ESR of a nitroxide radical and its radical pair in irradiated 1-hydroxyxanthine single crystals

Single crystals of 1- hydroxyxanthine have been X-irradiated at 77 K and studied by ESR at 300 K. A nitroxide radical formed by abstraction of a hydrogen atom at N1 is the primary species formed by the radiation. A nitrogen hyperfine tensor with principal values of 0.8, 2.6, and 13.6 G and a g tensor with principal values of 1.9985, 2.0031, and 2.0099 have been determined. These results indicate a spin density of 0.24 on N1. Pairwise trapping of the nitroxide radical was also detected and principal values of the dipolar coupling tensor were found to be 140.7, -69.4, and -67.8 G. These data indicate effective radical separations of 7.3 A and are consistent with molecular layer spacings of 3.5 A in this crystal structure.