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

Abstract

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 ( G ax − G eq) values (Δ G ax–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 Δ G ax–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 Δ G ax–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.