@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{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{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{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{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{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{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{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{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{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{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{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{MuehlhaeuserEngelsMarianetal.1994,
  author    = {M{\"u}hlh{\"a}user, M. and Engels, Bernd and Marian, C. M. and Peyerimhoff, S. D. and Bruna, P. J. and Jansen, M.},
  title     = {Einfluß der Ladungsverteilung auf die Bindungsl{\"a}ngen im P\(_4\)O\(_6\) Ger{\"u}st bei Verbindungen des Typs P\(_4\)O\(_6\)X},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-59034},
  year      = {1994},
  abstract  = {No abstract available},
  subject      = {Organische Chemie},
  language  = {de}
}
@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{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{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{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{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}
}