Ab Initio Study of the H + HONO Reaction: Direct Abstraction versus Indirect Exchange Processes

Abstract

The mechanism of the H + HONO reaction (for which no experimental data are available) has been elucidated by ab initio molecular orbital calculations using modified G2 and BAC-MP4 methods. These results indicate that the reaction occurs predominantly by two indirect metathetical processes. One produces OH + HNO and H2O + NO from the decomposition of vibrationally excited hydroxyl nitroxide, HN(O)OH, formed by H atom addition to the N atom of HONO. The other produces H2O + NO from the decomposition of vibrationally excited dihydroxylamino radical, N(OH)2, formed by H atom addition to the terminal O atom. These indirect displacement processes are much more efficient than the commonly assumed, direct H-abstraction reaction producing H2 + NO2. A transition-state theory calculation for the direct abstraction reaction and RRKM calculations for the two indirect displacement processes give rise to the following rate constants, in units of cm3 molecule-1 s-1 for the 300−3500 K temperature range under atmospheric conditions: kH2 = 3.33 × 10-16T1.55 exp(−3328.5/T), kOH = 9.36 × 10-14T0.86 exp(−2500.8/T), kH2O = 1.35 × 10-17T1.89 exp(−1935.7/T), where the rate constant for H2O production represents the sum from both indirect displacement reactions.