Does π-σ-π through-bond coupling significantly increase C - C bond lengths?

Abstract

Orbital effects ascribed to π-σ-π through-bond coupling are the result primarily of destabilizing filled-filled interactions; little to no mixing occurs between the antisymmetric π combination and σ*. The magnitude of through-bond coupling does not correlate with bond lengthening. Previous conclusions that through-bond coupling induces long bonds relied in some cases on poor X-ray data and/or inappropriate molecular orbital theories to deduce geometries. Comparison of semiempirical and empirical force field data as a test for through-bond coupling is unwarranted. Standard force fields underestimate the steric bulk of the benzene face, but addition of so- called 'through-bond coupling' terms does not provide a physically significant measure of through-bond coupling.