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 3NO- and singlet 1HNO, respectively, which further react with NO to produce the N2O2- radical, albeit with the very different specific rates, k(3NO- + NO) = (3.0 (plus-minus) 0.8) × 10^9 and k(1HNO + NO) = (5.8 (plus-minus) 0.2) × 10^6 M^-1 s^-1. These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of 3NO- and 1HNO under all experimentally accessible conditions. As a result, 3NO- and 1HNO 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 (plus-minus) 1.4) × 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 ~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 × 10^3 M^-1 cm^-1). The spectra previously assigned to the NOanion 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 3NO- and 1HNO are generated by photolysis of the monoprotonated anion of Angeliʹs salt, HN2O3-, in NO-containing solutions.