One-Electron Oxidation and Reduction of Sulfites and Sulfinic Acids in Oxygenated Media: The Formation of Sulfonyl and Sulfuranyl Peroxyl Radicals

Abstract

Bisulfite anions and sulfinic acids are shown by ESR spectroscopy to undergo reduction with radiolytically produced free electrons and oxidation by hole centers (Cl2•-) in a low-temperature aqueous LiCl glass. Electron-reduced intermediates display g anisotropy within the 2.010−2.002 range and are considered as neutral or singly deprotonated forms of sulfuranyl radicals (HO)3S•, (RO)2SOH•, and RS(OH)2•. The oxidized species produced by hole trapping are sulfite (SO3•-) and sulfonyl (RSO2•) radicals. Dialkyl sulfites scavenge free electrons only and are not oxidized by Cl2•-. Both oxidized and reduced intermediates add oxygen on annealing forming peroxyl radicals which are identified by their characteristic oxygen-17 hyperfine couplings. A large 100 G oxygen-17 coupling with the terminal oxygen atom found for oxidized sulfite−oxygen adduct -O3SOO• is in accord with its known high reactivity. Sulfuranyl peroxyls formed by the reductive pathway show lower 17O couplings (e.g., 90 G with terminal oxygens) and are expected to be far less reactive than alkyl peroxyls. Ab initio MO calculations predict stable structures for these sulfuranyl peroxyls, suggesting a trigonal-bipyramid coordination of the central sulfur atom. However, the sulfuranyl intermediates formed through one-electron reduction of dimethyl and methylphenyl sulfoxides are found to be unstable even in low-temperature glass, fragmenting into a methyl radical and sulfenic acid or into a hydroxyl anion and sulfide radical cation, respectively.