Valence-Bond Studies of Contact Nuclear Spin–Spin Coupling. I. A Truncated Matrix Sum Method

Abstract

A theoretical description of contact nuclear spin–spin coupling is presented in terms of a valence‐bond formalism which does not invoke the approximation of a “mean excitation energy” in the second‐order perturbation sum. By explicitly including a sum over the valence‐bond triplet‐state wavefunctions, a general expression for the contact nuclear spin coupling constant is obtained in a theory which uses a bond diagram formalism. The relationship of this extended theory to existing valence‐bond descriptions is investigated. As examples of the applicability of the method, detailed results are given for the contributions of the individual triplet states to the long‐range proton coupling constants in eight electron fragments of the butadiene and allene molecules.