Cycloalkylmethyl radicals. Part 4.¹ electron spin resonance study of conformational equilibria in cyclohexenylmethyl and 4-alkylcyclohexenylmethyl radicals

For cyclohex-2-enylmethyl and 4-alkylcyclohex-2-enylmethyl radicals the quasi-axial and the quasiequatorial conformers can both be observed by e.s.r. spectroscopy. Similarly, the axial and equatorial conformers of cyclohex-3-enylmethyl radical can be distinguished by e.s.r. spectroscopy. The conformational free-energy difference of the CH₂’ group in the 2-position, -∆G°₃₀₀., was found to be 0.17 ± 0.03 kcal mol^-1 and in the 3-position -∆G°₃₀₀ = 0.0 ± 0.1 kcal mol^-1. The Arrhenius parameters for inversion of the half-chair conformation of cyclohex-2-enylmethyl radical were determined by line shape analysis of the exchange-broadened spectra and found to be: log(/k_f/s^-1) = 12.3 - (5.7 kcal mol^_1)/2.3RT and log(/k_b/s^-1) = 12.0 - (5.5 kcal mol^-1)/2.3RT. The barrier to rotation about the C_α-C_β bond in a cyclohexenylmethyl radical is much less than the barrier in a cyclohexylmethyl radical because the former radical has only one syn-axial hydrogen on C(5) to impede the rotation whereas the latter radical has two syn-axial hydrogens on C(3) and C(5).