Conformational flexibility of antiaromatic 1,4 heterocyclic analogues of 1,4-cyclohexadiene

Abstract

The molecular structure and conformational flexibility of antiaromatic six-membered 1,4-diheterocycles have been studied using an ab initio quantum-chemical method with a 6-31G basis set. Molecules without the sulfur atom have a planar equilibrium conformation. The bending strain in the presence of S atoms causes a transition to boat conformation. Generally, all rings are more flexible than other 1,4-dihydrocycles because of the anti-aromatic character of cyclic the π-system. An increase in bending strain results in a change in the shape of the energy minimum from flat single to double.