Molecular orbital theory of nuclear spin coupling constants

Abstract

Molecular orbital theory in the LCAO form is used in a study of the indirect coupling between nuclear spins through the electronic environment. By retaining only the largest one-centre integrals, approximate formulae involving the LCAO coefficients are derived for the three contributions due to (1) the electron orbital effect, (2) the dipolar interaction between nuclear and electron spins and (3) the Fermi contact effect. The theory is then applied in detail to the coupling between directly bonded atoms, when the contact term is usually dominant. Approximate calculations indicate that the reduced coupling constant K AB (defined as (2πtħγAγB)J AB, where γ A and γB are the nuclear magnetogyric ratios and J AB is the usual constant in cycles/second) is negative if one of the atoms is fluorine. Broad agreement is obtained with the available experimental data for atoms up to fluorine connected by single bonds and a tentative pattern for signs and magnitudes is suggested.