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

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 conformation s but CBS‐4 resulted in an equatorial preference for compound 6 . The Gibbs free energy difference ( G eq − G ax ) values (i.e., Δ G eq–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 Δ G eq–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( µ eq − µ ax )] decrease from compound 1 to compound 3 . The conflict between the increase of AE and the decrease of Δ( µ eq − µ ax ) values could explain the variation of the calculated Δ G eq–ax for compounds 1–3 . The Gibbs free energy difference values between the axial and equatorial conformations (i.e., Δ G ax–ax and Δ G eq–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), Δ G eq–ax , Δ G ax–ax , Δ G eq–eq , dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–6 have been investigated. Copyright © 2010 John Wiley & Sons, Ltd.