ESR and ENDOR studies of the 2-methylbenzophenone anion radical: unusual spin density distribution due to a sterical hindrance by methyl substitution

Well-resolved ESR and ENDOR spectra of the 2-methylbenzophenone anion radical were observed at low temperature in DMF. The hyperfine coupling constant values obtained are consistent with each other, and indicate an unusual spin-density distribution in a molecule that is largely different from that of the pristine benzophenone anion radical. The most characteristic is the nonequivalency on the two ortho-positions of the phenyl ring. These findings are also discussed based on a comparison with the case of the 2-methylphenyl phenyl nitroxide, and a lack of a free rotation of the phenyl rings is concluded. (author)