The predominant free radical trapped in single crystals of a hydrogen-bonded complex of 1-methylcytosine and 5-fluorouracil x-irradiated between 77°K and room temperature has been identified. It is characterized by hydrogen atom extraction from N(1) of the 5-fluorouracil moiety, the unpaired electron interacting with the N(1) and F nuclei of this molecule. The principal values of the hyperfine and g-tensors are:$[A_{x}{}^{{\rm N}}=14.4,A_{y}{}^{{\rm N}}=A_{z}{}^{{\rm N}}=0.0];[A_{x}{}^{{\rm F}}=156.4,A_{y}{}^{{\rm F}}=16.4,A_{z}{}^{{\rm F}}=16.1\ {\rm G}];[g_{u}=2.00114,g_{v}=2.00564,g_{w}=2.00620]$. The radical is present at 77°K and decays upon warming to room temperature. There is at least one additional radical present at 77°K, but its structure has not been determined.

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