NUCLEAR-SPIN SPIN COUPLING AND NUCLEAR MOTION

The vibration and rotation of molecules affects nuclear spin-spin coup ling constants. This manifests itself as a temperature dependence of t he coupling and also as an isotope effect (after allowing, where neces sary, for differing magnetogyric ratios of the two nuclei involved in the isotopic substitution). Within the Born-Oppenheimer approximation, a nuclear spin-spin coupling surface can be defined for each pair of coupled nuclei. This surface is sampled by the nuclei as they undergo the excursions about equilibrium geometry that are governed by the for ce field. An accurate ab initio carbon-proton spin-spin coupling surfa ce for the methane molecule has been calculated. This was obtained by summing the surfaces for each of the four contributions-Fermi contact, spin-dipolar, orbital paramagnetic, and orbital diamagnetic-expressed as power series in terms of symmetry coordinates. Preliminary calcula tions for (CH4)-C-13 and (CD4)-C-13 give a difference of only 6% betwe en the calculated and observed nuclear motion contributions. The obser ved temperature dependence is also accounted for by the calculations. For these isotopomers, bond stretching plays the dominant role. (C) 19 94 John Wiley & Sons, Inc.