Abstract
Ab initio molecular orbital theory with the 6‐31G(d), 6‐31G(d,p), 6‐31+G(d), 6‐31+G(d,p), 6‐31+G(2d,p), 6‐311G(d), 6‐311G(d,p), and 6‐311+G(2d,p) basis sets and density functional theory (BLYP, B3LYP, B3P86, B3PW91) have been used to locate transition states involved in the conformational interconversions of 1,4‐dithiacyclohexane (1,4‐dithiane) and to calculate the geometry optimized structures, relative energies, enthalpies, entropies, and free energies of the chair and twist conformers. In the chair and 1,4‐twist conformers the CHax and CHeq bond lengths are equal at each carbon, which suggest an absence of stereoelectronic hyperconjugative interactions involving carbon–hydrogen bonds. The 1,4‐boat transition state structure was 9.53 to 10.5 kcal/mol higher in energy than the chair conformer and 4.75 to 5.82 kcal/mol higher in energy than the 1,4‐twist conformer. Intrinsic reaction coordinate (IRC) calculations showed that the 1,4‐boat transition state structure was the energy maximum in the interconversion of the enantiomers of the 1,4‐twist conformer. The energy difference between the chair conformer and the 1,4‐twist conformer was 4.85 kcal/mol and the chair‐1,4‐twist free energy difference (Δ__G__°~c‐t~) was 4.93 kcal/mol at 298.15 K. Intrinsic reaction coordinate (IRC) calculations connected the transition state between the chair conformer and the 1,4‐twist conformer. This transition state is 11.7 kcal/mol higher in energy than the chair conformer. The effects of basis sets on the 1,4‐dithiane calculations and the relative energies of saturated and unsaturated six‐membered dithianes and dioxanes are also discussed. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 909–919, 2003