Disproportionation Pathways of Aqueous Hyponitrite Radicals (HN2O2•/N2O2•-)статья
Статья опубликована в высокорейтинговом журнале
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Аннотация:Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2W2O2 -) 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) × 10 7 M-1 s-1 (at zero ionic strength) effectively in a redox reaction regenerating N2O2 2- 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 N3O 3 - ion, which slowly decomposes at 300 s-1 to the final N2O + NO2 - products. The N 2O2 - 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 HN2O 2/N2O2 - is discussed in the context of these new kinetic and mechanistic results