CONFORMATIONAL FLEXIBILITY OF ANTIAROMATIC 1,4-HETEROCYCLIC-ANALOGS OF 1,4-CYCLOHEXADIENE

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 an more flexible than other 1,4-dihydrocycles bec ause of the anti-aromatic character of cyclic the pi-system. An increa se in bending strain results in a change in the shape of the energy mi nimum from flat single to double. (C) 1998 Elsevier Science B.V.