Abstract
Ab initio molecular orbital theory with the 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), 6-311+G(d,p), 6-31G(2d), 6-311G(2d), and 6-311G(2d,p) basis sets have been used to calculate the conformational enthalpies (Δ H°), entropies (Δ S°), and free energies (Δ G°) of the axial and equatorial conformers of 2-methyl-, 3-methyl-, and 4-methyltetrahydro-2 H-pyran (tetrahydropyran, oxacyclohexane, oxane) and methylcyclohexane (toluene). Although HF and MP2 generally gave higher conformational free energies (Δ G°) than the experimentally reported values, other MP2 calculations gave Δ G° values in excellent agreement with experimental results for methylcyclohexane [6-311G(d,p)] and 3-methyltetrahydro-2 H-pyran [6-31+G(d), 6-311+G(d,p)]. Consistent with solution studies, the MP2 calculations gave larger Δ G° values for 4-methyltetrahydro-2 H-pyran than for methylcyclohexane.
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