Molecular orbital calculations of substituent effects on directly bonded 13C-H coupling constants

The effect of substituents on 13C-H coupling constants is discussed, within the framework of the Pople-Santry MO theory of nuclear spin coupling, by making a theoretical analysis of the contributions due to differences of coulomb and resonance integrals as well as due to the presence of electron lone-pairs. Earlier treatments based on correlations of J CH with s characters are shown to be less justified mainly because the description of CH bonds in terms of localized MO's is not valid, and no physical meaning can be rigorously attributed to concepts such as rehybridization.