The Role of Phosphorus Dioxide in the H + OH Recombination Reaction: Ab Initio Quantum Chemical Computation of Thermochemical and Rate Parameters

Abstract

A theoretical investigation of the effects of PO2 on the H + OH radical recombination reaction is reported. The focus of the study is the computation of rate coefficients by ab initio quantum chemical and RRKM methods for the H + PO2 and OH + PO2 recombination reactions and for the H + HOPO → H2 + PO2, OH + H2 → H + H2O, and H + HOPO2 → H2O + PO2 abstraction reactions, which constitute a catalytic pathway for the H + OH recombination reaction (Twarowski, A. Combust. Flame 1993, 94, 91). These are a subset of Twarowski's reaction model (Twarowski, A. Combust. Flame 1995, 102, 41) that consists of 175 individual reactions and includes 17 phosphorus-containing molecules. The thermochemistry of this complete reaction model was computed using the Gaussian methods: G2, G3, and G3X. While G3X was found to be superior to G2 and G3 for the prediction of heats of formation of phosphorus-containing molecules, it underestimates the heat of formation of HOPO2 by at least 3.6 kcal mol-1, when compared with the extensive coupled cluster computations of Bauschlicher (Bauschlicher, C. W., Jr. J. Phys. Chem. 1999, 103, 11126). Consequently, the rate coefficients reported in this work are based on Bauschlicher's thermochemical data for PO2, HOPO, and HOPO2. The computed rate coefficients are consistent with the available experimental data and the results of modeling studies by Twarowski (Twarowski, A. Combust. Flame 1995, 102, 41) and Korobeinichev et al. (Korobeinichev, O. P.; Ilyin, S. B.; Bolshova, T. A.; Shvartsberg, V. M.; Chernov, A. A. Combust. Flame 2001, 125, 744). The recombination reactions are found to be substantially into the fall-off region at near atmospheric pressures.