Electron spin resonance studies of electron capture processes. Part 7. σ*(3-Electron covalent bond) radical anions of halogenoimidazoles

Abstract

Exposure of dilute solutions of 4-bromo- and 4-iodo-2-methylimidazole, 5-bromo- and 5-iodo-1,2-dimethylimidazole, and 4,5-dibromo- and 4,5-di-iodo-1,2-dimethylimidazole [(1)–(6)] in methanol (CD₃OD) or methyltetrahydrofuran (MeTHF) to ⁶⁰Co γ-rays at 77 K resulted in electron addition to yield not the expected π* radical anions but σ* radicals anions. In these radical anions the SOMO is primarily the C–Hal σ* molecular orbital with little delocalisation into the imidazole ring, i.e. a 3-electron σ bond. On slowly increasing the temperature of the MeTHF solutions of the bromoimidazoles the hyperfine features for the σ* radical anions were lost but with no clear concommitant growth of features assignable to 4- or 5-imidazolyl radicals. However, for the iodoimidazoles (2), (4), and (6) in CD₃OD a new species was observed on annealing. The same species was observed for all three iodo compounds and is assigned to iodine atoms weakly bonded to CD₃OD molecules [I[graphic omitted]O(D)CD₃]. The results are discussed in the light of the σ* and/or π* species obtained from electron capture by other aromatic halides and other substituted imidazoles.