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

Abstract

Rate constants for the reaction H + NO2 → 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 C2H5I. The first method uses atomic resonance absorption spectrometry (ARAS) to follow the temporal behavior of H-atoms. Experiments were performed under such low [C2H5I]0 that the title reaction could be chemically isolated, and the decay of H-atoms was strictly first-order. The results from these experiments can be summarized as k = (1.4 ± 0.3) × 10-10 cm3 molecule-1 s-1 for 1100 ≤ T ≤ 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 cm3 molecule-1 s-1 for 1250 ≤ T ≤ 2000 K. Because the results from the two methods statistically overlap, they can be combined giving k = (1.64 ± 0.30) × 10-10 cm3 molecule-1 s-1 for 1100 ≤ T ≤ 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 cm3 molecule-1 s-1 for 195 ≤ T ≤ 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.