Computational study of the chair–chair interconversion and stereoelectronic interactions in 1,2,3‐trithiacyclo‐hexane (1,2,3‐trithiane)

Abstract

AbstractAb initio theory, density functional theory (DFT) and Møller–Plesset perturbation theory (MP2) with the 6–31G(d), 6–31++G(d), 6–31G(d,p), 6–31+G(d,p), 6–31++G(d,p), 6–311G(d,p) and 6–311+G(d,p) basis sets were used to study stereoelectronic hyperconjugative interactions and the mechanism of the chair–chair conformational interconversion in 1,2,3‐trithiacyclohexane (1,2,3‐trithiane). The relative energies, enthalpies, entropies, free energies and structural parameters of the chair, 1,4‐twist and 2,5‐twist conformers, a distorted 1,4‐boat transition state and a 2,5‐boat transition state were calculated. The HF calculated energy difference (ΔE) between the chair conformer of 1,2,3‐trithiane and the distorted 1,4‐boat transition state was 10.59 kcal mol−1 (1 kcal=4.184 kJ). The 1,4‐twist conformer and the 2,5‐boat transition state are close in energy, as are the 2,5‐twist conformer and the distorted 1,4‐boat transition state. B3LYP/6–311+G(d,p) calculated the chair conformer of 1,2,3‐trithiane to be 5.83, 10.09, and 5.96 kcal mol−1, respectively, lower in energy than the 1,4‐twist conformer, 2,5‐twist conformer and 2,5‐boat transition state. Intrinsic reaction coordinate (IRC) calculations were used to connect the transition state between the chair conformer and the 1,4‐twist conformer. B3LYP/6–31+G(d,p) and B3LYP/6–311+G(d,p) calculated this transition state to be 14.25 kcal mol−1 higher in energy than the chair conformer. In the chair conformer, the respective C4—H and C6—H bond lengths are equal, but the C5—Heq bond is longer than the C5—Hax bond. In the 1,4‐twist conformer, the C4—Hiso bond lengths are equal, the C5—Hψeq bond is longer than the C5—Hψax bond and the C6—H bond lengths are equal. In the 2,5‐twist conformer, equal C—H bond lengths are found at C4 and at C5, but the C6—Hψeq bond is longer than the C6—Hψax bond. Copyright © 2003 John Wiley & Sons, Ltd.Additional material for this paper is available in Wiley Intersciene