photophysical properties of the triarylborane monomers were studied in detail in order to estimate possible influences on the behaviour of the corresponding polymer. The first part of this work aimed at the synthesis and investigation of conjugated donor-acceptor polymers which combine the prerequisites of an OLED within one material: the transport of positive and negative charges and the formation of emissive excited states. With the carbazole-substituted oxadiazoles 1-3 it was shown that on the one hand the carbazole functionality is suitable for enabling the electrochemical polymerisation of the monomers and on the other hand it facilitates reversible p-doping of the resultant polymers. Although n-doping of poly-1-poly-3 is possible due to the electron-deficient oxadiazole rings, it causes the continuous degradation of these electron-acceptor units. Interestingly, this process does not influence the capability of p-doping of the polymers. With respect to its electrochemical and spectroelectrochemical properties the behaviour of the borane polymer poly-4 is absolutely identical with that of the oxadiazole polymers. Moreover, the optical excitation of poly-4 in the solid state leads to the emission of blue-green light which suggests that this polymer might also possess electroluminescent properties. AFM-measurements of poly-4 films on ITO-coated glass substrates revealed, that the film thickness can be controlled to a certain extent by the number of polymerisation redox cycles. It was shown from the electrochemical and photophysical properties of the triarylboranes 4-6 that the pi-pi-interaction between boron and nitrogen atoms is comparably weak in these molecules. This leads to an unexpected ground-state polarisation with a partially positive boron atom and a partially negative nitrogen atom. Moreover, it was found that TAB 4 possesses a lower symmetry than D3 in solution and that excitation energy can be transferred amongst the three subchromophores of 4. By titration experiments it was also demonstrated that TAB 4 can reversibly bind fluoride ions and that the binding event significantly influences the optical absorption characteristics of the chromophore. It can be assumed, that the above mentioned properties, which have a profound influence on the photophysical behaviour of these triarylborane chromophores, also determine the behaviour of the corresponding polymer in a solid state environment. The aim of the second part of this work was the investigation of purely n-conducting materials based on electron-deficient borane and viologen polymers. The corresponding precursor molecules should be polymerised on platinum electrodes by reductive electropolymerisation. However, a reductive polymerisation was not possible for the borane monomer 19 which is thought to be due to a strong localisation of the unpaired electron on the central boron atom of the radical anion. An electropolymerisation of the cyano-substituted bispyridinio-compound 17 failed because of the poor quality of CN- as a leaving group. Thus, a synthesis of the analogous isomer 18 was developed, in which the cyano-substituents were exchanged by the better leaving group Cl-. The viologen polymer poly-18, which can be regarded as an electron-deficient iso-electronic analogue of poly(para-phenylene), was successfully deposited on a platinum electrode by reductive electropolymerisation of 18. Poly-18 can be reversibly n-doped at comparably low potentials; however, at higher potentials the polymer is overcharged and destroyed irreversibly. As the synthetic strategy for 18 allows the variation of both spacer unit and leaving group in the last two steps of the reaction sequence, a series of analogous compounds can be easily synthesised using this route.},
subject = {Polymerhalbleiter},
language = {en}
}
@phdthesis{Chen2006,
author = {Chen, Zhijian},
title = {pi-Stacks Based on Self-Assembled Perylene Bisimides : Structural, Optical, and Electronic Properties},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-19940},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2006},
abstract = {As a traditional industrial pigment, perylene bisimide (PBI) dyes have found wide-spread applications. In addition, PBI dyes have been considered as versatile and promising functional materials for organic-based electronic and optic devices, such as transistors and solar cells. For these novel demands, the control of self-organization of this type of dye and the investigation of the relationship between the supramolecular structure and the relevant optical and electronic properties is of great importance. The objective of this thesis focuses on gaining a better understanding of structural and functional properties of pi-stacks based on self-assembling PBIs. Studies include the synthesis and characterization of new functional PBI dyes, their aggregation in solution, in liquid crystalline state and on surfaces, and their fluorescence and charge transport properties. An overview of the formation, thermodynamics and structures of pi-stacks of functional pi- conjugated molecules in solution and in liquid crystalline phases is given in Chapter 2. Chapters 3 and 4 deal with the pi-pi aggregates of new, highly fluorescent PBIs without core-substituents. In Chapter 3, the self-assembly of a PBI with tridodecylphenyl substituents at imide N atoms both in solution and condensed phase has been studied in great detail. In condensed state, the dye exhibits a hexagonal columnar liquid crystalline (LC) phase as confirmed by DSC, OPM and X-ray diffraction analysis. The columnar stacking of this dye has been further confirmed by atomic force microscopy (AFM) where single columns could be well resolved The charge transport properties this dye have been investigated by pulse radiolysis-time resolved microwave conductivity (PR-TRMC) measurements. To shed more light on the nature of the pi-pi interaction of the unsubstituted PBIs, solvent depend aggregation properties have been investigated in Chapter 4. The studies are further extended from core-unsubstituted PBIs to core-substituted ones (Chapter 5 and 6). In Chapter 5, a series of highly soluble and fluorescent core-twisted PBIs that bear the same trialkylphenyl groups at the imide positions but different bay-substituents and were synthesized. These compounds are characterized by distortions of the perylene planes with dihedral angles in the range of 15-37° according to crystallographic data and molecular modeling studies. In contrast to the extended oligomeric aggregates formed for planar unsubstituted PBIs, this family of dyes formed discrete pi-pi-stacked dimers in apolar methylcyclohexane as concentration-dependent UV/Vis measurements and VPO analysis revealed. The Gibbs free energy of dimerization can be correlated with the twist angles of the dyes linearly. In condensed state, several of these PBIs form luminescent rectangular or hexagonal columnar liquid crystalline phases with low isotropization temperatures. The core-twisting effect on semiconducting properties has been examined in Chapter 6. In this chapter, a comparative study of the electrochemical and the charge transport properties of a series of non-substituted and chlorine-functionalized PBIs was performed. While Chapters 3-6 focus on one-component dye systems, Chapter 7 explored the possibility of a supramolecular engineering of co-aggregates formed by hydrogen-bonded 2:1 and 1:1 complex of oligo(p-phenylene vinylene)s (OPVs) and PBIs. Covalently linked donor-acceptor dye arrays have been prepared for comparison. Concentration and temperature-dependent UV/Vis spectroscopy revealed all hydrogen-bonded and covalent systems form well-ordered J-type aggregates in methylcyclohexane. With these hydrogen-bonded OPV-PBI complexes, fibers containing p-type and n-type molecules can be prepared on the nano-scale (1-20 nm). For the 2:1 OPV-PBI hydrogenbonded arrays hierarchically assembled chiral superstructures consisting of left-handed helical pi-pi co-aggregates (CD spectroscopy) of the two dyes that further assemble into right-handed nanometer-scale supercoils in the solid state (AFM study) have been observed. All of these well-defined OPV-PBI assemblies presented here exhibit photoinduced electron transfer on sub-ps timescale, while the electron recombination differs for different systems.Thus, it was suggested that such assemblies of p- and n-type semiconductors might serve as valuable nanoscopic functional units for organic electronics.},
subject = {Perylenderivate},
language = {en}
}
@phdthesis{Sturm2006,
author = {Sturm, Christian},
title = {Theoretical Investigation of the Geometrical Arrangements of alpha-alanyl-peptide Nucleic Acid Hexamer Dimers and the Underlying Interstrand Binding Motifs},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-20363},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2006},
abstract = {Die Funktionalit{\"a}ten der DNA oder RNA werden haupts{\"a}chlich durch die verschiedenen Wechselwirkungen der paarenden Nucleinbasen bestimmt. Um die komplexen Zusammenh{\"a}nge dieser verschiedenen Wechselwirkungen zu verstehen, werden Modellsysteme ben{\"o}tigt, die weniger Restriktionen durch das R{\"u}ckgrat besitzen. Ein Beispiel f{\"u}r solche Systeme sind Peptidnucleins{\"a}uren (PNA), in denen das Zuckerphosphatr{\"u}ckgrat der DNA oder RNA durch ein Peptidr{\"u}ckgrat ersetzt wird. Diederichsen et al. gelang es, eine große Anzahl solcher Systeme mit einen alpha-Alanyl-R{\"u}ckgrat zu synthetisieren, an das kanonische und nicht-kanonische Nucleins{\"a}uren gebunden sind. Diese Systeme aggregieren in verschiedenen Bindungsmotiven, die nicht in der DNA oder RNA auftauchen. Diese ungew{\"o}hnlichen Paarungsmotive k{\"o}nnten einen tiefen Einblick in das Zusammenspiel der Wechselwirkungen der Nucleinbasen geben, aber die geringen L{\"o}slichkeit der alpha-Alanyl-PNA Oligomere verhinderte eine experimentelle Charakterisierung der geometrischen Anordnung durch R{\"o}ntgenstruktur- oder NMR-Experimente. Lediglich die absolute Stabilit{\"a}t der verschiedenen Aggregate konnte durch Messungen der Schmelztemperatur mit Hilfe der UV-Spektroskopie bestimmt werden. Da die Kenntnis der geometrischen Strukturen sowie der ausgebildeten Bindungsmotive wichtig ist, um einen Einblick in das Zusammenspiel der einzelnen Wechselwirkungen zu erlangen, besteht das Ziel der vorliegenden Arbeit darin, solche Informationen mit der Hilfe von theoretischen Methoden zu erlangen. Zus{\"a}tzlich sind Effekte von Interesse, aus denen sich Trends bez{\"u}glich der Stabilit{\"a}t bestimmen lassen. Solche Untersuchungen sind einfacher zu realisieren als die Berechnung der absoluten Stabilit{\"a}ten, da viele Beitr{\"a}ge zur absoluten Energie f{\"u}r {\"a}hnliche Systeme (entropische und dynamische Effekte) in etwa gleich groß sind. Somit sind diese entropischen und dynamischen Effekte f{\"u}r das Ziel dieser Arbeit weniger wichtig. Zur Untersuchung der Bindungseigenschaften und der Stabilit{\"a}ten von alpha-Alanyl-PNA Oligomeren war es notwendig, bis dato nicht parametrisierte Nucleinbasen in den Parametersatz des Amber4.1 Kraftfelds zu integrieren. Die fehlenden Ladungen wurden durch Berechungen mit dem R.E.D-Programm-Paket ermittelt. Das Programm bestimmt aus dem elektrostatischen Potential einer optimierten Struktur die atomzentrierten Ladungen. Die fehlenden Bindungsparameter wurden der Literatur entnommen. Die Untersuchungen der einzelnen Dimere begannen jeweils mit der Konstruktion der alpha-Alanyl-PNAs f{\"u}r alle m{\"o}glichen Paarungsmodi. Es konnte gezeigt werden, dass bestimmte Paarungsmodi aufgrund der geometrischen Gegebenheiten der Dimere und des R{\"u}ckgrats nicht realisierbar waren. F{\"u}r andere Dimere war ein Aufbau der alpha-Alanyl-PNA-Dimere zwar m{\"o}glich, jedoch zerfielen die Dimere wieder w{\"a}hrend einer ersten Geometrieoptimierung aufgrund der hohen Spannung im R{\"u}ckgrat. Die stabilen Systeme wurden zun{\"a}chst in verschiedenen Molekulardynamik-(MD)-L{\"a}ufen simuliert. Informationen {\"u}ber die Geometrie bei T=0 K wurden durch Geometrieoptimierungen erhalten, die an verschieden Punkten der MD L{\"a}ufe gestartet wurden. Die resultierenden Geometrien aus den verschiedenen Anfangspunkten waren identisch. F{\"u}r die geometrieoptimierten Strukturen wurden f{\"u}r das T=0 K Modell die Wechselwirkungsenergien zwischen den Nucleinbasen und der Einfluss der R{\"u}ckgrats auf die Stabilit{\"a}t der Dimer in zwei separaten Schritten bestimmt. Im ersten Schritt wurde das R{\"u}ckgrat entfernt und die Schnittstellen mit Methylgruppen abges{\"a}ttigt. Die Wechselwirkungsenergie zwischen den Nucleinbasen wurde durch die Differenz der Energien des gesamten Systems und der Summe der Energien der einzelnen Nucleinbasen in der Geometrie des Dimers bestimmt. Aufgrund der durchgef{\"u}hrten Untersuchungen und die sich daraus ergebenen Korrelation der berechneten Stabilisierungsenergien mit der Schmelztemperatur konnte gezeigt werden, dass mit der vorgeschlagenen Methode eine verl{\"a}ssliche Beschreibung der PNA Systeme m{\"o}glich ist. F{\"u}r eine weitere Verbesserung des vorgestellten Modells bedarf es zus{\"a}tzliche R{\"o}ntgenstruktur- oder NMR-Experimente, die zur Strukturaufkl{\"a}rung der alpha-Alanyl-PNA Dimere entscheidend beitragen. Weitere detaillierte Daten {\"u}ber die Enthalpiebeitr{\"a}ge zur absoluten Energie der verschiedenen Komplexe w{\"a}ren sehr hilfreich, um die vorgestellte Methode zu best{\"a}tigen und zu verbessern. Diese Informationen k{\"o}nnten zum einen durch die Auswertung der Form der Schmelzkurve sowie durch Mikrokalorimetrie erhalten werden. F{\"u}r den Fall, dass die Vorhersagen durch die experimentellen Befunde best{\"a}tigt w{\"u}rden, k{\"o}nnte der Ansatz auf verwandte Systeme wie zum Beispiel beta-Alanyl-PNA, DNA oder RNA angewandt werden. Durch diese weiteren Informationen k{\"o}nnte unser Ansatz zus{\"a}tzlich durch die Ber{\"u}cksichtigung von dynamischen und/oder entropischen Effekte erweitert werden.},
subject = {Peptid-Nucleins{\"a}uren},
language = {en}
}
@phdthesis{Hippius2007,
author = {Hippius, Catharina},
title = {Multichromophoric Arrays of Perylene Bisimide Dyes - Synthesis and Optical Properties},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-24767},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {The present work deals with the synthesis and the investigation of the photophysical properties of covalently constructed calix[4]arene-perylene bisimide dye arrays containing various PBI units. The obtained conjugates are characterized with respect towards their application in a new, zigzag-type architecture of artificial light-harvesting systems. For this purpose, orange (core-unsubstituted), red (6,7,11,12-tert-butylphenoxy-functionalized) and green (1,7-pyrrolidino-substituted) perylene bisimide building blocks have been attached to the calix[4]arene scaffold. First, the monochromophoric reference systems have been studied, and second, the photophysical properties of a comprehensive series of newly synthesized, multichromophoric calix[4]arene-perylene bisimide conjugates showing efficient energy transfer processes between the individual dye subunits have been investigated. Furthermore, a series of bichromophoric compounds containing identical chromophoric units has been obtained. Towards this goal, a variety of spectroscopic techniques such as UV/vis absorption, steady state and time-resolved fluorescence emission, and femtosecond transient absorption spectroscopy as well as a spectrotemporal analysis of the obtained data has been applied. This work presents a new concept for an artificial light-harvesting system positioning the dye units by means of calix[4]arene spacers along a zigzag chain. The investigations start with the syntheses and optical properties of the monochromophoric building blocks and result in an elaborate study on the energy and electron transfer processes occurring after photoexcitation in a comprehensive series of multichromophoric calix[4]arene-perylene bisimide conjugates. Finally, the photophysical properties of a series of compounds containing each two identical PBI units are discussed.},
subject = {Fluoreszenz-Resonanz-Energie-Transfer},
language = {en}
}
@phdthesis{Osswald2007,
author = {Osswald, Peter Uwe},
title = {Perylene Bisimide Atropisomers : Synthesis and Optical and Chiroptical Properties},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-23248},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {Zur Herstellung von atropo-enantiomeren Perylenbisimiden wurde die kovalente Verkn{\"u}pfung von Aryloxysubstituenten durch Makrocyclisierung eingesetzt. Die Darstellung makrocyclischer Perylenbisimide erfolgte ausgehend von einem vierfach (3-Hydroxyphenoxy)-funktionalisierten Perylenbisimid mit achiralen 2,6-Diisopropylphenylsubstituenten an den Imidpositionen durch Williamsonsche Ethersynthese. Die Synthese konnte f{\"u}r vier unterschiedliche Oligoethylenglykol-Kettenl{\"a}ngen realisiert werden, wobei f{\"u}r jede Br{\"u}ckenl{\"a}nge jeweils zwei regioisomere Makrocyclen, n{\"a}mlich das diagonal verbr{\"u}ckte (1,7- und 6,12-Verk{\"u}pfung) und das seitlich verbr{\"u}ckte Isomer (1,12- und 6,7- Verkn{\"u}pfung), isoliert werden konnten. Die strukturelle Zuordnung der isolierten Makrocyclen zu einem der beiden Regioisomeren gelang zweifelsfrei anhand von R{\"o}ntgenstrukturanalysen f{\"u}r zwei Makrocyclen und 1H NMR-Spektroskopie f{\"u}r alle Isomere. Der konformative Einfluß der Aryloxy-Substituenten auf die funktionenellen Eigenschaften dieser Farbstoffklasse konnte durch Vergleich der optischen und elektrochemischen Eigenschaften aller isolierter Makrocyclen mit einer offenkettigen Referenzverbindung abgeleitet werden. Hierbei zeigte sich, dass die Aryloxy-Substituenten dieser Farbstoffe in L{\"o}sung bevorzugt in einer horizontalen Konformation vorliegen. Durch l{\"o}sungsmittelabh{\"a}ngige Fluoreszenzmessungen konnte gezeigt werden, dass ein photoinduzierter Elektronentransferprozess f{\"u}r die Fluoreszenzl{\"o}sung elektronenreicher Aryloxy-Substituenten von Bedeutung ist. Die Trennung der Atropo-Diastereomere konnte f{\"u}r eine diagonal {\"u}berbr{\"u}ckte makrocyclische Verbindung mit chiralen 2-(R)-Octylamin Imidsubstituenten und Diethyleneglykol als Br{\"u}ckenkette mittels semi-preparativer HPLC an einer chiralen station{\"a}ren Phase realisiert werden. Die chiroptischen Eigenschaften der isolierten epimerenreinen Makrocyclen wurden mittels CD-Spektroskopie untersucht. Die Zuordnung der absoluten Stereochemie konnte anhand der erhaltenen CD-Spektren durch Anwendung der „Theorie der excitonischen Kopplung" abgeleitet und durch quantenchemische Berechnung der CD-Spektren best{\"a}tigt werden. Dieses Synthesekonzept wurde auf 1,7-diaryloxy-substituierten Perylenbisimide erweitert. Die Struktur der erhaltenen diagonal verbr{\"u}ckten monocyclischen Verbindung konnte erneut durch NMR-Spektroskopie und R{\"o}ntgenstrukturanalyse eindeutig bestimmt werden. Die Trennung der Atropo-Enantiomere gelang mittels semi-preparativer HPLC an einer chiralen station{\"a}ren Phase. Die Zuordnung der Stereochemie konnte anhand des Vergleichs der CD-Spektren mit den zuvor f{\"u}r epimerenreine Bismakrocyclen erhaltenen CD-Spektren realisiert werden. Anhand der R{\"o}ntgenstrukturanalysen sowohl der racemischen Mischung als auch eines Atropo-Enantiomers ließen sich bedeutende Informationen {\"u}ber die p-Dimerisierung von Perylenbisimiden ableiten. Die Abh{\"a}ngigkeit der Razemisierungsbarriere von der Gr{\"o}ße der Bay-Substituenten wurde f{\"u}r vier halogensubstituierte Derivate untersucht. Die dynamischen Eigenschaften wurden mittels temperaturabh{\"a}ngiger NMR-Spektroskopie und kinetischer Messungen mittels CD-Spektroskopie bestimmt. Unter Anwendung des „Apparent Overlap"-Konzeptes konnte eine {\"u}berzeugende lineare Beziehung zwischen der Gr{\"o}ße der Substituenten und der Inversionsbarriere hergestellt werden. Dar{\"u}ber hinaus war es m{\"o}glich die Atropo-Diastereomere bzw. Enantiomere der tetrachlor- und tetrabrom-substituierten Derivate zu trennen, wobei vor allem das 1,6,7,12-tetrabrom-substituierte Perylenbisimid stabile Enantiomere bei Raumtemperatur lieferte. Die abgeleitete Struktur-Eigenschaftsbeziehung sollte zuk{\"u}nftig die Herstellung von stabilen Enantiomeren durch geeignete Wahl der Substituenten in den Bay-Positionen erm{\"o}glichen. Um die Reversibilit{\"a}t der Selbstorganisation zur quantitativen Synthese makrocyclischer Perylenbisimide ausn{\"u}tzen zu k{\"o}nnen, wurde ein tetra(zinkporphyrin)-funktionalisiertes Perylenbisimid synthetisiert. Die Ausbildung des angestrebten 1:2-Sandwichkomplexes aus Tetra-Zinkporphyrin-Perylenbisimid und Diazabicyclo-[2.2.2]-undecan wurde mittels UV/Vis und 1H NMR Spektroskopie untersucht und die makrocyclische Struktur des Komplexes konnte mittels diffusionsabh{\"a}ngiger NMR Spektroskopie (DOSY NMR) eindeutig bewiesen werden. Weiterhin konnte mittels rasterkraftmikroskopischer (AFM) Untersuchungen gezeigt werden, dass diese funktionellen makrocyclischen Verbindungen sehr geordnet auf einer Graphitoberfl{\"a}che (HOPG) abgeschieden werden k{\"o}nnen. Die Ausrichtung eines amino-funktionalsierten p-konjugierten Polymers durch Zugabe des bichromphoren Tetra-Zinkporphyrin-Perlyenbisimids wurde mittels UV/Vis-Spektroskopie und AFM-Messungen untersucht. Die Oberfl{\"a}chenanalyse mittels AFM zeigte, dass die bichromophore Verbindung die linearen p-konjugierten Polymere {\"u}ber weite Teile der Oberfl{\"a}che auszurichten vermag, so dass eine definierte Anordnung von drei p-Systemen auf der Graphitoberfl{\"a}che erm{\"o}glicht wurde.},
language = {en}
}
@phdthesis{Holzapfel2007,
author = {Holzapfel, Marco},
title = {Photoinduced Charge Transfer Processes in Triarylamine Based Redox Cascades},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-25276},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {In the first part of this work a new approach to measure transient absorption spectra of fluorescent compounds by means of laser flash photolysis technique was presented. Generally, the recorded transient absorption signal consists of transient absorption, fluorescence and ground state bleaching. Thus, for fluorescent chromophores a fluorescence correction is indispensable in order to obtain undisturbed absorption decay curves as well as accurate transient absorption spectra. Due to time response characteristics of the PMT detector the fluorescence contribution cannot be corrected by recording the fluorescence separately. Measuring two transient absorption signals with probe light differing in intensity, compounds with quantum yields up to ~ 35 \% can be investigated. This is a major improvement because transient absorption spectroscopy is a powerful method to gain insight into the kinetics and the energy of excited states and information in the time domain of fluorescence are no longer lost. In the second part the synthesis and the photophysical characterisation of redox cascades were reported. These cascades consist of an acridine acceptor and up to three triarylamine donor subunits. The redox potentials of the triarylamines were tuned by adequate substituents in the para-position of the phenyl ring to ensure a directed redox gradient. Upon photoexcitation a locally excited state or a CT state is populated which then injects a hole onto the adjacent donor and consequently results in a CS state. Fluorescence and transient absorption measurements revealed that HT depends strongly on donor strength and solvent polarity. Formation of a CS state was only observed in case of strong terminal donors or polar solvents. A low lying localised triplet state acts as an energy trap and quenches all CS states even in case of the cascade with the strongest terminal donor in very polar solvents. Furthermore, population of a CS state catalyses the formation of this triplet states which results in a shorter lifetime of the CS state compared to the lifetime of the CT state of the corresponding reference compound. Compared to redox cascades already reported in literature, the electronic coupling between the redox centres was decreased by sterical as well as electronic effects. To prolong the lifetime of the CS state saturated spacers on the one hand and a perpendicular orientation of the acceptor and the adjacent donor on the other hand were selected. The twisting of the subunits forming the CT state results in a higher degree of charge separation but its contribution to increase the lifetimes of the CS states is of minor importance. The longer lifetime of the CS states can be ascribed to the saturated spacers. Experimental data in combination with calculated values indicate that charge recombination takes place in the Marcus normal region by a superexchange mechanisms. Although charge recombination of the known cascades is located in the Marcus inverted region, these CS states decay faster than the CS states of the compounds investigated in this work.},
subject = {Ladungstransfer},
language = {en}
}
@phdthesis{Arnone2007,
author = {Arnone, Mario},
title = {Theoretical Characterization and Optimization of Photochemical Alkoxyl Radical Precursors},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-23815},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {Oxygen-centered radicals are important intermediates in photobiological, mechanistic and synthetic studies. The majority of precursors of reactive oxyl radicals are labile and thus delicate to handle. Therefore N-(alkoxy)-pyridinethiones and N-(Alkoxy)-thiazolethiones have attracted attention as "mild'' photochemical source of alkoxyl radicals, in the last few years. A disadvantage of the pyridine compounds, is their sensibility to daylight. Despite of their similarities, both molecules behave surprisingly different, if photolyzed in the absence of trapping reagents. The pyridinethione compounds undergo highly efficient radical chain reactions under such conditions while the corresponding thiazolethiones react surprisingly sluggish and give rise to several unwanted side products. The properties of both compounds should be understood and optimized in the frame of this work. Additionally new compounds should be suggested that can also be applied in the photochemical alkoxyl radical generation. Some background information about the generation and application of alkoxyl radicals is provided in chapter 2. Electronic excitations and UV/vis spectroscopy together with a description of quantum chemical approaches that are able to calculate such phenomena are outlined in chapter 3. Chapter 4 deals with the description of the vertical excitation spectra. During the validation CASSCF, CASPT2, TD-DFT and RI-CC2 were tested with respect to their ability to describe the vertical excitations in both compounds. The CASPT2 approach gives accurate descriptions of the electronic excitation spectra of all compounds. The time-dependent DFT results are very sensitive on the choice of the functional and a validation of the results should be always done. On the basis of these computations the spectroscopic visible absorption bands of both compounds were assigned to a pi-->pi* transition in the thiohydroxamic acid functionality. In chapter 5 the mechanism of the thermally and the photochemically induced N,O homolysis in both compounds is unveiled. The near UV-induced N,O homolysis will start from the S2 state. The expected relaxation from the S2- to the S1-state and the dissociation process is expected to be very fast in the case of the thiazolethione compound. The potential surfaces of the pyridine compound in contrast point to a slower N,O bond dissociation. Due to the resulting faster dissociation process the excess energy which results from the photochemical activation is quenched only to small amounts. The maximal possible excess energy of the fragments is lower and a quenching is much more likely in the case of the pyridinethione compounds. This explaines the different reactivities of both compounds. For the also already successfully applied precursor system N-(alkoxy)-pyridineones the computed dissociation paths show courses that clearly predict a slow bond dissociation process. Chapter 6 deals with the tuning of the initial excitation wave length of the known pyridinethiones und thiazolethiones. In the first part the effects of substituents on the thiazolethione heterocycle was examined. The UV/vis spectra of 4 and 5 substituted thiazolethiones can be interpreted like the spectrum of the parent compound. The second part of chapter 6 deals with the identification of a substitution pattern on the pyridine heterocycle which induces a blue shift of the photo active band. The computations showed that electron rich and electron poor substituents result the same effects on the electronic excitation spectra. These substituent effects are additive, but the steric orientation of the substituents has to be taken into account. Chapter 7 describes a computer aided design of new alkoxyl radical precursors. Combining the advantages of both compounds the radical formation should be initiated by an irradiation with light at about 350 nm, and the amount of side products during the radical formation process should be small. To achieve this 18 test candidates were obtained by a systematic variation of the parent compound of the thiazolethione precursor. To identify the promising new precursor systems a screening of the lower electronic excitations of all resulting 18 systems was performed with TD-DFT. For promising systems the N,O or P,O dissociation paths, respectively, were analyzed according to the developed model. N-(methoxy)-azaphospholethione and N-(methoxy)-pyrrolethione seem to be the most promising candidates. The computations predict a strong absorption at about 350 nm respectively 320 nm. Due to the amounts of maximal excess energy and the shapes of the potential surfaces of the N,O bond dissociation paths their reactivity should resemble more the behavior of the pyridinethiones.},
language = {en}
}
@phdthesis{Faber2007,
author = {Faber, Johan Henrik},
title = {Naphthylisoquinoline Alkaloids : Structural Elucidation, Metabolism and Functional Analysis of their Bioactivities},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-22789},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {This thesis deals with the isolation and structural elucidation of bioactive naphthylisoquinoline alkaloids and related analogs. The mode of action of the antiplasmodial activity exhibited by the naphthylisoquinoline alkaloids was explored and compared to that of the antimalarial drug chloroquine. Furthermore, the phase 1 and 2 metabolism of dioncophyllines A and C and dioncopeltine A were investigated. In detail the following results have been obtained: • From the leaves of the recently discovered East African liana A. tanzaniensis six naphthylisoquinoline alkaloids were isolated. • The leaves of a botanical yet undescribed Ancistrocladus species, collected by Prof. Dr. V. Mudogo in the Democratic Republic of Congo in the habitat Yeteto near the town Ikela, were analyzed for naphthylisoquinoline alkaloids for the first time. The isolation work led to the first identification of an N,C-coupled naphthyldihydroisoquinoline alkaloid; ancistrocladinium B. Phytochemical investigation of the roots of the Congolese Ancistrocladus species (habitat Yeteto), , afforded five new derivatives of known naphthylisoquinoline alkaloids, namely 5'-O-demethylhamatine, 5'-O-demethylhamatinine, 6-O-demethylancistroealaine A, 6,5'-O,O-didemethylancistroealaine A, and 5-epi-6-O-methylancistrobertsonine A, along with six known naphthylisoquinoline alkaloids. • The antiplasmodial activity guided purification of 60Co irradiated samples containing commercially available naphthylisoquinoline related substances, afforded the isolation of the irradiation products 3,4-dihydro-1-isoquinolinone, 3,4-dihydro-1-isoquinolineamine, and 1,2,3,4-tetrahydro-1,2-diazirino-isoquinoline. The compounds were found to be more active than the starting material, although only exhibiting weak antiplasmodial activity against P. falciparum. • The effect on the absorption spectrum of FPIX due to complex formation with the naphthylisoquinoline alkaloids dioncophyllines A and C, dioncopeltine A korupensamine A, and ancistrocladine was examined by a titration study. Job's plot analyses by UV-spectroscopy determined the stoichiometry for the complex formation of FPIX and naphthylisoquinoline alkaloids to be 2:1. Furthermore, the dissociation constants for the complexation with FPIX were determined for each of the naphthylisoquinoline alkaloids investigated. Dioncophylline C and dioncopeltine A were found to possess dissociation constants, which are comparable to the one reported for the antimalarial drug chloroquine. The ability of ESI to transfer noncovalent solution-phase assemblies intact into the gas phase, was conducted on solution mixtures of naphthylisoquinoline alkaloid and FPIX, as well as on mixtures of chloroquine and FPIX. The mass spectrometry analyses revealed several peaks, which corresponded to the complex formation of FPIX to the respective ligands investigated. The most interesting results obtained were the detection of peaks corresponding to the complex formation between a chelated dimer of FPIX and dioncophylline Cand of peaks corresponding to a double protonated tetramer of FPIX - consisting of two chelated \&\#61549;-oxo dimers of FPIX - in complex formation with two molecules of chloroquine. • Two phase 1 metabolism products of dioncophylline A were identified. Coelution in combination with HPLC-MS/MS, NMR, and CD investigations assigned the major metabolic product as 5'-O-demethyldioncophylline A. The minor metabolic product was only present in small amounts, which disabled an unambiguous structural characterization of the compound. However, as deduced from the mass spectrometry analyses and exclusion of a possible metabolic oxidation product by coelution with authentic reference material, the metabolite should possess a 4-hydroxylated isoquinoline portion and is assumed to be represented by structure. Dioncophylline C and dioncopeltine A were found to be stable to phase 1 metabolism reactions caused by rat liver microsomes.},
subject = {Naphthylisochinolinalkaloide},
language = {en}
}
@phdthesis{Roeger2007,
author = {R{\"o}ger, Cornelia},
title = {Bioinspired Light-Harvesting Zinc Chlorin Rod Aggregates Powered by Peripheral Chromophores},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26760},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {Artificial light-harvesting (LH) systems have been obtained by self-assembly of naphthalene diimide-functionalized zinc chlorin dyads and triad in nonpolar, aprotic solvents. UV-vis, CD, and steady-state emission spectroscopy as well as atomic force microscopy showed that rod-like structures are formed by excitonic interactions of zinc chlorin units, while the appended naphthalene diimide dyes do not aggregate at the periphery of the cylinders. In all cases, photoexcitation of the enveloping naphthalene diimides at 540 and 620 nm, respectively, was followed by highly efficient energy-transfer processes to the inner zinc chlorin backbone, as revealed by time-resolved fluorescence spectroscopy on the picosecond time-scale. As a consequence, the LH efficiencies of zinc chlorin rod aggregates were increased by up to 63\%. The effective utilization of solar energy recommends these biomimetic systems for an application in electronic materials on the nanoscale.},
subject = {Farbstoff},
language = {en}
}
@phdthesis{Schlund2007,
author = {Schlund, Sebastian},
title = {Quantifying Non-covalent Interactions - Rational in-silico Design of Guanidinium-based Carboxylate Receptors},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-24388},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2007},
abstract = {Die nat{\"u}rlichen Vorbilder effektiver Anionenrezeptoren sind Enzyme, welche oftmals Arginin als entscheidende Aminos{\"a}ure in der Bindungstasche tragen. Die positiv geladenene Guanidiniumgruppe, wie sie in der Seitenkette von Arginin vorkommt, ist daher das zentrale Strukturmerkmal f{\"u}r viele k{\"u}nstliche Anionenrezeptoren. Im Jahre 1999 gelang es Schmuck und Mitarbeitern eine neue Klasse von Guanidinium-basierten Oxoanionenrezeptoren zu entwickeln, die Carboxylate sogar in w{\"a}ssrigen Medien binden k{\"o}nnen. Die Bindungsmodi der 2-(Guanidiniocarbonyl)-1H-pyrrole basieren auf einer Kombination von einzeln betrachtet schwachen nicht-kovalenten Wechselwirkungen wie Ionenpaarbildung und multiplen Wasserstoffbr{\"u}ckenbindungen zwischen k{\"u}nstlichem Rezeptor und Substrat. Durch Substitution einer Carboxylatgruppe in Position 5 des Pyrrolringes erh{\"a}lt man ein zwitterionisches Derivat welches sich in Wasser mit einer Assoziationskonstante von sch{\"a}tzungsweise 170 M-1 zu einzelnen Dimeren zusammenlagert (Dimer 1). Um das Strukturmotiv hinsichtlich einer noch effektiveren Anionenbindung weiter verbessern zu k{\"o}nnen, ist es daher von großem Interesse, die verschiedenartigen intermolekularen Wechselwirkungen zwischen den beiden monomeren Einheiten von Dimer 1 zu quantifizieren. Vor diesem Hintergrund wurden verschiedene theoretische ab initio Studien durchgef{\"u}hrt, um die Einfl{\"u}sse von intrinsischen Eigenschaften sowie von Solvenseffekten auf die Stabilit{\"a}t sich selbst zusammenlagernden Dimeren aufzukl{\"a}ren. In Kapitel 4.1 wurden die molekularen Wechselwirkungen im Dimer 1 durch Vergleich mit verschiedenen „Knock-out" Analoga untersucht. In diesen Analoga wurden einzelne Wasserstoffbr{\"u}ckenbindungen durch Substitution von Wasserstoffdonoren mit Methylengruppen oder Etherbr{\"u}cken ausgeschaltet. Es konnte gezeigt werden, dass die Anwendung eines vereinfachten Kontinuum-Solvensmodells nicht ausreicht, die absoluten Energien der „Knock-out" Analoga in stark polaren L{\"o}sungsmitteln vorherzusagen, jedoch k{\"o}nnen die berechneten Trends Auskunft {\"u}ber die relativen Stabilit{\"a}ten geben. In Kapitel 4.2 wurde die strukturelle {\"A}hnlichkeit von Arginin mit Struktur 1 ausgenutzt, um die Abh{\"a}ngigkeit der St{\"a}rke der Dimerisierung von der Flexibilit{\"a}t der molekularen Struktur eingehender zu untersuchen. In Kapitel 4.2.1 wurden neue globale Minimumsstrukturen des kanonischen und zwitterionischen Arginins in der Gasphase bestimmt. Dies geschah mit Hilfe von umfangreichen kraftfeldbasierten Konformationssuchen in Verbindung mit ab initio Strukturoptimierungen der energetisch niedrigsten Konformere. Die meisten der neu identifizierten Minimumskonformere sowohl des zwitterionischen als auch des kanonischen Tautomers zeigten geometrische Anordnungen mit bis dahin unbekannten gestapelten Orientierungen der endst{\"a}ndigen Gruppen. Es wurde letztendlich eine neuartige globale Minimumsstruktur (N1) gefunden, welche eine um mehr als 8 kJ mol-1 niedrigere Energie besitzt als die bislang ver{\"o}ffentlichten Konformere. Die gleiche Strategie f{\"u}r das Auffinden von energetischen Minimumskonformeren, wie sie bereits f{\"u}r das Arginin Monomer benutzt wurde, wurde auch im Falle der Dimere von Arginin verwendet. Im Gegensatz zu vorhergehenden theoretischen Untersuchungen ist die neue globale Minimumsstruktur ungef{\"a}hr 60 kJ mol-1 stabiler und weist ebenfalls eine gestapelte Orientierung der Guanidinium- und Carboxylatgruppen auf. Der Einfluss der Rigidit{\"a}t auf die Dimerstabilit{\"a}t wurde durch Berechnungen eines k{\"u}nstlich versteiften Arginin Dimersystems bewiesen. Die hohe Bindungsaffinit{\"a}t des Dimers 1 ergibt sich daher zu etwa 50\% aus der Rigidit{\"a}t der Monomere, welche jegliche intramolekulare Stabilisierung verhindert. Um Vorschl{\"a}ge f{\"u}r ein verbessertes Carboxylatbindungsmotiv machen zu k{\"o}nnen, wurden in Kapitel 4.3 neuartige Strukturmotive mit ver{\"a}nderten Ringsystemen auf DFT Niveau untersucht. Die direkte Abh{\"a}ngigkeit der Dimerisierungsenergie von einem zunehmenden Dipolmoment wurde durch verschiedene anellierte Ringstrukturen bewiesen. Der Einfluss der Delokalisierung in den Monomeren auf die Dimerisierungsenergie wurde durch Ver{\"a}nderung der Elektronenstruktur von elektronisch entkoppelten Biphenylenen untersucht. Es konnte gezeigt werden, dass die Carbonylfunktion haupts{\"a}chlich f{\"u}r eine gute Pr{\"a}organisation verantwortlich ist, wohingegen der Effekt auf die Azidit{\"a}t eine geringere Bedeutung besitzt. Im letzten Kapitel wurden Kooperativit{\"a}tseffekte in supramolekularen Systemen untersucht. Als Modellsysteme dienten hierbei Adenosin-Carbons{\"a}ure-Komplexe, deren berechnete NMR Verschiebungen mit experimentellen Niedrigtemperatur-NMR-Studien verglichen wurden. Wir konnten zeigen, dass nur durch die Verwendung von schwingungsgemittelten NMR Verschiebungen die experimentelle Protonenverschiebung reproduziert werden kann, welche unter Tieftemperaturbedingungen im Austauschregime von Wasserstoffbr{\"u}ckenbindungen erhalten wurde.},
subject = {nicht-kovalente Wechselwirkungen},
language = {en}
}
@phdthesis{Mladenovic2008,
author = {Mladenovic, Milena},
title = {Theoretical Investigation into the Inhibition of Cystein Proteases},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-25763},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2008},
abstract = {Although known about and investigated since the late 1970's, the picture of the basic principles governing inhibitor strengths and the structure-activity relationships of the cysteine protease inhibition mechanism is still very incomplete. Computational approaches can be a very useful tool for investigating such questions, as they allow the inspection of single, specific effects in isolation from all others, in a manner very difficult to achieve experimentally. The ab initio treatments of such large systems like proteins are still not feasible. However, there is a vast number of computational approaches capable of dealing with protein structures with reasonable accuracy. This work presents a summary of theoretical investigations into cysteine protease cathepsin B using a range of methods. We have concentrated on the investigation of cysteine protease inhibition by epoxide- and aziridine-based inhibitors in order to obtain better insight into these important topics. Various model systems are simulated by means of pure quantum mechanical methods and by hybrid (QM/MM) methods. Both approaches provide a static picture. Dynamical effects are then accounted for by additional molecular dynamics (MD) simulations, using both classical and QM/MM MD approaches. The quantum mechanical approach was used to study very small model systems consisting only of the electrophilic warhead of the inhibitor (both substitituted and not) and molecular moieties simulating a very simplified protein active site (methylthiolate instead of Cys29 and methylimidazolium instead of His199 residue) and solvent surroundings (two waters or two ammonium ions, in combination with a continuum solvent model). Although simple, such a system provides a good description of the most important interactions involved in the inhibition reaction. It also allows investigation of the influence of the properties of the electrophilic warhead on the reaction rate. Beside the properties of the electrophilic warhead, the protein and solvent environment is also an important factor in the irreversible deactivation of the enzyme active site by the inhibitor. The non-covalent interactions of the inhibitor with the oxyanion hole and other subsites of the enzyme, as well as its interaction with the solvent molecules, need to be explicitly taken into account in the calculations, because of their possible impact on the reaction profile. As molecular modeling methods allow the treatment of such large systems, but lack the possibility of describing covalent interactions, our method of choice was the combined quantum mechanics/molecular modeling approach. By splitting the system into a smaller part that undergoes the bond cleavage/formation process and must be treated quantum mechanically, and a larger part, comprised of the rest of the protein, which could be treated using force fields, we managed to simulate the system at the desired precision. Our investigations concentrated on the role of His199 in the inhibition mechanism as well as on the structure-reactivity relationships between cysteine protease and various inhibitors, yielding new insight into the kinetics, regio- and stereospecificity of the inhibition. In particular, our calculations provide the following insights: i.) an explanation for the regioselectivity of the reaction, and original insight into which interactions affect the stereoselectivity; ii.) a clear model which explains the known structure-activity relationships and connects these effects with the pH-dependency of the inhibition; iii.) our computations question the generally accepted two-step model by showing that substituent effects accelerate the irreversible step to such an extent that the achievement of an equilibrium in the first step is doubtful; iv.) by way of theoretical characterizations of aziridine models, the reasons for similarities and differences in the mode of action of epoxide- and aziridine-based inhibitors are elucidated; and finally, v.) combining our results with experimental knowledge will allow rational design of new inhibitors. To account for dynamical effects as well, molecular dynamics (MD) computations were also performed. In these calculations the potential energy was computed at the force field level. The results not only supported and clarified the QM/MM results, but comparison with previous X-ray structures helped correct existing errors in the available geometrical models and resolved inconsistencies in the weighting of various factors governing the inhibition. In the work the first QM/MM MD calculations on the active site of the cysteine proteases are presented. In contrast to the MD simulations, these calculations used potential energies computed at the QM/MM-level. With the help of these computations we sought to address strongly disputed questions about the reasons for the existence of the active site ion pair and its role in the high activity of the enzyme.},
subject = {Quantenchemie},
language = {en}
}
@phdthesis{Stepanenko2008,
author = {Stepanenko, Vladimir},
title = {Self-Assembly of Bay-Substituted Perylene Bisimide by Ligand-Metal Ion Coordination},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-32063},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2008},
abstract = {The subject of this thesis is the synthesis and characterization of PBI-based fluorescent metallosupramolecular polymers and cyclic arrays. Terpyridine receptor functionalized PBIs of predesigned geometry have been used as building blocks to construct desired macromolecular structures through metal-ion-directed self-assembly. These metallosupramolecular architectures have been investigated by NMR, UV/Vis and fluorescence spectroscopy, mass spectrometry, and atomic force microscopy.},
subject = {Supramolekulare Chemie},
language = {en}
}
@phdthesis{Stepanenko2008,
author = {Stepanenko, Svetlana},
title = {Global Optimization Methods based on Tabu Search},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30605},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2008},
abstract = {This work encompasses three parts. The first part provides a concise review of the most prominent metaheuristic concepts currently available and gives essential preliminaries together with definition of the combinatorial optimization problems. It substantiates the choice of the investigation direction and basis idea of the developed methods. In the second part the new nonlinear global optimization routines based on the TS strategy are described. The new approaches are the Gradient Tabu Search (GTS), the Gradient Only Tabu Search (GOTS), and the Tabu Search with Powell's Algorithm (TSPA). In the last part of the work the GOTS is applied for such chemical optimization problems. The chapter provides a systematic approach how the variables are chosen and the adjustable parameters are set. As test cases the global minimum energy conformation of some amino acids, of two angiotensin converting enzyme (ACE) inhibitors, of 2-acetoxy-N,N,N-trimethylethanaminium, and of a HIV-1 protease inhibitor is determined.},
subject = {Tabusuche},
language = {en}
}
@phdthesis{Schmidt2008,
author = {Schmidt, R{\"u}diger},
title = {Perylene Bisimide and Acene Derivatives as Organic Semiconductors in OTFTs},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-29314},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2008},
abstract = {This thesis deals with the synthesis of improved organic semiconductors, the detailed investigation of the molecular properties and the solid state arrangements revealed by single crystal X-ray diffraction and finally the development of structureperformance dependencies by measuring of the charge carrier mobilities of the derivatives in thin film transistors. The two main-goals of this thesis were achieved. Well soluble acene derivatives for spin-coated TFTs were obtained, showing charge carrier mobilities in the range of polymer p-type materials. Novel core-fluorinated perylene bisimide dyes were synthesized particularly and the use of electron deficient substituents lead to PBIs with outstanding air-stable mobilities in thin film transistors prepared by vacuum deposition techniques. The relationship between performance, air stability and solid state packing was elucidated in detail by single crystal X-ray diffraction analysis.},
subject = {D{\"u}nnschichttransistor},
language = {en}
}
@phdthesis{Lohr2008,
author = {Lohr, Andreas},
title = {Self-Assembly of Merocyanines : Thermodynamic and Kinetic Insights into the Formation of Well-Defined Dye Aggregates},
url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28964},
school = {Universit{\"a}t W{\"u}rzburg},
year = {2008},
abstract = {The present thesis demonstrates the potential of dipolar aggregation of merocyanine dyes as novel directional and specific supramolecular binding motif for the creation of more elaborate supramolecular architectures beyond simple dimers. Furthermore, the self-assembly studies into bis(merocyanine) nanorods gave new insights into the kinetics of morphogenesis in supramolecular aggregates.},
subject = {Supramolekulare Chemie},
language = {en}
}