One-electron reduction of aqueous nitric oxide: A mechanistic revision

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (NO-)-N-3 and singlet (HNO)-H-1, respectively, which further react with NO to produce the N2O2- radical, albeit with the very different specific rates, k((NO-)-N-3 + NO) = (3.0 +/- 0.8) x 10(9) and k((HNO)-H-1 + NO) = (5.8 +/- 0.2) x 10(6) M-1 s(-1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (NO-)-N-3 and (HNO)-H-1 under all experimentally accessible conditions. As a result, (NO-)-N-3 and (HNO)-H-1 give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N2O2- radical extremely rapidly acquires another NO, k(N2O2- + NO) = (5.4 +/- 1.4) x 10(9) M-1 s(-1), producing the closed-shell N3O3- anion, which unimolecularly decays to the final N2O + NO2- products with a rate constant of similar to 300 s(-1). Contrary to the previous belief, N2O2- is Stable with respect to NO elimination, and so is N3O3-. The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N3O3- anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M-1 x cm(-1)). The spectra previously assigned to the NO- anion and to the N2O2- radical are due, in fact, to a mixture of species (mainly N2O2- and N3O3-) and to the N3O3- anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (NO-)-N-3 and (HNO)-H-1 are generated by photolysis of the monoprotonated anion of Angeli's salt, HN2O3-, in NO-containing solutions.