Single crystals of 5-nitro-6-methyluracil have been irradiated with 4.0-MeV electrons and studied at temperatures between 77 K and room temperature by the electron spin resonance technique. Four radicals are formed altogether and three of these have been identified. In the present work an intermediate radical, stabilized in the temperature region 110-250 K is found and its esr parameters determined. The radical is presumably formed from the primary radical by a double-proton tunneling process. MO calculations carried out at the INDO level of approximation supported the interpretations. The primary radical, formed at 77 K, was found to disappear via two independent reactions when the crystals were warmed to temperatures above 110 K. Both reactions were studied in detail. A previous identification of the room-temperature radical by Snipes and Benson was supported by some recent MO calculations which also showed that the NO2 group is slightly twisted out of the ring plane.

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