Irradiation of 9-methyladenine at room temperature produces two stable free radicals, one by a hydrogen addition reaction at either C-2 or C-8 of the purine ring, and one by a hydrogen abstraction reaction from the methyl group. In this paper the hydrogen abstraction radical is analyzed in detail by using electron spin resonance (ESR) spectroscopy. Hydrogen hyperfine interactions with the two protons have principal values, in gauss, of$A_{x}{}^{{\rm H}(1)}=10.5,A_{y}{}^{{\rm H}(1)}=30.0,A_{z}{}^{{\rm H}(1)}=18.5,A_{x}{}^{{\rm H}(2)}=8.5,A_{y}{}^{{\rm H}(2)}=32.5$, and$A_{z}{}^{{\rm H}(2)}=19.5$. The principal values for the hyperfine interaction with N-9 are$A_{x}{}^{{\rm N}}=3.5$,$A_{y}{}^{{\rm N}}=3.9$, and${\rm A}_{z}{}^{{\rm N}}=3.0$. In nondeuterated crystals, an additional hyperfine splitting due to the C-8-H proton is observed at some orientations. The g tensor is axially symmetric, with$g_{\parallel}=2.0021$ and$g_{\perp}=2.0029$. Some effects of observation temperature on the ESR spectra are also reported.

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