Oxide Radical Anion Reactivity with Aliphatic Amino Compounds in Aqueous Solution: Comparison of H−Atom Abstraction from C−H and N−H Groups by •O- and •OH Radicals

One-electron oxidants react with H 2 N−CHRR‘ amino compounds by electron transfer (ET), and direct H abstraction from N−H and C−H, giving respectively, aminium ( +• NH 2 CHRR‘), aminyl ( • NH−CHRR‘), and α-C-centered radicals (H 2 N- • CRR‘). The yields of these species from • O - reactions with the anions of glycine (Gly - ), alanine (Ala - ), and α-methylalanine (MeAla - ) and with methylamine (MeNH 2 ) have been investigated at pH ≥ 13. The results indicate an ET process is negligible. Aminyl and α-C-centered radicals appear to be formed only by direct H abstraction reactions. In line with this, the ratios of the overall rates of H abstraction from N−H and C−H, k N( - H • ) / k C( - H • ), for • O - reacting with different amino compounds decrease with C−H bond dissociation enthalpy (BDE) and thus follow the pattern expected for direct abstraction reactions. In contrast to • O -, the conjugate • OH radical produces significant yields of aminium radicals by ET, which evidently contribute to aminyl radical formation by subsequent proton loss from nitrogen. Thus, the k N( - H • ) / k C( - H • ) ratios for • OH are higher than those for • O - and do not decrease regularly with C−H BDE. Formation of α-C-centered radicals via ET and subsequent proton loss from a C−H group of the aminium radical is much less likely. The overall rates of H abstraction from C−H sites by both • OH and • O - are found to increase with the exothermicity of the reaction. Because of its spherical symmetry the steric factors for • O - reactions are larger than those for • OH, but in most cases this appears to be compensated by more favorable potential energy surfaces stemming from the 36 kJ mol -1 greater exothermicity of • OH reactions. • O - reactions with charged species are of course also susceptible to the effects of Coulombic interactions.