Heterocyclic analogues of cyclohexene: theoretical studies of the molecular structures and ring-inversion processes

The equilibrium geometry, ring-inversion barrier, and pathway for heterocyclic analogues of cyclohexene have been studied using the MP2/6-311G(d,p) level of theory. It is concluded that the replacement of one methylene group in cyclohexene by heteroatom results in significant changes in the character of the potential-energy surface in comparison with cyclohexene. The equilibrium conformation of ring strongly depends on the position of the heteroatom due to the existence of the n-pi conjugation. However, the character of the ring-inversion process is determined by the nature of the heteroatom. In the case of sulfur- and selenium-containing rings, the boat or twist-boat conformation corresponds to an additional minimum on the potential-energy surface. Moreover, the barriers of the conformational transition from this conformer to two different half-chair forms are significantly different. Nitrogen-containing heterocycles possess two pairs of minima corresponding to the different configurations of the nitrogen atom. However, the transition between the two minima with the same configuration of the heteroatom proceeds only in two steps that include ring inversion and nitrogen inversion.