Reaction of Organic Peroxyl Radicals with ^̇NO₂ and ^̇NO in Aqueous Solution: Intermediacy of Organic Peroxynitrate and Peroxynitrite Species

In this work, we studied the reactions of alkyl peroxyl radicals with ^̇NO₂ and ^̇NO using the pulse radiolysis technique. The rate constants for the reaction of ^̇NO ₂ with (CH₃)₂C(OH)CH₂OO ^̇, CH₃OO^̇, and c-C₅H ₉OO^̇ vary between 7 × 10⁸ and 1.5 × 10⁹ M^-1 s^-1. The reaction produces relatively long-lived alkyl peroxynitrates, which are in equilibrium with the parent radicals and have no appreciable absorption above 270 nm. It is also shown that ^̇NO adds rapidly to (CH₃) ₂C(OH)CH₂OO^̇ and CH₃OO ^̇ to form alkyl peroxynitrites. The rate constants for these reactions were determined to be 2.8 × 10⁹ and 3.5 × 10⁹ M^-1 s^-1, respectively. However, in contrast to alkyl peroxynitrates, alkyl peroxynitrites do not accumulate. Rather, they decompose rapidly via homolysis along the relatively weak O-O bond, initially forming a geminate pair. Most of this pair collapses in the cage to form an alkyl nitrate, RONO₂, and about 14% diffuses out as free alkoxyl and ^̇NO₂ radicals. A thermokinetic analysis predicts the half-life of CH₃OONO in water to be less than 1 μs, an estimate that agrees well with previous experimental findings of ours for other alkyl peroxynitrites. A comparison of aqueous and gaseous thermochemistry of alkyl peroxynitrates reveals that alkyl peroxyl radicals and the corresponding alkyl peroxynitrates are similarly solvated by water.