Electron Spin Resonance of X-Irradiated Single Crystals of Zinc Acetate Dihydrate: Hindered Rotation of Methylene Group in a Free Radical CH2COO−

Electron spin resonance spectra of X-irradiated single crystals of zinc acetate dihydrate have been studied at 9.2 Gc in the temperature range from 77°K to 380°K. The radicals trapped at 77°K are CH3 and CH2COO−. The former disappears above 150°K. The latter is stable and persists up to 370°K. The principal values of hyperfine coupling constants for two indetical hydrogens of the radical CH2COO− have been found to be 10.5, 20.6 and 33.4G. The CH2 group in the radical undergoes restricted rotation about the C–C bond. Analysis of the ESR spectra at different temperatures with a modified Bloch equation shows the activation energy and the frequency factor for the restricted rotation to be 5.8±0.2 kcal/mol and (4.4±2.7)×1012 sec−1, respectively.