Production of HCl in the OH + ClO Reaction: Laboratory Measurements and Statistical Rate Theory Calculations

Abstract

The branching ratio for the OH + ClO reaction has been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. In our earlier study, OD was used instead of OH due to the large HCl background produced by the ClO source (J. Chem. Soc., Faraday Trans. 1997, 93, 2665). Improvements to our experimental technique have significantly reduced this HCl background, thus making it possible to observe directly the production of very small concentrations of HCl (∼109 molecule cm-3) from the minor channel of the OH + ClO reaction. At room temperature and ∼100 Torr pressure, the rate constant for this minor channel was determined to be (9.5 ± 1.6) × 10-13 cm3 molecule-1 s-1 with a two standard deviation error limit, which remained unchanged when the pressure was increased to 200 Torr. The temperature dependence of the rate constant for this minor channel was also investigated between 207 and 298 K, and the data were fit to the following Arrhenius expression: (3.2 ± 0.8) × 10-13 exp[(325 ± 60)/T] cm3 molecule-1 s-1. The branching ratio for the HCl channel was determined to be 0.07 ± 0.03 at all pressures and temperatures investigated in this study. Statistical rate theory calculations were also performed on the OH + ClO reaction system and are in good agreement with the experimental results.