Kinetics of the Cl(p-2(J)+CH4 reaction:: Effects of secondary chemistry below 300 K

Absolute rate data for the Cl(P-2(J)) + CH4 --> HCl + CH3 reaction have been obtained from 218 to 298 K by using the discharge flow resonance fluorescence technique in helium at 1 Torr total pressure. The result at 298 K is (10.1 +/- 0.6) x 10(-14) cm(3) molecule(-1) s(-1). The temperature dependence in Arrhenius form is (6.5 +/- 0.9) x 10(-12) exp[(-1235 +/- 34)/T]. The errors given are one standard deviation; overall experimental error is estimated at +/-15%. Because of the relatively large disagreement among earlier measurements at low temperatures, the results were examined for possible effects of non-Boltzmann spin distribution and vibrational excitation of CH4, secondary chemistry of CH3 radicals, and impurities in the Cl atom and CH4 sources. There was no significant change in the observed rate constant when an efficient spin quencher, CF4, was added, and estimates indicate that vibrational partitioning in CH4 should be at the ambient reactor temperature before the start of the reaction. The results were also independent of the source of Cl atoms (microwave discharge or thermal decomposition of Cl-2) and whether CH4, was purified in situ. However, the observed rate constant did depend on initial Cl atom concentrations and to a lesser extent on CH4 concentrations. Numerical simulations were used to assess the importance of secondary chemistry over a range of reactant concentrations.