Cycloalkylmethyl Radicals. 2. Axial and Equatorial Cyclohexylmethyl and (4-Alkylcyclohexyl)methyl Radicals. First Determination of the Conformational Free Energy Difference of the CH₂• Group

At 140 K the EPR spectra of cyclohexylmethyl and (4-alkylcyclohexyl)methyl radicals in which the CH₂• group adopts the axial conformation have a^Hβ ~ 42–43 G, while those in which the CH₂. group adopts the equatorial conformation have a^Hβ= 30–31 G. Both conformers can be observed over a range of temperatures for cyclohexylmethyl and for (cis-4-methylcyclohexyl)methyl. For the former radical, the relative conformer concentrations under conditions where ring inversion is rapid relative to radical lifetime can be described by log ([axial]/[equatorial]) = 0.79 - 1.79/θ (where θ= 2.3RTkcal/mol). At 300 K, therefore, the conformational free energy difference of the CH₂• group in the axial and equatorial positions is -ΔG° = 0.71 kcal/mol. The greater a^Hβ value for the axial conformers is attributed to a higher barrier to rotation about the C_α-C_β bond.