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

Abstract

One-electron oxidants react with H2N−CHRR‘ amino compounds by electron transfer (ET), and direct H abstraction from N−H and C−H, giving respectively, aminium (+•NH2CHRR‘), aminyl (•NH−CHRR‘), and α-C-centered radicals (H2N-•CRR‘). The yields of these species from •O- reactions with the anions of glycine (Gly-), alanine (Ala-), and α-methylalanine (MeAla-) and with methylamine (MeNH2) 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, kN(-H•)/kC(-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 kN(-H•)/kC(-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.