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

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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.