ELECTRON-SPIN RESONANCE DETERMINATION OF HINDERED ROTATION IN BENZYL RADICALS - RESONANCE STABILIZATION ENERGY

ESR spectra of four substituted benzyl radicals (1-4) have been obtained in the 100-237°C temperature interval. Alternating line width effects appear and are shown to arise from hindered rotation about the C1-C7 partial double bond. The experimental line shapes are analyzed to yield values of the mean rate ω of interconversion between the two conformers. These interconversion rates fit well to the Arrhenius expression ω = k exp(-E/RT): log k = 13.2 ± 0.7, E = 13.9 ± 1.0 kcal/mol for 1; log k = 13.3 ± 0.8, E = 13.4 ± 1.0 kcal/mol for 2; and log k = 12.9 ± 1.2, E = 12.0 ± 2.0 kcal/mol for 3. Because the transition state of this interconversion reaction corresponds to zero overlap of the C1 and C7 π orbitals, the activation energies reported here are measures of the resonance stabilization in these benzyl radicals. The reported values are compared with a previous theoretical calculation as well as the simple Hückel model. © 1979 American Chemical Society.