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 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 alpha-C-centered radicals (H2N-(CRR)-C-.'). The yields of these species from O-.(-) reactions with the anions of glycine (Gly(-)), alanine (Ala(-)), and alpha-methylalanine (MeAla(-)) and with methylamine (MeNH2) have been investigated at pH greater than or equal to 13. The results indicate an ET process is negligible. Aminyl and alpha-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 (.)0(-) 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 (.)0(-), the conjugate (OH)-O-. 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.))/kc((-H.)) ratios for (OH)-O-. are higher than those for (.)0(-) and do not decrease regularly with C-H BDE. Formation of alpha-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)-O-. 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)-O-., 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)-O-. reactions. O-.(-) reactions with charged species are of course also susceptible to the effects of Coulombic interactions.