Rate Constants, 1100 ≤ T ≤ 2000 K, for H + NO₂ → OH + NO Using Two Shock Tube Techniques:  Comparison of Theory to Experiment

Abstract

Rate constants for the reaction H + NO 2 f OH + NO have been measured over the temperature range 1100-2000 K in reflected shock wave experiments using two different methods of analysis. In both methods, the source of H-atoms is from ethyl radical decomposition in which the radicals are formed essentially instantaneously from the thermal decomposition of C 2 H 5 I. The first method uses atomic resonance absorption spectrometry (ARAS) to follow the temporal behavior of H-atoms. Experiments were performed under such low [C 2 H 5 I] 0 that the title reaction could be chemically isolated, and the decay of H-atoms was strictly firstorder. The results from these experiments can be summarized as k ) (1.4 ( 0.3) × 10 -10 cm 3 molecule -1 s -1 for 1100 e T e 1650 K. The second method utilizes a multipass optical system for observing the product radical, OH. A resonance lamp was used as the absorption source. Because this is the first OH-radical kinetics investigation from this laboratory, extensive calibration was required. This procedure resulted in a modified Beer's law description of the curve-of-growth, which could subsequently be used to convert absorption data to OH-radical profiles. Rate constants by this method required chemical simulation, and the final result can be summarized as k ) (1.8 ( 0.2) × 10 -10 cm 3 molecule -1 s -1 for 1250 e T e 2000 K. Because the results from the two methods statistically overlap, they can be combined giving k ) (1.64 ( 0.30) × 10 -10 cm 3 molecule -1 s -1 for 1100 e T e 2000 K. The present results are compared to earlier work at lower temperatures, and the combined database yields the temperature dependence over the large range, 195-2000 K. The combined results can be summarized as k ) (1.47 ( 0.26) × 10 -10 cm 3 molecule -1 s -1 for 195 e T e 2000 K. The reaction is subsequently considered theoretically using ab initio electronic structure calculations combined with modern dynamical theory to rationalize the thermal rate behavior. † Part of the special issue "Donald Setser Festschrift".

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