Valence-Bond Studies of Contact Nuclear Spin–Spin Coupling. II. Long-Range Pi-Electron Coupling between Protons

A theoretical study of long-range proton coupling is presented in terms of a semiempirical valence-bond formalism which does not invoke the approximation of a “mean excitation energy” in the second-order perturbation sum. Using the spin coupling theory which includes an explicit sum over the triplet-state valence-bond wavefunctions, π-electron contributions to long-range couplings are calculated for a large number of unsaturated molecules in which the protons are separated by four to seven bonds. The calculated results are shown to be in reasonably good agreement with the available experimental data and are of importance in discussing mechanisms of contact nuclear spin–spin coupling.