Infrared intensities as a quantitative measure of intramolecular interactions. Part XXXV. Conjugation in para-substituted benzenes and its relation to strain energies and σ values

Abstract

ΔG^‡ Values for the rotational barriers of a variety of para-disubstituted benzenes are determined by the n.m.r. coalescence method: for two compounds ΔS^‡ and ΔH^‡ are found by complete line shape matching. These and other available barriers are compared with barriers determined by the i.r. intensity method. The work supports the previous conclusion that σ values can be related directly to the energy scale and points to a method for predicting substituent–substituent interactions in such compounds from i.r. derived parameters.