The imbalanced kekule structure contributions in the ring in 1,3,5-trisubstituted benzene derivatives: Low temperature X-ray study on 1,3,5-trimethoxybenzene, and ab initio calculations on 1,3,5-triformylo - benzene and trimethoxy-benzene and the related di-substituted systems

Abstract

A low temperature X-ray structural determination for 1,3,5-trimethoxybenzene reveals π-electron localisation in the ring. ab initio calculations for 1,3,5-trimethoxybenzene and 1,3,5-triformylobenzene show that these nonsymmetrical substituents induce opposite imbalance of Kekule structures in the ring. An empirical rule is proposed - that cis CC-bonds in the ring show increased double bond character (compared to the trans bond) when the substituent is of X-Y kind (with a single XY bond), and less double bond character if the substituent is of the X=Y type (with a double XY bond). Additional calculations for cis and trans conformers of 1,4-dimethoxybenzene differ dramatically in the degree of π-electron localisation, supporting the idea that the dominating factor of these distortion is a through-space effect.