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

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Abstract
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.

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