Valence-Bond Interpretation of Electron-Coupled Nuclear Spin Interactions; Application to Methane

A valence-bond theory, based on the approximate second-order perturbation method of Ramsey, has been developed for the calculation of the electron-coupled nuclear spin interactions in a 1Σ molecule. It has been found possible to derive a general formula which expresses the proton-proton coupling constant in terms of functions that are easily evaluated with the Rumer-Pauling diagrams. The theory is such that, for non-bonded protons, the coupling constant is a sensitive direct measure of deviations from perfect pairing. Application of the theory to methane leads to a numerical value for the coupling constant in agreement with the experimental result. This agreement indicates that contributions of nonperfect-pairing structures to the ground-state wave function of methane are so small that they can have little effect on the energy.