Electron Correlation and the Nuclear Spin—Spin Coupling Constant. I. An Alternant Molecular Orbital Description of Nuclear Spin—Spin Coupling

Abstract

A theoretical description of contact nuclear spin—spin coupling is presented in terms of the density‐matrix formalism and the approximation of an ``average excitation energy''. For a molecular system in a singlet state the density of spin coupling is proportional to the difference between the correlation functions for electrons of the same and opposite spins. Application of the one‐parameter alternant molecular orbital (AMO) method, which includes both types of correlation in an easily visualizable manner, yields expressions for the density of spin coupling in alternant systems of 2n electrons. In the case of directly bonded systems of two electrons, correlation between electrons of antiparallel spins make a significant contribution to the coupling constant. Coupling constants between nuclei separated by two and three bonds are examined by the AMO expressions in the tight‐binding approximation. In contrast to the single‐determinant results, a long‐range spin correlation is implicit in the AMO description and depending on whether the nuclei are in the same or different subsets of the alternant system, this term alternates in sign.