The OH + NO2 reaction, forming nitric acid (HONO2), is critically important to radical chain termination in tropospheric chemistry. However, it has now been confirmed that a minor channel leading to peroxynitrous acid (HOONO) formation has a branching ratio approaching 15% at STP. This minor channel reveals itself in several ways: through biexponential OH kinetics (HOONO can decompose on the timescale of some kinetics experiments); through 18OH isotopic scrambling experiments (H atom migration is possible in HONO2 but not HOONO); and through direct observation of HOONO products (there are two conformers, cis- and trans- HO-ONO). Our objective is to model both biexponential and isotopic scrambling kinetics data with a multiple-well master equation simulation.

Recent experiments for biexponential decay of OH in the reaction OH + NO2 were modeled using a multiple-well master equation. Based on the analysis of three-well system (HONO2, HOONO, and HO + NO2) and four-well system (HONO2, trans-HOONO, cis-HOONO, and HO + NO2), we have considered different critical energies and isomerization rate constants among various wells and thus constrained the reaction mechanism for the OH + NO2 reaction system. The model results agree well with the data.

H scrambling in HONO2 was then studied by running a nine-well master equation for 18OH, 16OH, and NO2 reaction system. A scrambling transition state was located using density function theory (B3lyp/6-31G+(d,p)). The model shows that H-atom scrambling is rapid and the rapid scrambling can explain the experimental results by D'Ottone et al. The sensitivity analysis for the critical energy and microcanonical rate constant for the isotopic scrambling transition state shows that the scrambling is complete.