Ab initio and NBO analysis of the conformational properties of 1,2-oxathiane mono-S-oxide, 1,2-dithiane mono-S-oxide and 1,2-thiaselenane mono-S-oxide

NBO analysis, ab initio molecular orbital (MP2/6-311+G∗∗//MP2/6-311+G∗∗) and hybrid-density functional theory (B3LYP/6-311+G∗∗//MP2/6-311+G∗∗) based methods were used to study the anomeric effects (AE) on the conformational properties of 1,2-oxathiane mono-S-oxide (1), 1,2-dithiane mono-S-oxide (2) and 1,2-thiaselenane mono-S-oxide (3). Both methods showed that the axial conformations of compounds 1-3 are more stable than their equatorial conformations. The MP2/6-311+G∗∗//MP21/6-311+G∗∗ results showed that the Gibbs free energy difference (Gax − Geq) values (ΔGax-eq) between the axial and equatorial conformations of compound 1-3 are 4.08, 2.93 and 2.36 kcal mol−1, respectively. The calculated ΔGax-eq value for compound 2 is in excellent agreement with the reported experimental result (which has been obtained from a variable-temperature NMR experiment). Based on the optimized ground state geometries using the MP2/6-311+G∗∗ level of theory, the NBO analysis of donor-acceptor (bond-antibond) interactions revealed that the AE for compounds 1-3 are −44.57, −3.22 and −2.40 kcal mol−1, respectively. The decrease of the AE could explain the decrease of ΔGax-eq from compound 1 to 3. The correlations between the AE associated with the electronic delocalizations, dipole-dipole interactions, donor and acceptor orbital occupancies, donor and acceptor energies, the pairwise steric exchange energies (PSEE), structural parameters and conformational behavior of compounds 1-3 have been investigated.