Conformational behaviors of trans-2,3-bis(methylthio)-1,4-dioxane, -dithiane and –diselenane. A hybrid-DFT study and NBO interpretations

The conformational behaviors of 2,3-bis(methylthio)-1,4-dioxane (1), 2,3-bis(methylthio)- 1,4-dithiane (2) and 2,3-bis(methylthio)-1,4-diselenane (3) have been analyzed by means of hybrid-density functional theory (B3LYP/Def2-TZVPP) based method and NBO interpretation. B3LYP/Def2-TZVPP results showed that the axial conformations of compounds 1-3 are more stable than their equatorial conformations. The calculated Gibbs free energy difference (Geq–Gax) values (i.e. ΔGeq-ax) at 298.15 K and 1 atm between the axial and equatorial conformations decrease from compound 1 to compound 3. The NBO analysis of donor-acceptor (LP→σ*) interactions showed that the anomeric effects (AE) decrease from compound 1 to compound 3. On the other hand, the calculated dipole moment values between the axial and equatorial conformations [Δ(μeq - μax)] increase from compound 1 to compound 2 but decrease from compound 2 to compound 3. However, the variations of the calculated Δ(μeq - μax) values are not in the same trend observed for the corresponding AE and ΔG values. Therefore, the calculated Δμ values do not seem to be sufficient to account for the axial preferences in compounds 1-3. These findings led to the proposal that the AE, due to donor→acceptor hyperconjugation effect, is more significant for the explanation of the axial conformational preferences of compounds 1-3 than the electrostatic effect.