Electron Spin Resonance of a Gamma-Irradiated Single Crystal of 5-Nitro-6-Methyluracil

Free radicals in gamma-irradiated 5-nitro-6-methyluracil have the unpaired electron localized on C(4) and C(6) of the pyrimidine ring. Principal values for the spectroscopic splitting factor are gu = 2.002, gυ = 2.005, and gw = 2.008. Hyperfine interaction between the spin density at C(6) and the methyl protons is nearly axially symmetric with principal values A‖CH3 = 10.7 G and A⊥CH3 = 8.8 G. Hyperfine interaction between the spin density at C(4) and the hydroxyl proton has approximate principal values of A‖OH = 9.5 G and A⊥OH = − 0.5 G. The unpaired spin densities are 0.32 on C(6) and 0.63 on C(4). The localization of spin density on C(4) and C(6) is unusual by comparison to the radicals in most other pyrimidines where the spin density is on C(5).