Electron Spin Resonance of Irradiated Single Crystals of Alanines: Hindered Rotation of the Methyl Group in a Free Radical

Abstract

Electron spin resonance absorption of irradiated single crystals of alanines has been observed at 9 kMc/sec. dl-Alanine and l-alanine showed the same absorption pattern. When treated with heavy water, l-alanine gave the same hyperfine structure but showed a considerably sharper linewidth. A notable dependence on temperature of the absorption was observed in the range from 110° to 210°K. It was concluded that the reorientation of the methyl group of the radical H3CĊHR is quenched at low temperature. The life time τ of the reorientation was estimated from the absorption pattern, using a modified Bloch equation. The classical theory of rate processes was applied to analyze the relation between τ and temperature. The potentential barrier and the frequency factor for l-alanine were found to be 3.6±0.2 kcal/mole and (10±4)×1012 sec—1, respectively. The same values were obtained for dl-alanine, within experimental error. The direction of each C–H bond of the methyl group was determined by analyzing the hyperfine constant for 77°K. It was also shown that some atoms in the parent molecule in the crystal shift considerably on irradiation.