PNNL

Abstract

Aqueous hyponitrite radical (N2O2-) and nitrosyl hyponitrite anion (N3O3-) species are important intermediates in the reductive chemistry of NO. The structures and absorption spectra of various hydrated isomers of these compounds were investigated in this work using high-level quantum mechanical calculations combined with the explicit classical description of the aqueous environment. For N2O2-, comparison of the calculated spectra and energetics with the experimental data reveals that: (1) upon the one-electron oxidation of trans-hyponitrite (ONNO2-), the trans configuration of the resulting ONNO- radical is preserved; (2) although cis- and trans-ONNO- are energetically nearly equivalent, the barrier for the trans-cis isomerization is prohibitively high due to the partial double character of the NN bond; (3) the UV spectrum of ONNO- was misinterpreted in the earlier pulse radiolysis work and its more recent revision has been justified. For the N3O3- ion, the symmetric isomer is the dominant observable species, and the asymmetric isomer contributes little to the experimental spectrum. Coherent analysis of the calculated and experimental data suggests a re-interpretation of the N2O2- + NO reaction mechanism, according to which the reaction evenly bifurcates to yield both the symmetric and asymmetric isomers of N3O3-. While the latter isomer rapidly decomposes to the final NO2 - + N2O products, the former isomer is stable toward this decomposition but its formation is reversible with the homolysis equilibrium constant Khom = 2.2 × 10-7 M. Collectively, these results demonstrate that advanced theoretical modeling can be of significant benefit in structural and mechanistic analysis of UV spectra.