Solvent Cage Concept for the Homolytic Fragmentation of the Peroxynitrite-CO 2 Adduct, ONOOCO 2 .

The toxicity of peroxynitrite, ONOO - , is directed by carbon dioxide via the formation of the corresponding adduct, ONOOCO 2 - . Entity ONOOCO 2 - is believed to be a highly unstable compound that primarily decomposes to nitrate and carbon dioxide, but it also undergoes fractional homolysis to generate carbonate radical anion, CO 3 •- , and nitrogen dioxide, NO 2 • , in a so-called solvent (radical) cage reaction. Recently, Koppenol et al. reviewed their proposal that ONOOCO 2 - is a relatively long-lived intermediate, arguing that "the solvent cage as proposed is physically not realistic". To further address whether ONOOCO 2 - could be a long-lived species, bond dissociation enthalpies (BDE) were calculated by the composite reference method (SMD)W1BD. Anion ONOOCO 2 - can exist in two conformers, s-cis-gauche and s-trans-gauche with predicted gas-phase O-O BDEs of about 10.8 and 9.5 kcal mol -1 , respectively. Therefore, both conformers should have very short lifetimes. The (SMD)W1BD method was also used to evaluate the thermodynamic parameters of interest, revealing that the homolytic decomposition of ONOOCO 2 - is the most reasonable pathway. Moreover, previously reported experimental chemically induced dynamic nuclear polarization data also support the intermediacy of the radical cage and the formation of products CO 2 and NO 3 - at a total yield of about 70%. Because the solvent radical cage concept for the decay of ONOO - in the presence of CO 2 is supported by a variety of spectrometric methods as well as by quantum chemical calculations at high levels of theory, it provides strong evidence against the "out-of-cage" construct. For clarification of the nature of the transient UV/vis absorption(s) between 600 and 700 nm, as observed by Koppenol et al., several experimental approaches are suggested.