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

The toxicity of peroxynitrite, ONOO, is directed by carbon dioxide via the formation of the corresponding adduct, ONOOCO. Entity ONOOCO 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, and nitrogen dioxide, NO, in a so-called solvent (radical) cage reaction. Recently, Koppenol et al. reviewed their proposal that ONOOCO is a relatively long-lived intermediate, arguing that "the solvent cage as proposed is physically not realistic". To further address whether ONOOCO could be a long-lived species, bond dissociation enthalpies (BDE) were calculated by the composite reference method (SMD)W1BD. Anion ONOOCO can exist in two conformers, and with predicted gas-phase O-O BDEs of about 10.8 and 9.5 kcal mol, 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 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 and NO at a total yield of about 70%. Because the solvent radical cage concept for the decay of ONOO in the presence of CO 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.