Nitrogen and small hydrogen hyperfine couplings are analyzed in the hydrogen adduct of 1-methyluracil, and the 5,6-dihydro-1-methylcytosin-6-yl and -5-yl radicals. The information derived from these small hyperfine couplings is used to eliminate the lactim form of 5,6-dihydro-1-methyluracil-5-yl as a possible free radical assignment for the dominant room-temperature radical in 1-methyluracil and, thereby, unambiguously assign C5 as the site of hydrogen addition. Comparison of nitrogen hyperfine coupling in the 6-yl radical of 1-methyluracil and 1-methylcytosine with other proposed radicals of this type lends further support to the other assignment.

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