Experimental and Theoretical Study of the Kinetics of the CH3 + HBr → CH4 + Br Reaction and the Temperature Dependence of the Activation Energy of CH4 + Br → CH3 + HBr

The rate coefficient of the reaction of CH3 with HBrwas measured and calculated in the temperature range 225-960K. The results of the measurements performed in a flow apparatus withmass spectrometric detection agree very well with the quasiclassicaltrajectory calculations performed on a previously developed potentialenergy surface. The experimental rate coefficients are described wellwith a double-exponential fit, k (1)(exp)= [1.44 x 10(-12) exp(219/T)+ 6.18 x 10(-11) exp(-3730/T)] cm(3) molecule(-1) s(-1). The individual rate coefficients below 500 K accord with the availableexperimental data as does the slightly negative activation energyin this temperature range, -1.82 kJ/mol. At higher temperatures,the activation energy was found to switch sign and it rises up toabout an order of magnitude larger positive value than that below500 K, and the rate coefficient is about 50% larger at 960 K thanthat around room temperature. The rate coefficients calculated withthe quasiclassical trajectory method display the same tendencies andare within about 8% of the experimental data between 960 and 300 Kand within 25% below that temperature. The significant variation ofthe magnitude of the activation energy can be reconciled with thetabulated heats of formation only if the activation energy of thereverse CH4 + Br reaction also significantly increaseswith the temperature.