AN ABINITIO MO STUDY OF THE PSEUDOROTATION IN CYCLOHEXASULFUR, S-6, AND CYCLOHEPTASULFUR, S-7

An ab initio MO study of different conformations of both cyclohexasulfur and cycloheptasulfur has been carried out using the 3-21G* basis set for sulfur. After the full geometry optimization the electron correlation was taken into account using second-order Moller-Plesset perturbation theory. The chair conformations were, as expected, found to be the most stable of all conformations for both molecules, and the calculated molecular parameters are in good agreement with the experimental ones. In S6 the inversion from the chair conformation to the boat conformation can be thought to proceed either by direct inversion or by pseudorotation about any of the six SS bonds. While the energy difference between the chair and boat conformations is only 63.3 kJ mol-1, the barrier to the inversion in both cases was found to be very high (132-137 kJ mol-1), thus explaining why the chair conformation of S6 is very rigid. In contrast to this behaviour, S7 is fluxional. The calculated barriers to pseudorotation (ca. 6 kJ mol-1 between two chair conformations via an intermediate of symmetry C2, and ca. 18 kJ mol-1 between the chair and boat conformations) agree well with the predicted barriers to pseudorotation of the molecule, and imply the coexistence of both the chair, twist and boat conformations in solution or in the liquid or gaseous state.