HO 2 + NO 2 : Kinetics, Thermochemistry, and Evidence for a Bimolecular Product Channel.

A master equation (ME) analysis of available experimental data has been carried out on the reaction HO 2 + NO 2 + M ⇋ HO 2 NO 2 + M (1a)/(-1a). The analysis, based on the ME code MESMER, uses both the association and dissociation kinetic data from the literature, and provides improved thermochemistry on reaction 1a. Our preferred model assigns two low-frequency vibrations of HO 2 NO 2 as hindered rotors and couples these to the external rotations. This model gives Δ r H ° 0 (1a) = -93.9 ± 1.0 kJ mol -1 , which implies that Δ f H ° 0 HO 2 NO 2 = -42.0 ± 1.0 kJ mol -1 (uncertainties are 2σ). A significant contributor to the uncertainty derives from modeling the interaction between the internal and external rotors. Using this improved kinetics for reaction 1a/-1a, data at elevated temperatures, 353-423 K, which show no evidence of the expected equilibration, have been reanalyzed, indicating that an additional reaction is occurring that masks the equilibration. Based on a published ab initio study, this additional channel is assigned to the bimolecular reaction HO 2 + NO 2 → H-NO 2 + O 2 (1b); H-NO 2 is nitryl hydride and has not previously been directly observed in experiments. The output of the master equation analysis has been parametrized and Troe expressions are provided for an improved description of k 1a ( p , T ) and k -1a ( p , T ).