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

摘要

Electron spin resonance absorption of irradiated single crystals of alanines has been observed at 9 kMc/sec.dlhyphen;Alanine andlhyphen;alanine showed the same absorption pattern. When treated with heavy water,lhyphen;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 110deg; to 210deg;K. It was concluded that the reorientation of the methyl group of the radical H3CCdot;HR is quenched at low temperature. The life time tgr; 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 tgr; and temperature. The potentential barrier and the frequency factor forlhyphen;alanine were found to be 3.6plusmn;0.2 kcal/mole and (10plusmn;4)times;1012secmdash;1, respectively. The same values were obtained fordlhyphen;alanine, within experimental error. The direction of each Csngbnd;H bond of the methyl group was determined by analyzing the hyperfine constant for 77deg;K. It was also shown that some atoms in the parent molecule in the crystal shift considerably on irradiation.