Valence-Bond Calculation of Geminal Spin-Spin Coupling Constants in Substituted Methanes

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Abstract
A theoretical study is presented of geminal proton spin‐spin coupling constants for a class of substituted methanes. Using a valence‐bond approach and a six electron model, consisting of a methylene group with an adjacent π electron pair, the hyperconjugation energy and the contact contribution to the geminal coupling constant are calculated. These data, which are in agreement with the experimental findings, indicate that hyperconjugative effects give larger coupling constants than would be expected on the basis of the H–C–H angular dependence alone. This is shown to arise out of delocalized structures, which in the ground state correlate the spins of the geminal protons. It is noted that the geminal coupling constant depends on the orientation of the methylene group with respect to the adjacent π electron system, and this data is reported for two values of the C–C internuclear distance.

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