Kinetics of Cl Atom Reactions with Methane, Ethane, and Propane from 292 to 800 K

Absolute rate coefficients for the reactions of chlorine atom with methane and ethane between 292 and 800 K and with propane between 292 and 700 K have been determined using the laser photolysis/continuous wave infrared long-path absorption method, LP/cwIRLPA. A novel reactor design and optical arrangement allow long absorption paths with precise control of the temperature in the probed volume. The rate coefficient for methane exhibits significant curvature between 292 and 800 K and can be described over this temperature range by a modified Arrhenius expression, k CH 4 ( T ) = [3.7( ) × 10 -13 cm 3 molecule -1 s -1 ]( T/ 298) 2.6(±0.7) exp[−385(±320)/ T ] (all error bars are ±2σ precision only). In the temperature range 292−600 K the rate with ethane agrees well with earlier investigations, fitting a simple Arrhenius expression k C 2 H 6 ( T ) = 8.6(±0.5) × 10 -11 exp[−135(±26)/ T ] cm 3 molecule -1 s -1 . However, as the temperature increases beyond 600 K the Arrhenius plot exhibits significant upward curvature. Over the range 292−800 K a three-parameter Arrhenius fit, k C 2 H 6 ( T ) = 3.4(±1.4) × 10 -11 ( T /298) 0.7(±0.3) exp[150(±110)/ T ] cm 3 molecule -1 s -1, models the experimental data adequately. The rate coefficient for propane is found to be independent of temperature and equal to 1.38(±0.03) × 10 -10 cm 3 molecule -1 s -1 .