DOES PI-SIGMA-PI THROUGH-BOND COUPLING SIGNIFICANTLY INCREASE C-C BOND LENGTHS

Orbital effects ascribed to pi-sigma-pi through-bond coupling are the result primarily of destabilizing filled-filled interactions; little t o no mixing occurs between the antisymmetric pi combination and sigma . The magnitude of through-bond coupling does not correlate with bond lengthening. Previous conclusions that through-bond coupling induces l ong bonds relied in some cases on poor X-ray data and/or inappropriate molecular orbital theories to deduce geometries. Comparison of semiem pirical and empirical force field data as a test for through-bond coup ling is unwarranted. Standard force fields underestimate the steric bu lk of the benzene face, but addition of so-called ''through-bond coupl ing'' terms does not provide a physically significant measure of throu gh-bond coupling.