High-pressure flow reactor product study of the reactions of HOx+NO2: The role of vibrationally excited intermediates

The gas-phase reactions between HOchi and NOchi are critical in determining the chemical composition of both the troposphere and stratosphere. They do minate both interconversion among radical species and formation of stable r eservoir species for both HOchi acid NOchi. In many cases, the rates of the se reactions are known, but the products and mechanisms are less well under stood. In particular, the distribution of products among available channels as a function of temperature and pressure is very uncertain for several cr ucial reactions. One important reaction is that of OH with NO2; some fracti on of reactions may lead to an isomer of nitric acid, peroxynitrous acid (H OONO), though this species has not been observed in the gas phase. We prese nt an investigation of that possibility. With reaction modulation FTIR spec troscopy in our high-pressure flow system, we are able to examine the behav ior of various (HOchi + NO2 --> products) reactions with independent contro l over system temperature and pressure. Application of strict mass-balance in our wall-less reactor allows for quantitative analysis of reactant and p roduct concentrations, even in those cases where the integrated bandwidths are unavailable. We examine the reactions HO2 + NO2 --> HOONO2 and OH + NO2 --> products. Each reaction proceeds through at least one vibrationally ex cited intermediate, and in each case there is a potential for the dynamics of those intermediates to produce unexpected behavior. In the case of HO2 NO2, there is no evidence that a hydrogen atom transfer in the intermediat e produces any HONO, even at low pressure. This is consistent with previous work. In the case of OH + NO2 there are almost certainly two intermediates , HOONO and HONO2, but we see no evidence for stable HOONO formation, even at 230 K.