An ab initio molecular orbital theory and density functional theory study of the conformational free energies of methyltetrahydro-2H-thiopyrans

Abstract

Ab initio Hartree–Fock calculations using the 6-31G(d), 6-31G(2d), 6-31G(d,p), 6-311G(d,p), 6-31+G(d), and 6-311+G(d,p) basis sets, second-order Møller–Plesset perturbation theory (MP2) using the same basis sets, and density functional theory [SVWN/DN ∗, SVWN/DN ∗∗, pBP/DN ∗, pBP/DN ∗∗, BLYP/6–31G(d), B3BLYP/6–31G(d)] were used to calculate the geometry optimized structure of tetrahydro-2 H-thiopyran (tetrahydrothiopyran, thiacyclohexane, thiane) and the conformational enthalpy (Δ H°), entropy (Δ S°), and free energy (Δ G°) of the chair conformers of methylcyclohexane and 2-methyl-, 3-methyl-, and 4-methyltetrahydro-2 H-thiopyran. The DFT methods generally overestimate the conformational free energies (−Δ G°) while some of the MP2 methods give values closer to the experimental results. The MP2/6-311G(d,p) calculated value (− ΔG°=1.46 kcal/mol) for 2-methyltetrahydro-2 H-thiopyran is in excellent agreement with the experimentally reported value and the MP2/6-21G(2d) calculated value (− ΔG°=1.46 kcal/mol) for 3-methyltetrahydro-2 H-thiopyran is also in excellent agreement with the experimentally reported value. The equatorial preference of the methyl group is discussed in terms of the repulsive nonbonded interactions in the equatorial conformer, gauche butane (torsional) interactions in the axial conformer, and repulsive nonbonded interactions of the axial methyl group with the ring carbons and hydrogens.