Disproportionation pathways of aqueous hyponitrite radicals (HN2O2•/N2O2•-)

Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN O,'/N,O,-) 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 ofNO release-consumption steps. In the first, rate-determining step, two N2O2 center dot- radicals disproportionate with the rate constant 2k = (8.2 +/- 0.5) x 10(7) 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 center dot-, yieldin,, the N3O3- ion, which slowly decomposes at 300 s(-1) to the final N2O + NO2- products. The N2O2 center dot- radical protonates with pK(a) = 5.6 +/- 0.3. The neutral HN2O2 center dot radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 +/- 0.1) x 10(9) M-1 s(-1). The N2O2 center dot- radical shows surprisingly low reactivity toward O-2 and O-2(center dot-), with the corresponding rate constants below 1 x 10(6) and 5 x 10(1) M-1 s(-1). The previously reported rapid dissociation of N2O2 center dot- into N2O and O center dot- does not occur. The thermocherilistry of HN2O2 center dot-)/N2O2 center dot- is discussed in the context of these new kinetic and mechanistic results.