DMPO-Alkyl radical spin trapping: an in situ radiolysis steady-state ESR study

Short-lived free radicals formed in the reaction of 11 substrates and radiolytically produced hydroxyl radicals were trapped successfully with 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) in dilute aqueous solution. The in situ radiolysis steady-state ESR spectra of the spin adducts were analyzed to determine accurate ESR parameters for these spin adducts in a uniform environment. Parent alkyl radicals include methyl, ethyl, 1-propyl and 2-propyl (1-methylethyl). Hydroxyalkyl parent radicals were hydroxymethyl, hydroxyethyl, 2-hydroxy-2-propyl (1-methyl-1-hydroxyethyl), 1-hydroxypropyl and 2-hydroxy-2-methylpropyl. Carboxyl radical (carbon dioxide anion, formate radical) and sulfite anion radical were the sigma radicals studied. The DMPO spin adduct of 1-propyl was identified for the first time. For most spin adducts, g factors were also determined for the first time. In DMPO spin adducts of hydroxyalkyl radicals, nitrogen and C(2)-proton hyperfine coupling constants are smaller than those of alkyl radical adducts; the hydroxyalkyl spin adducts possess larger g values than their unsubstituted counterparts. These changes are ascribed to the spread of pi conjugation to include the hydroxyl group. Strong evidence of spin addend-aminoxyl group interaction can be seen in the asymmetrical line shapes in the hydroxyethyl and the hydroxypropyl spin adducts.