Temperature-Dependent Kinetics of the Gas-Phase Reactions of OH with Cl2, CH4, and C3H8

Abstract

The reactions of OH with molecular chlorine (reaction 1), methane (reaction 2), and propane (reaction 3) have been studied experimentally using a pulsed laser photolysis/pulsed-laser-induced fluorescence technique over wide ranges of temperatures (297−826, 298−1009, and 296−908 K, respectively) and at pressures between 6.68 and 24.15 kPa. The rate coefficients obtained for reactions 1−3 demonstrate no dependence on pressure and exhibit positive temperature dependences that can be represented with modified three-parameter Arrhenius expressions within their corresponding experimental temperature ranges: k1 = 3.59 × 10-16 T1.35 exp(−745 K/T) cm3 molecule-1 s-1, k2 = 3.82 × 10-19 T2.38 exp(−1136 K/T) cm3 molecule-1 s-1, and k3 = 6.64 × 10-16 T1.46 exp(−271 K/T) cm3 molecule-1 s-1. For the OH + Cl2 reaction, the potential energy surface has been studied using quantum chemical methods, and a transition-state theory model has been developed on the basis of calculations and experimental data. Model predictions suggest OH + Cl2 → HOCl + Cl as the main channel of this reaction. The model results in the expression k1 = 1.35 × 10-16 T1.50 exp(−723 K/T) cm3 molecule-1 s-1 for the temperature dependence of the reaction 1 rate coefficient extrapolation outside the experimental range to low temperatures down to 200 K and to high temperatures up to 3000 K. A temperature dependence of the rate coefficient of the HOCl + Cl → OH + Cl2 reaction has been derived on the basis of the experimental data, modeling, and thermochemical information.