TY  - JOUR
A1  - Reuter, W.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
T1  - The Reaction of Singlet and Triplet Methylen with Ethene - A Multi-Reference CI Study
N2  - 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.
KW  - Organische Chemie
Y1  - 1992
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58923
ER  - 
TY  - JOUR
A1  - Peric, M.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
T1  - 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
N2  - 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.
KW  - Organische Chemie
Y1  - 1991
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58901
ER  - 
TY  - JOUR
A1  - Peric, M.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
T1  - Ab initio investigation of the vibronic structure of the C\(_2\)H spectrum Computation of the vibronically-averaged values for the Hyperfine Coupling Constants
N2  - 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.
KW  - Organische Chemie
Y1  - 1991
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58915
ER  - 
TY  - JOUR
A1  - Metropoulos, B.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
T1  - On the chemi-ionization reaction O + CH ----> HCO\(^+\)+  e\(^-\). Coollinear O-CH Approach
N2  - 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.
KW  - Organische Chemie
Y1  - 1993
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58963
ER  - 
TY  - JOUR
A1  - Karna, S.P.
A1  - Grein, F.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
T1  - Ab initio configuration-interaction studies of the ground state potential energy and hyperfine coupling constants of \(^{35}\)Cl\(_2^-\)
N2  - 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;.
KW  - Organische Chemie
Y1  - 1990
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58869
ER  - 
TY  - JOUR
A1  - Karna, S.
A1  - Grein, F.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
T1  - The hyperfine coupling constants of \(^{19}\)F\(_2\)
N2  - 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\(_{dip}\)·
KW  - Organische Chemie
Y1  - 1989
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58817
ER  - 
TY  - JOUR
A1  - Funken, K.
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
A1  - Grein, F.
T1  - Study of the hyperfine coupling constants of the moleculs NH<sub>2</sub>, NHD and ND<sub>2</sub>
N2  - 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.
KW  - Organische Chemie
Y1  - 1990
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58875
ER  - 
TY  - JOUR
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
A1  - Skell, P.S.
T1  - Theoretical study of the potential energy surface governing the stereochemistry in ClC\(_2\)H\(_4\) reactions
N2  - 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.
KW  - Organische Chemie
Y1  - 1990
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58851
ER  - 
TY  - JOUR
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
A1  - Karna, S.P.
A1  - Grein, F.
T1  - The hyperfine coupling constants of the five lowest states of CH : An ab initio MRD-CI study
N2  - No abstract available
KW  - Organische Chemie
Y1  - 1988
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58807
ER  - 
TY  - JOUR
A1  - Engels, Bernd
A1  - Peyerimhoff, S.D.
A1  - Davidson, E.R.
T1  - Calculation of hyperfine coupling constants : An ab initio MRD-CI study for nitrogen to analyse the effects of the basis sets and CI parameter
N2  - 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.
KW  - Organische Chemie
Y1  - 1987
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-58784
ER  -