Concerning the Electronic Control of Torsion Angles in Biphenyls

Abstract

AbstractIn the search for evidence to establish the effect of π conjugation on the variation of the equilibrium torsion angle (θeq) in push–pull 4,4′‐disubstituted biphenyls, a systematic Hammett DFT study has been carried out. The study shows that although reasonable and intuitive, the through‐conjugation between donor and acceptor substituents in these systems is only of some relevance when strong donor and strong charged acceptors are involved. In these cases, changes in the torsion angle from 39 (average) to 10.2° have been observed in the gas phase. Otherwise, θeq is mainly dominated by the (steric) ortho–ortho effect. The results of this study, based on the electronic control of the torsion angle, are complementary to those that rely on planarization by means of reduction of the steric constriction at the 2,2′‐positions of the system.