Electron spin resonance data for mono- and di-anions of m-dinitrobenzene : radiation-induced addition of two electrons at 77 K

Abstract

Exposure of dilute solutions of m-dinitrobenzene or s-trinitrobenzene in methyltetrahydrofuran to ⁶⁰Co γ-rays at 77 K gave two species detectable by e.s.r. spectroscopy. One, favoured at low doses, had features characteristic of monoanions with the unpaired electrons primarily localised on one nitro-group. Similar anions were formed from the nitro-derivatives in methanol (CD₃OD). At high γ-ray doses a second species grew in at the expense of the monoanions. These had half-field (ΔM_s± 2) transitions characteristic of triplet states, and well defined features in the parallel regions of the anisotropic ΔM_s± 1 transitions. These parallel features, which exhibited no resolved hyperfine coupling to ¹⁴N were used to estimate average effective separations between the two unpaired electrons, giving values in the region of 5–6 Å. The ΔM_s± 1 perpendicular features were poorly defined but gave clear evidence of ¹⁴N hyperfine coupling. The (¹⁴N) parallel features were better resolved in the ΔM_s± 2 transitions, which showed five shoulders indicating two equivalent nitrogen nuclei with about half the coupling of the monoanions. These results are all in accord with the postulate that the species involved are dianions in triplet states. Each unpaired electron must be strongly localised on one nitro-group which cannot be twisted far from the limit of coplanarity. Absence of ¹⁴N coupling on the ΔM_s± 1 parallel features can then be understood since the ¹⁴N parallel axes are perpendicular to the principal axis for the zero-field coupling. The alternative explanation for these results, that solvent radicals are trapped close to the radical anions is considered, but ruled out on the grounds that the ΔM_s± 2 features should then be triplets rather than quintets, and because there would be no barrier to chemical reactions between such radicals and the nitrobenzene anions.