Carbon-13 Hyperfine Interactions in Nitroaromatic Radical Anions

A number of nitroaromatic compounds have been synthesized with enrichment in carbon-13 at positions 1 and 4 and in two cases enrichment at both positions has been achieved using mixtures of 1 and 4 labeled molecules. It has, therefore, been possible to examine the carbon-13 isotropic hyperfine interactions in the corresponding anion radicals by electron spin resonance spectroscopy. In the case of 4-cyanonitrobenzene anion the effect of changing the solvent medium has been explored. In hexamethyl-phosphoramide we find a _c1 = -3.42 G and α _c4 = 7.06 G. In dimethylformamide containing 0.234 mole fraction of water these values are altered to -5.94 and 6.51 G, respectively. Throughout the series of radicals studied, we find that a _c1 depends largely upon the spin density on nitrogen, a result which confirms the conclusion reached previously for nitrobenzene anion.