A Computational Study of Conformers of 2-Thiaoxacyclohexane (1,2-Oxathiane)

Ab initio molecular orbital theory with the 6-31G(d), 6-31G(2d), 6-31+G(d), 6-31G(d,p), 6-31+G(d,p), and 6-311G(d,p) basis sets and the hybrid density functionals B3LYP, B3P86, and B3PW91 have been used to calculate the optimized geometries and relative energies of the chair, half-chair, sofa, twist, and boat structures of 2-thiaoxacyclohexane (1,2-oxathiane). The values of the energy difference (ΔE, kcal/mol) between the chair and 3,6-twist structures of 1,2-oxathiane were 4.92 (HF), 4.73 (MP2), and 4.66 (DFT). The HF chair–twist energy difference (ΔG c–t o) for 1,2-oxathane was 5.16 kcal/mol. Intrinsic reaction coordinate (IRC) calculations connected a transition state (TS-A) between the chair conformation and the less stable 2,5-twist form and connected two transition states (TS-B, TS-C) between the chair conformation and the more stable 3,6-twist conformer. The DFT energy differences between the chair and TS-A, TS-B, and TS-C were 11.4, 10.8, and 12.6 kcal/mol, respectively. Hyperconjugative stereoelectronic interactions were observed in the chair (n o → $$\sigma _{{\text{C}}6 - {\text{H}}_{{\text{ax}}} }^*$$ and $$\sigma _{{\text{C}} - {\text{H}}_{{\text{ax}}} } $$ → $$\sigma _{{\text{C}} - {\text{H}}_{{\text{ax}}} }^* $$ ) and 3,6-twist (n S → $$\sigma _{{\text{C3}} - {\text{His}}_{{\text{oa}}} }^* $$ and n O → $$\sigma _{{\text{C6}} - {\text{His}}_{{\text{oa}}} }^* $$ ) conformers of 1,2-oxathiane. The chair conformation of 1,2-oxthiane is 9.6 and 10.0 kcal/mol, respectively, less stable than the chair conformations of 3-thiaoxacyclohexane (1,3-oxathiane) and 4-thiaoxacyclohexane (1,4-oxathiane, thioxane).