INVESTIGADORES
PROVASI patricio Federico
artículos
Título:
The effect of lone pairs and electronegativity on the indirect nuclear spinspin coupling constants in CH2X (X =CH2 , NH, O, S ): Ab initio calculations using optimized contracted basis sets
Autor/es:
PATRICIO F. PROVASI; GUSTAVO A. AUCAR; STEPHAN P. A. SAUER
Revista:
JOURNAL OF CHEMICAL PHYSICS
Editorial:
American Institute of Physics
Referencias:
Año: 2001 vol. 115 p. 1324 - 1334
ISSN:
0021-9606
Resumen:
The indirect nuclear spinspin coupling constants of C2H4 , CH2NH, CH2O, and CH2S were investigated by means of correlated ab initio calculations at the level of the second order polarization propagator approximation SOPPA and the second order polarization propagatorapproximation with coupled cluster singles and doubles amplitudes-SOPPA(CCSD) using large basis sets, which are optimized for the calculation of coupling constants. It is found that at the self-consistent-field SCF level CH2NH and CH2S exhibit triplet instabilities whereas CH2CH2 and CH2O show triplet quasi-instabilities, which renders the SCF results meaningless. Our best results deviate between 0.3 and 2.7 Hz from the experimental values. We find that although the one-bond CH and YH couplings as well as the two- and three-bond HH couplings are dominated by the Fermi contact term, significant contributions of the orbital paramagnetic and sometimes even spindipolar terms are observed for the one-bond CY and two-bond CH and YH coupling constants.Similarly the changes in the couplings caused by the electronegativity and the lone-pair of Y are mostly due to changes in the Fermi contact all couplingsand the orbital paramagnetic contribution CY and two-bond YH couplings. However, the trend in the changes are neither the same for both terms not for all couplings. In particular, the position of CH2S in the series varies indicating that, either the electronegativity or the lone pairs are the dominating perturbation. Furthermore, small but optimized Gaussian basis sets for the calculation of indirect nuclear spinspin coupling constants are presented. They were obtained by contraction of the s- and p-type basis functions for C, N, O, and S and of the s-type basis functions for H of the large uncontracted basis sets. Molecular orbital coefficients of self-consistent-field calculations on CH4, NH3, H2O, H2S, and H2 with & the uncontracted basis sets were used as contraction coefficients. Applied in the calculation of allcoupling constants in C2H4, CH2NH, CH2O, and CH2S the contraction leads to a maximum basis set error of 0.5 Hz.