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.

A Low temperature X-ra! structural determination for 1,3,5-trimethoxybenzene reveals pi-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 ns and trans conformers of 1.4-dimethoxybenzene differ dramatically in the degree of pi-electron localisation. supporting the idea that the dominating factor of these distortion is a through-space effect Copyright (C) 1996 Published by Elsevier Science Ltd