Viscosity effects on the reaction of peroxynitrite with CO₂: Evidence for radical formation in a solvent cage

Peroxynitrite (ONOO^-) reacts rapidly with excess of CO₂ to yield in water 30-35% CO₃(·)^- and (·)NO₂ radicals, which oxidize 2,2'-azino- bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and ferrocyanide. The yield of ABTS⁺ decreased upon increasing the glycerol concentration, independent of [ABTS]₀, and was, within experimental error, identical to that of ferricyanide at the same concentration of glycerol. These results demonstrate that the decrease in the oxidation yields upon increasing the glycerol concentration does not result from a competition between glycerol and ABTS/ferrocyanide for the oxidizing radicals but is due to an increase in viscosity. The oxidation yields were less affected by poly(ethylene glycol) than by glycerol, though the macroscopic viscosities of the polymer solutions were much higher. We conclude that the mechanism of the decomposition of peroxynitrite in the presence of CO₂ takes place through the formation of CO3(·)^- and (·)NO₂ in a solvent cage. In addition, in the glycerol-water system, the microscopic viscosity is essentially equal to the macroscopic viscosity, whereas in the polymer systems the microscopic viscosity does not approximate the macroscopic viscosity.