Electron spin resonance studies of inorganic radicals in irradiated aqueous solutions. II. Radical trapping with nitromethane

Radical trapping of the aci anion of nitromethane (CH₂=NO₂^-) has been used in a study of the intermediates in irradiated aqueous solutions of a number of inorganic salts. Steady-state, in situ radiolysis was carried out with a 2.8-MeV electron beam. Two types of reaction with this reagent were observed: direct addition of the inorganic radical (X·) to the CN double bond to form the adduct XCH₂ṄO₂^- and an electron transfer from CH₂=NO₂^- to X·, which results in the formation of ·CH₂NO₂ (which in turn reacts with another nitromethane molecule to form O₂NCH₂CH₂ṄO₂^-). Variation of the nitrogen and methylene proton hyperfine constants of the adduct radicals, XCH₂ṄO₂^-, is sufficient that a number of different spectra could be distinguished. In some cases hyperfine splittings by magnetic nuclei in the substituent X were also observed (e.g., where X· was CN· or HṖO₂^-). The radicals were identified by reference to the expected radiation chemistry of the inorganic solute. Accurate spectral parameters are given for 14 adducts to CH₂=NO₂^-. The successful application of CH₂=NO₂^- as a radical trap in these various cases demonstrates the potential for general use both in other studies of these same radicals and in studies of other radicals both inorganic and organic. The results contribute to the understanding of the radiation chemistry of a number of these inorganic systems. Of particular importance is the demonstration of the reaction e_aq^- + S₂O₃^2- → ·S^- + SO₃^2-.