Disproportionation Pathways of Aqueous Hyponitrite Radicals (HN2O2•/N2O2•−)
journal contributionposted on 11.09.2008, 00:00 by Gregory A. Poskrebyshev, Vladimir Shafirovich, Sergei V. Lymar
Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2•/N2O2•−) by one-electron oxidation of the hyponitrite in aqueous solution. Although the radical decay conforms to simple second-order kinetics, its mechanism is complex, comprising a short chain of NO release−consumption steps. In the first, rate-determining step, two N2O2•− radicals disproportionate with the rate constant 2k = (8.2 ± 0.5) × 107 M−1 s−1 (at zero ionic strength) effectively in a redox reaction regenerating N2O22− and releasing two NO. This occurs either by electron transfer or, more likely, through radical recombination−dissociation. Each NO so-produced rapidly adds to another N2O2•−, yielding the N3O3− ion, which slowly decomposes at 300 s−1 to the final N2O + NO2− products. The N2O2•− radical protonates with pKa = 5.6 ± 0.3. The neutral HN2O2• radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 ± 0.1) × 109 M−1 s−1. The N2O2•− radical shows surprisingly low reactivity toward O2 and O2•−, with the corresponding rate constants below 1 × 106 and 5 × 107 M−1 s−1. The previously reported rapid dissociation of N2O2•− into N2O and O•− does not occur. The thermochemistry of HN2O2•/N2O2•− is discussed in the context of these new kinetic and mechanistic results.