MECHANISTIC STUDY OF THE ELECTROCYCLIC RING-OPENING REACTION OF THIIRANE

The electrocyclic ring opening of the C-C bond in thiirane is investigated in detail within the context of ab initio molecular orbital theory. The conrotatory and disrotatory reaction pathways are studied with three different basis sets ranging from double-zeta plus polarization (DZP) through triple-zeta plus double polarization (TZ2P) in conjunction with self-consistent-field (SCF), two-configuration SCF (TCSCF), and configuration interaction with single and double excitations (CISD) levels of theory. The conrotatory and disrotatory stationary points were located on the potential energy surface and characterized via harmonic vibrational frequency analyses. At all levels of theory considered, the conrotatory stationary point is found to be a genuine transition state. The disrotatory stationary point, located with the two configuration methods, is found to have two imaginary vibrational frequencies. The conrotatory process is shown to be the favored mechanistic route for thiirane ring opening by 13.2 +/- 4.0 kcal mol-1 with the zero point vibrational energy correction, relative to thc disrotatory pathway at the highest level of theory employed (TZ2P TCSCF-CISD+Q). The potential energy hypersurface for thiirane ring opening is compared with the corresponding surface for oxirane.