CALCULATION OF NUCLEAR SPIN-SPIN COUPLINGS .8. VICINAL PROTON-PROTON COUPLING-CONSTANTS IN ETHANE

Ab initio self-consistent-field (SCF) and electron correlation calculations have been carried out for the dihedral angle dependence of the vicinal proton-proton coupling constants, (3)J(HH), in ethane molecule. The four contributions to (3)J(HH), (J(FC), J(SD), J(OP), and J(OD)) have been computed with the three different basis sets, [5s2p1d/2s1p], [5s3p1d/3s1p], and [7s4p2d/5s2p]. The Fermi contact (FC) contribution was largest and the spin-dipole (SD) contribution was smallest. The FC and orbital paramagnetic (OP) contributions showed large basis set dependence, but the SD and orbital diamagnetic (OD) contributions presented little basis set dependence. The calculated total SCF contribution to (3)J(HH) was higher than the experimental coupling. Using the Moller-Plesset perturbation theory we have introduced electron correlation effects on the FC and OP terms. The correlation effects on the OP term was shown to be negligible. The second-order correlation in the FC term was very large and amounted to half of its SCF value in magnitude with opposite sign. However, the third-order correlation in the FC contribution was small. Unfortunately, the calculated (3)J(HH) value including correlation corrections through third order was too small compared to the experimental one. The poor agreement between calculation and experiment is claimed to be due to higher than third-order correlations in the FC term. (C) 1995 American Institute of Physics.