Free radicals and diradicals in the reaction between nitrous acid and bisulfite in acid aqueous media

Abstract

Rates of the prompt formation of (HSO_3)_2NO•(F) in the reaction of nitrous acid with bisulfite were measured by stopped-flow electron spin resonance spectrometry at pH = 3.0-5.8, 298 K. Initial rates have a positive dependence on (HONO] and [HSO_3^-] but decrease markedly with pH. The induction periods observed in air-saturated solutions are suppressed by addition of 2 mM hydroxylaminedisulfonate, (SO_3^-)_2NOH (FH), revealing a free-radical precursor to F. The spin trap 5.5-dimethyl-1-pyrroline N-oxide (DMPO) also inhibits the formation of F but without producing new paramagnetic species, ruling out monoradical intermediates. We show that these observations are consistent with the previously proposed participation of a nitrene, HON:, in this system. Present evidence suggests that the title reaction is a potential source of free radicals in natural environments.