Electron paramagnetic resonance study of conformational effects in alkyl-substituted 2-cyclohexanonyl radicals in an adamantane matrix

Electron paramagnetic resonance spectra have been obtained for radicals produced by x-irradiation of cyclohexanone and various alkyl-substituted cyclohexanones trapped in an adamantane matrix. Temperature variation of these spectra permits determination of the enthalpy and entropy of activation for interconversion between the two half-chair conformations. In those cases where the two conformations have intrinsically different energies, the enthalpy and entropy differences between conformations are determined. For 2-cyclohexanonyl radical, the enthalpy of activation is 3.90 +- 0.07 kcal/mole and the entropy of activation is -2.3 +- 0.3 e.u. Methyl substitution on C3 or C5 gives a radical with activation parameters similar to the parent radical, indicating moderate realignment of atoms during the conformational change. Methyl substitution on C4 gives a radical with lower activation parameters, which are interpreted to indicate conformational change mainly be a folding along the diagonal through the radical site. Larger groups attached to C3 influence enthalpy and entropy differences between conformations much less than when they are attached to C5. Very large groups attached to C5 apparently flatten the ring; it is not known whether or not this is a matrix effect. Deuteration seems to cause a slight reduction in the activation parameters for 2-cyclohexanonyl radical