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

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.