Electron Spin Resonance of Free Radicals and Radical Pairs in Irradiated Single Crystal of 1-Methyluracil

Abstract

Abstract Radical formation by irradiation with X-rays in single crystals of 1-methyluracil has been measured using ESR-spectroscopy at 9.5 GHz and 35 GHz. When irradiating and measuring at 77 °K radicals characterized by hydrogen abstraction from the methyl group were found to be predominating. For the hyperfine splittings principal values of 8.5 G, 18.2 G, 30.5 G were found for both α-protons and 3.3 G, 2.4 G, 1.7 G for the N (1)-splitting. Pairs of abstraction radicals were also found under these conditions showing quintet splittings of about 10 G and anisotropic dipolar coupling. Average distances parallel to c of 6.3 Å and 6.5 Å were found for two types of pairs while those perpendicular to c were not analyzed in detail. Irradiating and measuring at room temperature an additional sextet pattern was found in the spectrum and attributed to a radical formed by addition of hydrogen at C (5). For the hyperfine splittings principal values of 8.1 G, 17.5 G, 30.0 G were found for the α-proton and 31.2 G, 34.4 G, 34.1 G for the two equivalent β-protons. Radical concentrations were determined at 77 °K yielding about 90% abstraction radical, 15% of these in pairs and 10% of addition radical. About half of the abstraction radicals are converted to addition radicals between 200 °K and 280 °K. The latter disappear at about 460 °K leaving a fraction of abstraction radicals which is annealed at even higher temperatures.