@phdthesis{Kaiser2022,
  author    = {Kaiser, Dustin},
  title     = {Non-standard computational approaches applied to molecular systems},
  doi       = {10.25972/OPUS-27664},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-276641},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {In this thesis, several contributions to the understanding and modeling of chemical phenomena using computational approaches are presented. These investigations are characterized by the usage of non-standard computational modeling techniques, which is necessitated by the complex nature of the electronic structure or atomic fluctuations of the target molecules. Multiple biradical-type molecules and their spectroscopic properties were modeled. In the course of the investigation, it is found that especially the impact of correct molecular geometries on the computationally predicted absorption properties may be critical. In order to find the correct minimum geometries, Multi-Reference methods may have to be invoked. The impact of geometry relaxation on the excitonic properties of Perylene Bisimide dimers were investigated. Oftentimes, these geometry factors are neglected in Organic Semiconductor modeling as an approximation. This present investigation suggests that this approximation is not always valid, as certain regimes are identified where geometrical parameters have critical impact on the localization and energetic properties of excitons. The mechanism of the Triazolinedione (TAD) tyrosine bioconjugation reaction is investigated using quantum-chemical methods. By comparison of different conceivable mechanisms and their energetic ordering, the TAD tyrosine bioconjugation is found to proceed by means of a base-mediated electrophilic aromatic substitution reaction. The kth nearest neighbor entropy estimation protocol is investigated. This estimator promises accurate entropy estimates even for flexible molecules with multiple structural minima. Our granular investigation of formal and practical properties of the estimator suggests that the uneven variance of a molecule's vibrational modes is the cause of the observed slow convergence of the estimator. A rescaling procedure to reestablish fast convergence is suggested and benchmarks are performed.},
  subject      = {Quantenchemie},
  language  = {en}
}
@phdthesis{Schmid2022,
  author    = {Schmid, Paul},
  title     = {Quantenchemische Untersuchungen von Umgebungseinfl{\"u}ssen bei offen- und geschlossenschaligen Systemen},
  doi       = {10.25972/OPUS-26510},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-265106},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {In dieser Dissertation werden die Umgebungseinfl{\"u}sse auf die strukturellen und elektronischen Eigenschaften von verschiedenen offen- und geschlossenschaligen Systemen mittels quantenchemischer Methoden berechnet. Ein Kernpunkt umfasst die Untersuchung von verdreht angeordneten, biradikalischen Diborylalkenen, welche eine unges{\"a}ttigte C2R2-Br{\"u}cke (R = Et, Me) besitzen und durch cyclische (Alkyl)(amino)carbene (CAACs) stabilisiert werden. Quantenchemische Berechnungen zeigen, dass haupts{\"a}chlich sterische Effekte f{\"u}r die Ausbildung einer verdrehten Molek{\"u}lanordnung verantwortlich sind, w{\"a}hrend bei geringen sterischen Wechselwirkungen (R = H) die Delokalisationseffekte {\"u}berwiegen, wodurch eine planare Struktur beg{\"u}nstigt wird. Die Bevorzugung einer offenschaligen Singulettkonfiguration anstelle eines Tripletts ist auf den großen Energieunterschied der beiden einfach besetzten Molek{\"u}lorbitale zur{\"u}ckzuf{\"u}hren. Durch die Berechnung der L{\"o}sungsmitteleffekte mithilfe von polarisierbaren Kontinuumsmodellen kann gefolgert werden, dass mit zunehmender statischer Dielektrizit{\"a}tskonstante eine planare und geschlossenschalige Struktur st{\"a}rker stabilisiert wird als eine verdrehte Anordnung. Ein weiteres Thema dieser Dissertation befasst sich mit der quantenchemischen Analyse eines makrozyklischen Perylenbisimid-Trimersystems, welches eingebettet in einer Polymethylmethacrylat-Matrix bei Temperaturen nahe dem absoluten Nullpunkt eine Lokalisierung der ersten drei angeregten Zust{\"a}nde zeigt. Quantenchemische Vakuumberechnungen ergeben, dass unabh{\"a}ngig von der gegenseitigen geometrischen Orientierung der drei Perylenbisimid-Chromophore der {\"U}bergang vom Grundzustand in den S1-Zustand verboten ist und dass die ersten drei angeregten Zust{\"a}nde delokalisiert vorliegen. Mithilfe von expliziten L{\"o}sungsmittelmodellen kann jedoch gezeigt werden, dass das Auftreten dieser Lokalisierungen auf eine inhomogene Polymethylmethacrylat-Umgebung zur{\"u}ckzuf{\"u}hren ist, die zu einem Symmetriebruch und somit zu einer Zunahme der Oszillatorst{\"a}rke f{\"u}r S1 und der Lokalisierungsgrade f{\"u}r S1, S2 und S3 f{\"u}hrt. Dar{\"u}ber hinaus wird der L{\"o}sungsmitteleinfluss auf die angeregten Zust{\"a}nde des Azulens mittels impliziter und expliziter L{\"o}sungsmittelmodelle berechnet. Bei einer Erh{\"o}hung der dynamischen Dielektrizit{\"a}tskonstante im impliziten Modell nehmen die Anregungsenergien der vertikalen Singulettzust{\"a}nde ab, wobei der Effekt mit steigender Oszillatorst{\"a}rke zunimmt. Die Auswirkung der statischen Dielektrizit{\"a}tskonstante auf die Anregungsenergien ist dagegen deutlich schw{\"a}cher ausgepr{\"a}gt. Im expliziten Modell bewirkt das L{\"o}sungsmittel ebenfalls eine Abnahme der Anregungsenergie des hellen Singulettzustands, wenn auch in geringerem Umfang als im impliziten Modell. Als letztes Thema wird der Inhibitionsmechanismus der Cysteinprotease Rhodesain durch zwei modifizierte 1,4-Naphthoquinone untersucht. W{\"a}hrend beide Naphthoquinone an der 2-Position eine Dipeptideinheit aufweisen, besitzen sie an der 3-Position entweder einen Nitril- oder Chloridsubstituenten. Zwar erfolgt bei beiden Derivaten die Inhibition {\"u}ber einen kovalent-reversiblen Mechanismus, jedoch verl{\"a}uft die Hemmung im Falle des Nitrilderivats erheblich effektiver. Die quantenchemischen Berechnungen eines vereinfachten Modells zeigen, dass die Cysteineinheit (HS-R) bevorzugt in einer exothermen und reversiblen Additionsreaktion an die elektronenarme C-C-Doppelbindung der Naphthoquinone anlagert. Dabei werden kleinere Reaktionsenergien f{\"u}r die Reaktion des Chlorderivats als f{\"u}r die Reaktion des Nitrilderivats erhalten. Durch die Ber{\"u}cksichtigung von Wasser in einem impliziten L{\"o}sungsmittelmodell kommt es bei fast allen Reaktionsprodukten zu einer Energiezunahme, die bei der Reaktion des Nitrilderivats st{\"a}rker ausf{\"a}llt als bei der Reaktion des Chlorderivats.},
  subject      = {Umgebungseinfluss},
  language  = {de}
}
@phdthesis{Gruene2022,
  author    = {Gr{\"u}ne, Marvin},
  title     = {Solid-state NMR Spectroscopic, X-Ray Diffraction and Quantum Chemical Investigations of the Crystalline Cancer Drug Paclitaxel and Paclitaxel incorporated into Polymer Micelles},
  doi       = {10.25972/OPUS-23719},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-237199},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Paclitaxel (PTX) is one of the leading drugs against breast and ovarian cancer. Due to its low solubility, treatment of the patients with this drug requires a very well-suited combination with a soluble pharmaceutical excipient to increase the bioavailability and reduce the strong side ef-fects. One efficient way to achieve this in the future could be the incorporation of PTX into pol-ymeric micelles composed of poly(2-oxazoline) based triblock copolymers (POL) which ena-bles PTX loadings of up to 50 wt.\%. However, structural information at an atomic level and thus the knowledge of interaction sites within these promising but complex PTX-POL formula-tions were not yet available. Such results could support the future development of improved excipients for PTX and suitable excipients for other pharmaceutical drugs. Therefore, a solid-state MAS NMR investigation of these amorphous formulations with different POL-PTX com-positions was performed in this thesis as this gives insights of the local structure at an atomic level in its solid state. NMR in solution showed very broad 13C signals of PTX for this system due to the reduced mobility of the incorporated drug which exclude this as an analytical meth-od. In a first study, crystalline PTX was structurally characterized by solid-state NMR as no com-plete 13C spectrum assignment and no 1H NMR data existed for the solid state. In addition, the asymmetric unit of the PTX crystal structure consists of two molecules (Z'=2) that can only be investigated in its solid state. As crystalline PTX in total has about 100 different 13C and 1H chemical shifts with very small differences due to Z'=2, and furthermore, its unit cell consisting of more than 900 atoms, accompanying GIPAW (CASTEP) calculations were required for NMR signal assignments. These calculations were performed using the first three available purely hydrous and anhydrous PTX structures, which were determined by XRD and published by Vel-la-Zarb et al. in 2013. Within this thesis, is was discovered that two investigated batches of commercially available PTX from the same supplier both contained an identical and so far un-known PTX phase that was elucidated by PXRD as well as solid-state NMR data. One of the two batches consists of an additional phase that was shown to be very similar to a known hy-drated phase published in 2013.[1] By heating the batch with the mixture of the two phases un-der vacuum, it is transformed completely to the new dry phase occurring in both PTX batches. Since the drying conditions to obtain anhydrous PTX in-situ on the PXRD setup described by Vella-Zarb et. al.[1] were much softer than ours, we identify our dry phase as a relaxed version of their published anhydrate structure. The PXRD data of the new anhydrate phase was trans-ferred into a new structural model, which currently undergoes geometry optimization. Based on solid-state NMR data at MAS spinning frequencies up to 100 kHz, a 13C and a partial 1H signal assignment for the new anhydrous structure were achieved. These results provided sufficient structural information for further investigations of the micellar POL-PTX system. In a second study, the applicability and benefit of two-dimensional solid-state 14N-1H HMQC MAS NMR spectra for the characterization of amorphous POL-PTX formulations was investi-gated. The mentioned technique has never been applied to a system of similar complexity be-fore and was chosen because around 84\% of the small-molecule drugs contain at least one nitrogen atom. In addition, the number of nitrogen atoms in both POL and PTX is much smaller than the number of carbons or hydrogens, which significantly reduces the spectral complexity. 14N has a natural abundance of 99.6\% but leads to quadrupolar broadening due to its nuclear spin quantum number I = 1. While this is usually undesirable due to broadening in the resulting 1D 14N NMR spectra, this effect is explicitly used in the 2D 14N-1H HMQC MAS experiment. The indirect 14N measurement can avoid the broadening while maintaining the advantage of the high natural abundance and making use of the much more dispersed signals due to the additional quadrupolar shifts as compared to 15N. This measurement method could be successfully applied to the complex amorphous POL-PTX mixtures. With increasing PTX loading of the formulations, additional peaks arise as spatial proximities of the amide nitrogens of POL to NH or OH groups of PTX. In addition, the 14N quadrupolar shift of these amide nitrogens decreases with increasing PTX content indicating a more symmetric nitrogen environment. The latter can be explained by a transformation of the trigonal planar coordination of the tertiary amide nitrogen atoms in pure POL towards a more tetrahedral environment upon PTX loading induced by the formation of hydrogen bonds with NH/OH groups of PTX. In the third and last project, the results of the two abovementioned studies were used and ex-tended by solid state 13C and two-dimensional 1H-13C as well as 1H-1H MAS NMR data with the aim to derive a structural model of the POL-PTX formulations at an atomic level. The knowledge of the NMR signal assignments for crystalline PTX was transferred to amorphous PTX (present in the micelles of the formulations). The 13C solid-state NMR signals were evalu-ated concerning changes in chemical shifts and full widths of half maximum (FWHM) for the different PTX loadings. In this way, the required information about possible interaction sites at an atomic level becomes available. Due to the complexity of these systems, such proximities often cannot be assigned to special atoms, but more to groups of atoms, as the individual de-velopments of line widths and line shifts are mutually dependent. An advantageous aspect for this analysis was that pure POL already forms unloaded micelles. The evaluation of the data showed that the terminal phenyl groups of PTX seem to be most involved in the interaction by the establishment of the micelle for lowest drug loading and that they are likely to react to the change in the amount of PTX molecules as well. For the incorporation of PTX in the micelles, the following model could be obtained: For lowest drug loading, PTX is mainly located in the inner part of the micelles. Upon further increasing of the loading, it progressively extends to-ward the micellar shell. This could be well shown by the increasing interactions of the hydro-phobic butyl chain of POL and PTX, proceeding in the direction of the polymer backbone with rising drug load. Furthermore, due to the size of PTX and the hydrodynamic radius of the mi-celles, even at the lowest loading, the PTX molecules partially reach the core-shell interface of the micelle. Upon increasing the drug loading, the surface coverage with PTX clusters increas-es based on the obtained model approach. The latter result is supported by DLS and SANS data of this system. The abovementioned results of the 14N-1H HMQC MAS investigation of the POL-PTX formulations support the outlined model. As an outlook, the currently running geometry optimization and subsequently scheduled calcu-lation of the chemical shieldings of the newly obtained anhydrous PTX crystal structure can further improve the solid-state NMR characterization through determination of further spatial proximities among protons using the existing 2D 1H(DQ)-1H(SQ) solid-state MAS NMR spec-trum at 100 kHz rotor spinning frequency. The 2D 14N-1H HMQC MAS NMR experiments were shown to have great potential as a technique for the analysis of other disordered and amor-phous drug delivery systems as well. The results of this thesis should be subsequently applied to other micellar systems with varying pharmaceutical excipients or active ingredients with the goal of systematically achieving higher drug loadings (e.g., for the investigated PTX, the similar drug docetaxel or even different natural products). Additionally, it is planned to transfer the knowledge to another complex polymer system containing poly(amino acids) which offers hy-drogen bonding donor sites for additional intermolecular interactions. Currently, the POL-PTX system is investigated by further SANS studies that may provide another puzzle piece to the model as complementary measurement method in the future. In addition, the use of MD simu-lations might be considered in the future. This would allow a computerized linking of the differ-ent pieces of information with the aim to determine the most likely model.},
  subject      = {Wirkstoff-Tr{\"a}ger-System},
  language  = {en}
}
@phdthesis{Hader2017,
  author    = {Hader, Kilian},
  title     = {Lokalisierungsdynamik unter Ber{\"u}cksichtigung von Molek{\"u}l-Feld-Wechselwirkung, Kern-Elektron-Kopplung und Exziton-Exziton-Annihilierung},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-146735},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {Diese Arbeit befasst sich mit verschiedenen Aspekten der Dynamik von Kernen, Elektronen und gekoppelten Kern-Elektron-Systemen, wobei je nach System unterschiedliche Herangehensweisen gew{\"a}hlt wurden. Zentrale Punkte sind bei allen drei Kapiteln einerseits die Lokalisierung von Teilchen und Energie und andererseits eine hohe Sensitivit{\"a}t in Bezug auf die Wahl der Anfangsbedingungen. Im ersten Teil wurden von der Carrier-Envelope-Phase (CEP) abh{\"a}ngende, laser-induzierte Lokalisierungen betrachtet. Das zentrale Element ist dabei das entwickelte Doppelpulsschema, mit welchem eine CEP-Abh{\"a}ngigkeit in beobachtbaren Gr{\"o}ßen erzeugt wird. Als Beispielsysteme wurden die Fragmentation im D₂⁺-Modellsystem und eine Isomerisierung im Doppelminimumpotential (DMP) untersucht. Als Observable wird die Asymmetrie betrachtet Im DMP kann die Asymmetrie mit dem Entantiomeren/Isomeren{\"u}berschuss gleich gesetzt werden kann und im D₂⁺-Modellsystem mit der Lokalisierung des Elektrons auf einem der beiden dissoziierenden Kerne. Eine Phasenabh{\"a}ngigkeit der Asymmetrien besteht nur f{\"u}r die CEP des zweiten Pulses φ₂, f{\"u}r welchen keine Begrenzungen f{\"u}r die Anzahl an Laserzyklen auftreten. Im DMP wurde die CEP-Abh{\"a}ngigkeit der Asymmetrien auch bei unterschiedlichen Startkonfigurationen untersucht. F{\"u}r alle untersuchten Startkonfigurationen konnte ein Laserparametersatz gefunden werden, der f{\"u}r zumindest eine der beiden Asymmetrien eine CEP-Abh{\"a}ngigkeit liefert. Aufgrund der aufgehobenen energetischen Entartung der Paare gerader und ungerader Symmetrie ist die resultierende Lokalisierung zeitabh{\"a}ngig. Zur Messung der vorhergesagten Dynamiken ist z.B. die Aufnahme eines Photoelektronen-Spektrums denkbar. In n{\"a}chsten Kapitel wurden unterschiedliche Dynamiken innerhalb eines 4d Kern-Elektron-Modells in der N{\"a}he einer konischen Durchschneidung (CI) zweier Potentiale betrachtet. Hierbei ist hervorzuheben, dass eine solche gleichzeitige Untersuchung von Kern- und Elektron-Dynamik in Systemen mit CIs in der Literatur, nach Wissen des Autors, bisher nicht ver{\"o}ffentlicht ist. Das 4d-Potential wurde mit Hilfe des sogenannten Potfit-Algorithmus gefittet. Dieser Fit wurde anschließend verwendet, um die Dynamik des gekoppelten Systems mit Hilfe der "Multi-Configuration Time-Dependent Hartree"(MCTDH)-Methode zu berechnen. Aus der Analyse der gekoppelten Kern-Elektron-Wellenfunktion ergaben sich zwei grundlegend unterschiedliche Klassen von Dynamiken: • Diabatisch: Kern- und Elektrondynamik sind nahezu entkoppelt. Der Kern bewegt sich und das Elektron bleibt statisch. • Adiabatisch: Kern- und Elektrondynamik sind stark gekoppelt. Die Kerndynamik findet auf Kreisbahnen statt. Mit der Rotation der Kerndichte um den Winkel φ geht eine Rotation der Elektron-Dichte einher. Die diabatische Bewegung entspricht der Dynamik durch die konische Durchschneidung und die adiabatische Bewegung der Dynamik auf der unteren Potentialfl{\"a}che. Welche der beiden Dynamiken stattfindet, wird durch die Wahl der Anfangsbedingung bestimmt. Der wesentliche Unterschied zwischen den beiden Startzust{\"a}nden ist dabei die Lage des Knotens im elektronischen Anteil der Wellenfunktion. In den diabatischen Bewegungen bleibt z.B. der pₓ -artige Charakter der elektronischen Wellenfunktion konstant, wohingegen sich bei der adiabatischen Dynamik der Charakter mit der Kernbewegung {\"a}ndert. Die Zeitersparnis durch die Verwendung des MCTDH-Ansatzes im Vergleich zur Split-Operator-Methode liegt etwa bei einem Faktor 5. Das letzte Kapitel widmet sich der mikroskopischen Beschreibung von Exziton-Exziton- Annihilierung (EEA). Dabei werden numerische L{\"o}sungen der aus einem mikro- skopischen Modell hergeleiteten Ratengleichungen mit Messungen ( transienter Absorption) verglichen. Es wurden zwei Systeme untersucht: ein Squarain-basiertes Heteropolymer (SQA-SQB)ₙ und ein [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylen]-Polymer, auch bekannt als MEH-PPV. In beiden F{\"a}llen gelang die systematische Parameterbestimmung mit Hilfe einer Aufteilung in lokalisierte Subsysteme. Diese Subsysteme werden einzeln gewichtet und anschließend aufsummiert, wobei die Gewichte optimiert werden k{\"o}nnen. Aus den so erhaltenen Parametern ergibt sich f{\"u}r beide Systeme ein {\"a}hnliches Bild: • Durch ultraschnelle Lokalisierung der Anregung im fs-Bereich auf kleinere Aggregateinheiten bilden sich voneinander getrennte Subsysteme. • Die in den Subsystemen lokalisierten Exzitonen k{\"o}nnen sich nur innerhalb dieser Bereiche frei bewegen. Es ist ausreichend, direkt benachbarte Mono-, Bi-, Tri- und Tetra-Exzitonen in bis zu zwei Dimensionen zu ber{\"u}cksichtigen. • Auf einer fs-Zeitskala annihilieren direkt benachbarte Exzitonen. • Im MEH-PPV ergibt sich der Signalzerfall im fs-Bereich als Mittelwert aus einer schnellen (zwischen Ketten) und einer langsamen (innerhalb von Ketten) Annihilierung. • Im ps- bis ns-Bereich wird sowohl durch Diffusion vermittelte Annihilierung, also auch der Zerfall der ersten angeregten Zust{\"a}nde bedeutsam.},
  subject      = {Quantenmechanik},
  language  = {de}
}
@phdthesis{Walter2015,
  author    = {Walter, Christof},
  title     = {Excitonic States and Optoelectronic Properties of Organic Semiconductors - A Quantum-Chemical Study Focusing on Merocyanines and Perylene-Based Dyes Including the Influence of the Environment},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123494},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {The scope of computational chemistry can be broadened by developing new methods and more efficient algorithms. However, the evaluation of the applicability of the methods for the different fields of chemistry is equally important. In this thesis systems with an unusual and complex electronic structure, such as excitonic states in organic semiconductors, a boron-containing bipolaron and the excited states of pyracene were studied and the applicability of the toolkit of computational chemistry was investigated. Concerning the organic semiconductors the focus was laid on organic solar cells, which are one of the most promising technologies with regard to satisfying the world's need for cheap and environmentally sustainable energy. This is due to the low production and material costs and the possibility of using flexible and transparent devices. However, their efficiency does still not live up to the expectations. Especially the exciton diffusion lengths seem to be significantly too short. In order to arrive at improved modules, a fundamental understanding of the elementary processes occurring in the cell on the molecular and supramolecular level is needed. Computational chemistry can provide insight by separating the different effects and providing models for predictions and prescreenings. In this thesis, the focus was laid on the description of excitonic states in merocyanines and perylene-based dyes taking the influence of the environment into account. At first, the photochemical isomerization between two configurations of 6-nitro BIPS observed experimentally was studied by first benchmarking several functionals against SCS-ADC(2) in the gas phase and subsequently calculating the excited-state potential energy surface. The geometries obtained from a relaxed scan in the ground state as well as from a scan in the excited state were used. The environment was included using different polarizable continuum models. It was shown that the choice of the model and especially the question of the state specificity of the approach is of vital importance. Using the results of the calculations, a two-dimensional potential energy surface could be constructed that could be used to explain the experimental findings. Furthermore, the importance of the excited-state isomerization as a potential deactivation channel in the exciton transport was pointed out. Then the assessment of the suitability of different merocyanines for optoelectronic applications with quantum-chemical methods was discussed. At first, the effect of the environment on the geometry, especially on the bond length alternation pattern, was investigated. It was shown that the environment changes the character of the ground-state wave function of several merocyanines qualitatively, which means that the results of gas-phase calculations are meaningless - at least when a comparison with solution or device data is desired. It was demonstrated that using a polarizable continuum model with an effective epsilon, a qualitative agreement between the calculated geometry and the geometry in the crystal structure can be obtained. Therefore, by comparing the bond length alternation in solution and in the crystal, a rough estimate of the effect of the crystal environment can be made. It was further shown that the connection between the HOMO energy and the open-circuit voltage is not as simple as it is often implied in the literature. It was discussed that it is not clear whether the HOMO of a single molecule or a \$\pi\$-stack containing several monomers should be used and if the environmental charges of the bulk phase or the interface should be included. Investigating the dependence of the HOMO energy on the stack size yielded no definitive trend. Furthermore, it was discussed that the effect due the optimization of the modules (solvent, bulk heterojunction) during the production masks any potential correlation between the HOMO energy and measured open-circuit values. Therefore, a trend can only be expected for unoptimized bilayer cells. It was concluded that ultimately, the importance of the HOMO energy should not be overestimated. The correlation between the exciton reorganization energy and the so-called cyanine limit, which is predicted by a simple two-state model, was also discussed. By referring to the results of VB calculations, it was discussed that the correlation indeed exists and is non-negligible, although the effect is not as strong as one might have expected. In this context, a potential application of a VB/MM approach was covered briefly. The importance of the molecular reorganization energy and the device morphology was also discussed. It was concluded that the optimization of merocyanines for organic optoelectronic devices is inherently a multiparameter problem and one cannot expect to find one particular parameter, which solely controls the efficiency. The perylene-based dyes were studied with a focus on the description of a potential trapping mechanism involving an intermolecular motion in a dimer. The aim was to find methods which can be applied to larger model systems than a dimer and take the effect of the environment into account. As a test coordinate the longitudinal shift of two monomers against each other was used. At first, it was demonstrated how the character of an excited state in a dimer can be defined and how it can be extracted from a standard quantum-chemical calculation. Then several functionals were benchmarked and their applicability or failure was rationalized using the character analysis. Two recipes could be proposed, which were applied to a constraint optimization (only intermolecular degrees of freedom) in the excited states of the PBI dimer and to the description of the potential energy surfaces of ground and excited states along a longitudinal displacement in the perylene tetramer, respectively. It was further demonstrated that the semi-empirical OMx methods fail to give an accurate description of the excited-state potential energy surfaces as well as the ground-state surface along the test coordinate. This failure could be attributed to an underestimation of overlap-dependent terms. Consequently, it could be shown that the methods are applicable to large intermolecular distances, where the overlap is negligible. The results of DFT calculations with differently composed basis sets suggested that adding an additional single p-function for each atom should significantly improve the performance. QM/MM methods are ideally suited to take the effect of the environment on a a dimer model system into account. However, it was shown that standard force fields also give an incorrect description of the interaction between the monomers along the intermolecular coordinate. This failure was attributed to the isotropic atom-atom interaction in the repulsion term of the Lennard-Jones potential. This was corroborated using two simple proof-of-principle anisotropy models. Therefore, a novel force field called OPLS-AA_O was presented that is based on OPLS-AA, but uses an anisotropic model for the repulsion. The model involves the overlap integral between the molecular densities, which are modeled as a sum of atom-centered p-type Gaussian functions. It was shown that using this force field an excellent agreement with the DFT results can be obtained when the correct parameters are used. These parameters, however, are not very generalizable, which was attributed to the simplicity of the model in its current state (using the same exponential parameter for all atoms). As a short excursion, the applicability of an MO-based overlap model was discussed. It was demonstrated that the repulsion term based on the density overlap can be used to correct the failure of the OMx methods for the ground states. This is in accord with the assumption that an underestimation of the overlap terms is responsible for the failure. It was shown that OPLS-AA_O also gives an excellent description of the longitudinal shift in a PBI tetramer. Using the tetramer as a test system and applying the recipe obtained in the TDDFT benchmark for the QM-part and OPLS-AA_O for the MM-part in conjunction with an electrostatic embedding scheme, a QM/MM description of the excited states of the PBI dimer including the effect of the environment could be obtained. In the last chapter the theoretical description of the Bis(borolyl)thiophene dianion and the excited states of pyracene were discussed. The electronic structure of the Bis(borolyl)thiophene dianion - a negative bipolaron - was elucidated using DFT and CASPT2 methods. Furthermore, an estimation of the extent of triplet admixture to the ground state due to spin-orbit coupling was given. In the second project the S1 and S2 states of pyracene were computed using SCS-CC2 and SCS-ADC(2) and an estimation for the balance between aromaticity and ring strain was given. This also involved computing the vibrational frequencies in the excited states. In both studies the results of the computations were able to rationalize and complete experimental results.},
  subject      = {Exziton},
  language  = {en}
}
@phdthesis{Settels2012,
  author    = {Settels, Volker},
  title     = {Quantum chemical description of ultrafast exciton self-trapping in perylene based materials},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-69861},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2012},
  abstract  = {Im Rahmen dieser Dissertation wurden sehr lange Exzitonen-Diffusionsl{\"a}ngen (LD) unter idealen Bedingungen f{\"u}r Perylen-basierte Materialien simuliert. Dies ist ein Indiz daf{\"u}r, dass die sehr kurzen LD in realen Materialien aus einer extrinsischen sowie einer intrinsischen Immobilisierung resultieren. Letztere basiert auf einer Relaxation in sogenannten „Self-Trapping"-Zust{\"a}nde. Ein tieferes Verst{\"a}ndnis der dem Self-Trapping zugrunde liegenden atomistischen Prozesse ist notwendig, um zuk{\"u}nftig Materialien mit langen LD entwickeln zu k{\"o}nnen, bei denen eine intrinsische Exzitonen-Immobilisierung verhindert wird. F{\"u}r die Entwicklung eines solchen mechanistischen Verst{\"a}ndnisses ist das Vorliegen einer eindeutigen Korrelation zwischen der molekularen Anordnung und der LD unabdingbar. Diese weisen Einkristalle von Diindenoperylen (DIP) und α-Perylen-tetracarboxyl-anhydrid (α-PTCDA) auf. Bei ersteren wurde eine außergew{\"o}hnlich lange LD von 90 nm und bei letzteren nur 22 nm gemessen. Teil dieser Arbeit war es, Gr{\"u}nde f{\"u}r diesen Unterschied in der LD zu finden. Nur Self-Trapping kommt als Ursache in Frage. Aus diesem Grund eignen sich diese Materialien, um ein atomistisches Verst{\"a}ndnis des Self-Trappings exemplarisch an ihnen zu erarbeiten. Mutmaßlich k{\"o}nnten Differenzen in der elektronischen Struktur in DIP und α-PTCDA f{\"u}r das unterschiedliche Self-Trapping verantwortlich sein. Allerdings konnte gezeigt werden, dass es f{\"u}r viele Perylen-basierte Materialien keine signifikanten Unterschiede in der elektronischen Struktur gibt, wodurch diese f{\"u}r die Aufkl{\"a}rung von Immobilisierungsmechanismen zu vernachl{\"a}ssigen sind. Eine weitere m{\"o}gliche Begr{\"u}ndung w{\"a}re in Polarisationseffekten im Kristall zu suchen, welche die elektronische Struktur in Perylen-basierten Materialien unterschiedlich beeinflussen. Vor allem ihr Einfluss auf Ladungstrennungs-Zust{\"a}nde (CT), die oberhalb des optisch hellen Frenkel-Zustandes liegen, war fraglich, weil sie energetisch abgesenkt werden k{\"o}nnten. Ein signifikanter Einfluss von Polarisationseffekten konnte aber f{\"u}r alle Zust{\"a}nde mittels eines polarisierbaren Kontinuum-Modells ausgeschlossen werden. Die geringe LD im α-PTCDA ist folglich ein Indiz f{\"u}r ein Self-Trapping, das durch die Kristallstruktur aus π-Stapeln evoziert wird, welche in DIP fischgr{\"a}tenartig ist. Da Polarisationseffekte auszuschließen sind, {\"u}bt der Kristall lediglich durch sterische Restriktionen einen Einfluss auf das Dimer aus. Daher muss die Methode f{\"u}r die Beschreibung von Self-Trapping nur diese Effekte ber{\"u}cksichtigen, so dass sich f{\"u}r den Einsatz des mechanical embedding QM/MM-Ansatzes entschieden wurde. Nun konnten Potentialfl{\"a}chen berechnet werden, auf denen anschließend eine Wellenpaketdynamik durchgef{\"u}hrt wurde. Diese Methode erlaubt es erstmals, Mechanismen der Exzitonen-Immobilisierung in organischen Materialien auf einer atomistischen Ebene zu beschreiben. Als Erkl{\"a}rung f{\"u}r Self-Trapping in α-PTCDA dienten Potentialfl{\"a}chen, die eine intermolekulare Verschiebung des Dimers im Kristall abbilden. So wurde eine Exzitonen-Immobilisierung innerhalb von 500 fs gefunden, die aus einem irreversiblem Energieverlust und einer lokalen Verzerrung der Kristallstruktur resultiert und auf diese Weise den weiteren Transport des Exzitons verhindert. Im Fall von DIP kann diese Immobilisierung aufgrund hoher Energiebarrieren nicht stattfinden. Diese Barrieren resultieren aus der fischgr{\"a}tenartigen Kristallstruktur des DIP. Diese Diskrepanzen in der Dynamik erkl{\"a}ren die unterschiedlichen LD-Werte f{\"u}r DIP und α-PTCDA. In einem weiteren Fall wurde eine Exzitonen-Immobilisierung in helikalen π Aggregaten von Perylen-tetracarboxyl-bisimid (PBI) Molek{\"u}len festgestellt. Hier wird Self-Trapping durch einen Relaxationsmechanismus verursacht, in dem das Exziton durch geringe asymmetrische Schwingungen des Aggregats innerhalb von 200 fs von dem hellen Frenkel- in den dunklen Frenkel-Zustand transferiert wird, wobei dieser {\"U}bergang von einem CT-Zustand vermittelt wird. Der gesamte Vorgang ist nur bei helikalen Aggregaten m{\"o}glich, weil nur hier CT-Zust{\"a}nde sehr dicht bei dem hellen Frenkel-Zustand vorhanden sind. Im finalen Frenkel-Zustand tritt eine Torsionsbewegung um die π-Stapelachse ein, so dass ein Energieverlust und eine lokale {\"A}nderung der Aggregatstruktur erfolgt - also ein Self-Trapping des Exzitons. Dieser modellierte Mechanismus steht im Einklang zu allen vorliegenden experimentellen Daten. Diese Erkenntnisse lassen die Schlussfolgerung zu, dass in k{\"u}nftigen Materialen f{\"u}r organische Solarzellen eine irreversible und ultraschnelle Deformation des Aggregats nach der Photoanregung vermieden werden muss - will man lange LD erreichen. Nur so kann Self-Trapping von Exzitonen verhindert werden.},
  subject      = {Exziton},
  language  = {en}
}
@phdthesis{Maksimenka2010,
  author    = {Maksimenka, Katsiaryna},
  title     = {Absolute Configuration by Circular Dichroism: Quantum Chemical CD Calculations},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56552},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {Quantum chemical calculations of circular dichroism (CD) spectra in combination with experimental CD studies are one of the most efficient analytical tools for the elucidation of the three-dimensional structure of a chiral molecule. In the present work 18 chiral compounds of most different molecular structures and origins were investigated using various theoretical methods (the semiempirical CIS methods, the time-dependent DFT and DFT/MRCI approaches). The advantages and limitations of the applied methods were discussed in the context of the studied compounds. Furthermore, the last part of this work deals with the CD investigations of a chiral compound in the crystalline state. A well-known natural product with a specific conformation/CD spectrum behavior was used as a model compound to examine a novel solid-state CD method and to investigate the possibility of its improvement to provide a higher reliability for the assignment of the absolute configuration.},
  subject      = {Circular-Dichroismus},
  language  = {en}
}
@phdthesis{Liu2011,
  author    = {Liu, Wenlan},
  title     = {Exciton Coupling in Valence and Core Excited Aggregates of pi-Conjugated Molecules},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56169},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2011},
  abstract  = {Im Rahmen dieser Arbeit werden theoretische Modelle zur Beschreibung von Valenz- und Rumpf-angeregten elektronischen Zust{\"a}nden diskutiert. Im Fall der Valenz-Anregungen wurden time-dependend Hartree-Fock (TD-HF) und timedependent Dichtefunktionaltheorie (TD-DFT)Methoden mit verschiedenen Funktionalen f{\"u}r ein Perylenbisimid (PBI) System validiert. Eine einfache Analyse der Charakt{\"a}re der angeregten Zust{\"a}nde wurde vorgeschlagen, die auf den berechneten {\"U}bergangsdipolmomenten basiert. Dieser Ansatz ist allerdings auf Zust{\"a}nde beschr{\"a}nkt, die ein signifikantes {\"U}bergangsdipolmoment aufweisen. Deshalb wurde eine allgemeinere und fundiertere Methode entwickelt, die auf einer Analyse der berechneten CISWellenfunktion basiert. Dar{\"u}berhinaus wurde ein literaturbekannter Model-Hamiltonoperator Ansatz von einem lokalisierten Molek{\"u}lorbitalbild (MO) abgeleitet, das aus der generelleren Analyse-Methode resultiert. Auf diesem Weg ist ein Zugang zu diabatischen angeregten Zust{\"a}nden und korrespondierenden Kopplungsparametern auf der Basis von ab initio Rechnungen gegeben. F{\"u}r rumpfangeregte elektronische Zust{\"a}nde wurden drei Methoden f{\"u}r C 1s-angeregte und ionisierte Zust{\"a}nde verschiedener kleiner Molek{\"u}le validiert. Dar{\"u}berhinaus wurde die Basissatzabh{\"a}ngigkeit dieser Zust{\"a}nde untersucht. Anhand der Resultate wurde die frozen core N{\"a}herung ausgew{\"a}hlt um rumpfangeregte Zust{\"a}nde von Naphthalintetracarbons{\"a}uredianhydrid (NTCDA) zu berechnen. Um experimentelle Ergebnisse zu erkl{\"a}ren, wurde ein Algorithmus entwicklet, der die Exzitonenkopplungsparameter im Fall von nicht-orthogonalen MOs berechnet.},
  subject      = {Exziton},
  language  = {en}
}
@phdthesis{Bahmann2010,
  author    = {Bahmann, Hilke},
  title     = {Implementation, Development and Assessment of Local Hybrid Density Functionals},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-55641},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {In order to describe complex molecular systems theoretically, an efficient and reliable solution to the underlying quantum mechanical equations of motion is required. Density functional theory (DFT) represents in most cases the best compromise between accuracy and efficiency for the treatment of electronic interactions. In Kohn-Sham DFT, the non-classical contribution to electron-electron interactions is gathered in the exchange-correlation functional, which has to be approximated in practice. While a large number of exchange-correlation functionals are of semi-empirical nature, some have been derived from physical considerations exclusively. In so-called global hybrid functionals a constant amount of the integrated DFT exchange-energy density is replaced by the exact-exchange energy from Hartree-Fock theory. The most popular functional, B3LYP, contains 20 \% exact exchange and several empirical parameters. It has been discovered that the optimal amount of exact exchange depends to a large extent on the molecular property to be computed. A possible solution to this problem is to use local hybrid functionals. Therein, the admixture of exact exchange is controlled by a position-dependent local mixing function (LMF), leading to molecule-specific amounts of exact exchange. In this work a semi-empirical approach is pursued for the development of new local hybrid functionals. Parameterized LMFs are introduced in the exchange-energy density integrals, for which the DFT contributions are taken from established approximations to the exchange-correlation functional. The LMFs developed here contain at least one empirical parameter and a variable that depends on the ratio of the von-Weizs{\"a}cker single-particle kinetic energy density to the correlated kinetic-energy density (the so-called t-LMFs), or on the reduced density gradient (referred to as s-LMF). Additional LMFs are obtained by inclusion of the spin polarization. All parameters are fitted to atomization energies and reaction barriers of well-established test sets. Visualization of the LMFs provides an additional tool for analyzing their physical and chemical behavior, potentially leading to further developments. As a general trend, an increasing exact-exchange admixture is observed upon bond stretching for all LMFs, with a more pronounced effect for t-LMFs. This observation correlates with a better performance for reaction barriers of t -LMF-based local hybrid functionals. Most of the local hybrid functionals discussed in this work are based on the exchange and correlation functional from the local spin density approximation (LSDA) and contain therefore no gradient correction such as in the generalized gradient approximation (GGA). The new functionals were initially implemented non-self-consistently into a development version of the quantum chemical Turbomole program package. That is, only the total energy is calculated for a given set of molecular orbitals or electron density, respectively. This is a reliable approximation that allows for significant time savings especially during parameter optimizations. In order to calculate orbital-dependent molecular properties, the local hybrid potential corresponding to the local hybrid energy is required as well. It is obtained as a functional derivative of the exchange-correlation energy with respect to the orbitals. Some of the resulting integrals contain the LMF-weighted non-local exact-exchange potential. These terms as well as the exact-exchange energy density itself cannot be calculated analytically. Following a well-established approach, they have been approximated using a basis set expansion of the exact-exchange potential. For simplicity, the underlying atomic basis set is employed in this resolution of the identity (RI) approximation. For comparison and in view of the optimization of auxiliary basis sets, the optional calculation of the potential by numerical integration has also been implemented in this work. The computational cost of local hybrid calculations for a given basis set, using the RI approximation is comparable to the one of gobal hybrid functionals: a slightly larger prefactor applies to a calculation with a local hybrid functional as compared to a meta-GGA global hybrid, while the scaling of computational effort as a function of system size is the same. Several molecular test sets including atomization energies, barrier heights, dissociation energies and equilibrium distances have been considered for the assessment. Some of them represent particular challenges for current density functional approximations. All of the discussed local hybrid functionals yield significantly better results for the 223 atomization energies of the G3 test set than the B3LYP functional. Especially local hybrid functionals with spin-polarized t -LMFs gives impressively small mean absolute errors for the G3 set. Most of our functionals are in addition significantly superior to B3LYP for the calculation of barrier heights. Some other global hybrid functionals perform even better than our functionals for barriers, but their intrinsic amount of exact exchange is inappropriately high for thermochemical property calculations. For the first time, LSDA-based local hybrid functional have thus been presented that gives accurate results for thermochemistry and reaction barriers simultaneously. The dissociation behavior of symmetric radical cations remains a challenge for the local hybrid functionals presented here. Dissociation energies are significantly overestimated, and the equilibrium distances are too short. The results are overall only slightly better than those obtained using the B3LYP functional. A larger amount of exact exchange is most likely needed for these systems to reduce self-interaction errors. Additionally, the performance of local hybrid functionals for 3d transition metal dimers and monohydrides has been studied. An accurate description of dynamical and nondynamical correlation is essential for the former. The poor performance of most exchange-correlation functionals for transition metal monohydrides can be attributed to self-interaction errors. Our local hybrid functionals perform similarly to B3LYP for the dimers and marginally better for the monohydrides. They do not provide any improvement for the atomic s-d transfer energies of 3d metals. The most suitable local hybrid functional for this particular property uses a s-LMF in the exchange functional and the LYP correlation functional. It yields, however, only average-quality results for thermochemistry and kinetics. Satisfactory results similar to B3LYP are obtained for the isotropic hyperfine coupling constants (HFCCs) of small main group compounds with a t-LMF-based local hybrid functional. The RI approximation to the local hybrid potential has been validated by comparing it to the numerically exact potential for the calculation of total energies, isotropic HFCCs and orbital energies. The error in total energies due to the RI approximation is comparatively small considering the rather large deviations from experimental values. Comparison of mean absolute errors from experimental values of the 26 isotropic HFCCs reveals only small differences between the RI and the numerically exact local hybrid potential. Further analysis shows that inaccuracies in the RI potential may have a larger impact on the isotropic HFCCs or the orbital energies of a particular molecule, especially if only small or medium-sized basis sets are employed. Several of the local hybrid functionals are suitable for the calculation of thermochemical and kinetic properties. Different functionals yield also results similar to other commonly used functionals for isotropic HFCCs of small main group compounds, as well as for the dissociation energies and equilibrium distances of 3d transition metal dimers and monohydrides. The local hybrid functionals studied in this work represent therefore an important step towards the development of universal approximations to the exchange-correlation functional. For a more accurate description of certain transition metal properties and the dissociation behavior of symmetric radical cations while maintaining a good performance for thermochemistry and kinetics, more complex LMFs will have to be considered. Ultimately a local hybrid functional with meta-GGA exchange and correlation energy densities that fulfills more exact constraints is desirable. Therefore further studies on the different gauges of the exchange energy densities are necessary. Another possibility would be the development of a specifically designed correlation functional to be combined with a local hybrid exchange functional based on the LSDA. More detailed studies on the quality of the RI approximation are recommended. Possible properties for this purpose include, e.g., ionization energies and electron affinities. Auxiliary basis sets should be implemented and optimized for the expansion of the exact-exchange potential in order to avoid additional deviations due to the RI-approximation or even fortuitously good results in the assessment of local hybrid functionals with normally contracted basis sets. Since density functional methods are applied extensively for structure optimizations, the gradient of the local hybrid energy with respect to the nuclear coordinates should be implemented to enable this feature in future versions of the code.},
  subject      = {Validierung},
  language  = {en}
}
@phdthesis{Seibt2009,
  author    = {Seibt, Joachim},
  title     = {Theoretical investigations on the spectroscopy of molecular aggregates},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37218},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2009},
  abstract  = {Die spektroskopischen Eigenschaften von Molek{\"u}laggregaten wurden mittels quantendynamischer Berechnungen untersucht. Hierbei wurden sowohl lineare als auch nichtlineare Spektroskopietechniken einbezogen. Zur Simulation von Absorptions- und CD-Spektroskopie wurden Kopplungseffekte sowie die relative Orientierung der Monomer-Einheiten in den Modellen ber{\"u}cksichtigt, um gemessene Spektren reproduzieren und so die entsprechenden Parameter zu bestimmen. Zur genaueren Beschreibung wurden auch Ergebnisse quantenchemischer Rechnungen verwendet. Dar{\"u}ber hinaus wurden Untersuchungen zur nichtlinearen optischen Spektroskopie an Dimeren durchgef{\"u}hrt.},
  subject      = {Theoretische Chemie},
  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{Auer2004,
  author    = {Auer, Dominik},
  title     = {Lithiosilane mit stereogenen Silicium-Zentren : Synthese, Struktur und Reaktionsverhalten},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-11300},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2004},
  abstract  = {Die vorliegende Arbeit liefert einen Beitrag zur Synthese und zum Reaktionsverhalten enantiomerenangereicherter Silyllithium- und Silylmagnesiumverbindungen. Dabei sind mehrere Punkte von Bedeutung: die Aufkl{\"a}rung der Struktur im Festk{\"o}rper wie in L{\"o}sung, die Bestimmung der Selektivit{\"a}ten und des stereochemischen Verlaufs hoch enantiomerenangereicherter polarer Silylmetallverbindungen in Umsetzungen mit Elektrophilen, die konfigurative Stabilit{\"a}t hoch enantiomerenangereicherter Silyllithium- und Silylmagnesiumverbindungen sowie die Aufkl{\"a}rung ungew{\"o}hlicher NMR-Verschiebungen. Im Rahmen dieser Arbeit durchgef{\"u}hrte quantenchemische Berechnungen haben sich vor allem auf die beiden letzten Punkte konzentriert. Insgesamt wurden folgende Themenkomplexe bearbeitet: 1.) Synthese einer hoch enantiomerenangereicherten Silyllithiumverbindung 2.) Reaktionsverhalten der hoch enantiomerenangereicherten Silyllithiumverbindung 2 gegen{\"u}ber Elektrophilen 3.) Konfigurative Stabilit{\"a}t der hoch enantiomerenangereicherten Silyllithiumverbindung 2 4.) Weitere Untersuchungen zur Darstellung von Silyllithiumverbindungen 5.) Untersuchungen zur Struktur von Silyllithiumverbindungen im Festk{\"o}rper und in L{\"o}sung 6.) Quantenchemische Analysen der Ursachen der NMR-Verschiebungen von Disilenen und heteroatom-substituierten Silanen},
  subject      = {Lithiumorganische Verbindungen},
  language  = {de}
}