For cyclohexylmethyl and 4-alkylcyclohexylmethyl radicals the conformer in which the CH2* group adopts the axial position and that in which the CH2* group adopts the equatorial position can both be observed by e.s.r. spectroscopy. At 140 K the axial conformers have a(H β) ca. 42-43 G; the equatorial conformers have a(H β) ca. 30-31 G. For cis-4-methylcyclohexylmethyl radicals the ratio of the concentrations of the two conformers was studied as a function of temperature and shown to depend on the rate of radical ring inversion vs. the radical lifetime; the rate constant for ring inversion was obtained. As a check on the e.s.r. results the conformational equilibrium of cis-4- methylcyclohexylmethyl bromide was studied by 1H n.m.r. spectroscopy, which gave -ΔG300°(CH2Br) = 1.91 kcal mol -1. The relative conformer concentrations were also measured as a function of temperature for cyclohexylmethyl radicals and the conformational free energy difference of the CH2* group (-ΔG 300°) was found to be 0.71 kcal mol-1. The preponderance of the conformer of the cis-4-methylcyclohexylmethyl radical with the CH3 group axial at T < ca. 175 K was attributed to the fact that the axial non-rotating CH3 group can adopt a staggered, minimum-energy conformation, whereas the axial non-rotating CH 2* group cannot because of its planarity. The barriers to rotation about the Cα-Cβ bonds in the axial radicals were found to be ca. 1.0 kcal mol-1 greater than those of the equatorial radicals; this is responsible for the greater a(H β) values of the axial radicals. The axial and equatorial conformers of cyclohexylmethyl radicals were investigated by semi-empirical SCF MO methods.
Journal of the Chemical Society, Perkin Transactions 2, no. 8 (1986): 1337–1344.