Mechanistic studies of the photochemical nitration of phenols, 1,2-dimethoxybenzene and anisoles with tetranitromethane by N-15 CIDNP

During the photochemical nitration of phenol, 3,5-dimethylphenol and 1,2-di methoxybenzene (8) with N-15-enriched tetranitromethane, the N-15 nuclear m agnetic resonance (NMR) signals of the nitro products 2-nitrophenol, 4-nitr ophenol, 2-nitro-3,5-dimethylphenol, 4-nitro-3,5-dimethylphenol and 1,2-dim ethoxy-4-nitrobenzene (9) appear in emission. The nuclear polarizations are built up in radical pairs formed by radical cations or in case of the phen ols by phenoxyl radicals and (NO2)-N-.. They are generated by photoreaction s from excited triplet states of the reactants or by free radical encounter s. With 8, 1,2-dimethoxy-4-trinitromethylbenzene is also formed which slowl y converts to 9. In contrast to this, the N-15 NMR signals of the nitration products of anisole (11) and 3,5-dimethylanisole (12), 2-nitroanisole, 4-n itroanisole, 3,5-dimethyl-2-nitroanisole and 3,5-dimethyl-4-nitroanisole ap pear in enhanced absorption indicating the appearance of singlet radical pa irs [11(+.), (NO2)-N-.](s) and [12(+.), (NO2)-N-.](s) in which the nuclear polarizations are built up. The singlet radical pairs are formed by decompo sition of an unstable intermediate, most probably a nitro-trinitromethyl ad duct.