X-irradiated 5-nitrobarbituric acid, 5-nitroorotic acid, and 5-nitro-4,6-dihydroxy-pyrimidine show distinctive ESR spectra with very anisotropic nitrogen coupling, essentially the same as 5-nitrouracil. The 5-nitrouracil radical has been identified as an iminoxyl radical formed by a mechanism which abstracts an oxygen from the nitro group leaving the unpaired electron coupling to the nitrogen of the nitro group. It is clear that these other nitropyrimidines yield similar iminoxyl radicals. The only reported nitropyrimidine which yields different radical structures is 5-nitro-6-methyluracil which has been investigated at 77°K and 300°K by other workers. On the basis of this experimental data, INDO molecular orbital calculations, and the proposed mechanism of formation of the 5-nitro uracil radical which fits also the other nitropyrimidines, we can now propose radical structures for the low temperature and room temperature 5-nitro-6-methyluracil radicals. Our proposed 77°K radical is formed by hydrogen addition to a nitro-oxygen leaving the unpaired electron coupling to this hydrogen and to the nitro-nitrogen. At room temperature the hydroxyl is abstracted from the nitro group leaving it a nitroso group, and a hydrogen is added to 0(4). INDO calculations on this structure give very good agreement to the reported experimental parameters. This radical differs from the previously reported structure by alteration of the nitro group into a nitroso.

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