Free Radical Formation in Pyrimidine Derivatives: A Study of 5-Nitrouracil at 77 K

The ESR spectra of 5-nitrouracil, irradiated at 77 K, yield evidence for the formation of several radical species. The two radicals which exhibit the strongest ESR signals are discussed in some detail. One of the resonances consists of a three line pattern with an isotropic splitting constant of 56.5 G and an average g-factor of 2.0007. These data are compatible with the interpretation that the NO₂ group is abstracted from the pyrimidine ring and trapped within the crystal lattice. The principal tensor values suggest that the oxygen atoms are kept at their original positions and that the nitrogen atom swings out of the pyrimidine plane. Furthermore, the data indicate some molecular motion around the O-O direction. The other resonance consists of a triplet of doublets due to hyperfine interactions with a nitrogen and a proton in a nitrogen centered π-radical. In an attempt to identify the responsible radical, extensive INDO-MO calculations were performed. The bulk of data support the hypothesis that a deprotonated radical anion is responsible for the resonance.