| Abstract
| - Full geometry optimizations were carried out at the HF/6-31G** and B3LYP/6-31G** levels formethylcyclohexane, 2-, 3-, and 4-methyltetrahydropyran, 2-, 3-, and 4-methylpiperidine, 2-, 3-, and4-methylthiane, 2-, 4-, and 5-methyl-1,3-dioxane, and 2-, 4-, and 5-methyl-1,3-dithiane and alsofor S-methyl thianium. Constrained geometry optimizations were carried out for methylcyclohexane,2-methyl-1,3-dioxane, and the axial conformers of 2- and 3-methyltetrahydropyran and 2- and3-methylpiperidine. The steric repulsion model, which is believed to account for the conformationalenergies of the cited compounds, was tested by stretching bonds and bending angles so that theaxial methyl group is either forced to approach the ring γ methylenes or get farther away fromthem. The calculated energies show that the energy costs of these perturbations are not dependenton the distances between the axial methyl group and the ring γ methylenes and are not dependenton whether the methyl is axial or equatorial. It is shown that, besides the steric repulsion model,the conformational energies of the compounds studied are dictated by hyperconjugative interactionsinvolving mainly the methine hydrogen. The C−C bond lengths of the axial and equatorialconformers of methylcyclohexane are shown to be related to hyperconjugation.
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