Conformational Studies in the Cyclohexane Series. 2. Phenylcyclohexane and 1-Methyl-1-phenylcyclohexane

The structures and relative energies of the conformers of phenylcyclohexane, and 1-methyl-1-phenylcyclohexane have been calculated at theoretical levels including HF/6-31G*, B3LYP/6-311G*, MP2/6-311G*, MP2/6−311(2df,p), QCISD/6-311G*, and QCISD/6-311G(2df,p). The latter gives conformational enthalpy (ΔH°), entropy (ΔS°), and free energy (ΔG°) values for phenylcyclohexane that are in excellent agreement with the experimental data. The calculations for 1-methyl-1-phenylcyclohexane find a free energy difference of 1.0 kcal/mol at −100 °C, favoring the conformation having an axial phenyl group, that is in only modest agreement with the experimental value of 0.32 ± 0.04 kcal/mol. The origin of the phenyl rotational profiles for the conformers of phenylcyclohexane and 1-methyl-1-phenylcyclohexane is discussed.