Carbonate radical ion is the only observable intermediate in the reaction of peroxynitrite with CO₂

The reaction of ONOO^- with CO₂ at alkaline pH was recently reported to form a transient absorption with a maximum at 640 nm and a half-life of ca. 4 ms at 10°C [Meli et al. (1999) Helv. Chim. Acta 82, 722-725]. This transient absorption was hardly affected by the presence of^•NO, and therefore was attributed to the adduct ONOOC(O)O^-. This conclusion contradicts all current experimental results as it suggests that the decomposition of this adduct via homolysis of the O-O bond into CO3•^- and ^•NO₂ is a minor pathway. In the present work the observations of Meli et al. will be shown to be artifacts resulting from light coming from the UV region. When these experiments are carried out in the presence of appropriate cutoff filters, the only observable intermediate formed in the reaction of ONOO^- with CO₂ at alkaline pH is the carbonate radical ion with a maximum at 600 nm. This transient absorption is not observed in the presence of ^•NO or ferrocyanide. In the latter case ferricyanide is formed, and its yield was determined to be 66 ± 2% of the initial concentration of peroxynitrite. The reaction of ONOO^- with 16 mM CO₂ with and without ferrocyanide was also studied at pH 5.6-7.7 in the presence of 0.1 M phosphate, where both the initial pH and [CO₂] remain constant. Under these conditions the rate constant of the decay of peroxynitrite was found to be identical to that of the formation of ferricyanide, indicating that ONOOC(O)^- does not accumulate. These results confirm our earlier observations, i.e., the reaction of peroxynitrite with excess CO₂ takes place via the formation of about 33% CO₃^•- and ^•NO₂ radicals in the bulk of the solution.