Electron transfer and addition reactions of free nitroxyl radicals with radiation induced radicals

Abstract

The mechanism of the reactions of free nitroxyl radicals [2,2,5,5-tetramethyl-1-pyrrolidinyl-oxy-3-carboxamide; 2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy-3-carboxamide; 4-hydroxy-2,2,6,6-tetramethyl-piperidino-1-oxy (TMPN); 2,2,6,6-tetramethyl-4-oxo-piperidino-oxy (TAN)] with radiation induced radicals has been investigated by pulse radiolysis conductivity measurements. The reaction [graphic omitted] proceeds by different simultaneous pathways. Electron transfer yields a [graphic omitted] ion which is stable over the acid and most of the basic pH range. At high pH it reacts according to [graphic omitted]. The OH radical also adds to the nitroxyl groups forming [graphic omitted] and to other activated sites of the nitroxyl radicals, e.g. to CC bonds. Reduction of [graphic omitted] and reducing α-hydroxy radicals yields [graphic omitted] in neutral and basic solutions. Radicals which are neither good oxidants nor reductants such as ·CH₂CH₂OH, ·CH₂C(CH₃)₂OH, c-C₅H₉·, CH₃· and CCl₃·, generally add to the nitroxyl group to form [graphic omitted]. In acid solution all the adducts are protonated according to the equilibrium [graphic omitted]. The corresponding pK values are dependent on X and the nature of the nitroxyl compound. The methanol radical, ·CH₂OH, forms an addition product [graphic omitted] which is either stabilized by protonation or decays unimolecularly (t_½≈ 1 ms) to [graphic omitted] and HCHO. The rate constants for the reactions of [graphic omitted] with the radiation induced radicals are in the range 10⁸–10¹⁰ dm³ mol^–1 s^–1.