Elucidation of the principal hyperfine tensor components for the methyl group interaction in semiquinone anion radicals utilizing powder ENDOR spectroscopy

Abstract

The feasibility of obtaining the principal values of the hyperfine tensor for a rotating methyl group from an organic radical’s powder ENDOR spectrum is illustrated. For both the 2,5-dimethylsemiquinone and the 1,2,4,5 tetramethylsemiquinone anion radicals, the ENDOR methyl group band for the powder sample is characterized by a buildup of intensity at the perpendicular and parallel principal components. The presence of sharp, well resolved bands near the free proton frequency is in striking contrast to the broad matrix band usually observed in this region indicating that more detailed information concerning radical–matrix interactions can be extracted from the powder spectrum.