Computational study of the conformational interconversion of 5,5-dimethyl-1,2,3-trithiacyclohexane (5,5-dimethyl-1,2,3-trithiane)

Ab initio molecular orbital theory and density functional theory (DFT) with the 6-31G(d), 6-31G(d,p), 6-31+G(d,p), and 6-311+G(d,p) basis sets have been used to study the mechanism of conformational interconversion of 5,5-dimethyl-1,2,3-trithiacyclohexane (5,5-dimethyl-1,2,3-trithiane). The relative energies, enthalpies, entropies, free energies, and structural parameters of the chair, 1,4-twist, and 2,5-twist conformers, the 1,4-boat transition state, and the 2,5-boat transition state were calculated. B3LYP/6-31+G(d,p) calculated the chair conformer of 5,5-dimethyl-1,2,3-trithiane to be 8.38 and 8.72 kcal/mol, respectively, lower in energy than the 1,4-twist and 2,5-twist conformers. The energy differences (DeltaE) between the chair conformer of 5,5-dimethyl-1,2,3-trithiane and the 1,4-boat transition state and the 2,5-boat transition state were 8.95 and 11.46 kcal/mol, respectively. Intrinsic reaction coordinate [IRC, minimum energy path (MEP)l calculations were used to connect the transition state between the chair conformer and 2,5-twist conformer. B3LYP/6-31+G(d,p) calculated energy differences (DeltaE) between the chair conformer and the transition state [TS-1]double dagger connecting the chair and 2,5-twist conformers to be 14.23 kcal/mol. In the chair, 1,4-twist, and 2,5-twist conformers, the C4-Heq bond is slightly longer than the C4-Hax bond. The C5-C8eq bond is longer than the C5-C7ax bond in the chair and 1,4-twist conformers. In the 1,4-twist conformer, the S1-S2 bond is longer than the S3-S3 bond and the S1-C6 bond is longer than the S3-C4 bond. (C) 2003 Wiley Periodicals, Inc.