Structural and mechanistic analysis through electronic spectra: aqueous hyponitrite radical (N2O2-) and nitrosyl hyponitrite anion (N3O3-).

Aqueous hyponitrite radical (N(2)O(2)(-)) and nitrosyl hyponitrite anion (N(3)O(3)(-)) 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 N(2)O(2)(-), comparison of the calculated spectra and energetics with the experimental data reveals that (1) upon the one-electron oxidation of trans-hyponitrite (ON═NO(2-)), the trans configuration of the resulting ON═NO(-) radical is preserved; (2) although cis- and trans-ON═NO(-) are energetically nearly equivalent, the barrier for the trans-cis isomerization is prohibitively high because of the partial double character of the NN bond; (3) the calculations confirm that the UV spectrum of ONNO(-) was misinterpreted in the earlier pulse radiolysis work, and its more recent revision has been justified. For the N(3)O(3)(-) ion, the symmetric isomer [Formula: see text] is the dominant observable species, and the asymmetric isomer [Formula: see text] contributes insignificantly to the experimental spectrum. Coherent analysis of the calculated and experimental data suggests a reinterpretation of the N(2)O(2)(-) + NO reaction mechanism according to which the reaction evenly bifurcates to yield both the symmetric and asymmetric isomers of N(3)O(3)(-). While the latter isomer rapidly decomposes to the final NO(2)(-) + N(2)O products, the former isomer is stable toward this decomposition, but its formation is reversible with the homolysis equilibrium constant K(hom) = 2.2 × 10(-7) M. Collectively, these results demonstrate that advanced theoretical modeling can be of significant benefit in structural and mechanistic analysis on the basis of the electronic spectra of aqueous transients.