Complete basis set, hybrid-DFT study and NBO interpretations of conformational behaviors of trans-2,3- and trans-2,5-dihalo-1,4-dithianes

Abstract

The conformational behaviors of trans-2,3-dihalo-1,4-dithiane [halo = F (1), Cl (2), Br (3)] and trans-2,5-dihalo-1,4-dithiane [halo = F (4), Cl (5), Br (6)] have been analyzed by means of complete basis set CBS-4, hybrid-density functional theory (B3LYP/6-311 + G**//B3LYP/6-311 + G**) based methods, and natural bond orbital (NBO) interpretation. Both methods showed that the axial conformations of compounds 1–5 are more stable than their equatorial conformations but CBS-4 resulted in an equatorial preference for compound 6. The 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 2 but increase from compound 2 to compound 3. Also, the calculated ΔGeq–ax values decrease from compound 4 to compound 6. The NBO analysis of donor–acceptor (LP σ*) interactions showed that the anomeric effect (AE) increase from compound 1 to compound 3 and also from compound 4 to compound 6. On the other hand, the calculated dipole moment values between the axial and equatorial conformations [Δ(µeq−µax)] decrease from compound 1 to compound 3. The conflict between the increase of AE and the decrease of Δ(µeq−µax) values could explain the variation of the calculated ΔGeq–ax for compounds 1–3. The Gibbs free energy difference values between the axial and equatorial conformations (i.e., ΔGax–ax and ΔGeq–eq) of compounds 1 and 4, 2 and 5 and also 3 and 6 have been calculated. The correlations between the AE, bond orders, pairwise steric exchange energies (PSEE), ΔGeq–ax, ΔGax–ax, ΔGeq–eq, dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–6 have been investigated. Copyright © 2010 John Wiley & Sons, Ltd.