Temperature and Pressure Dependence of the Multichannel Rate Coefficients for the CH₃ + OH System

Laser flash photolysis measurements on the kinetics of the title system in a He bath gas are reported for the temperatures 290, 473, and 700 K and a pressure range of 7.6−678 Torr. CH 3 and OH radicals were generated simultaneously by 193 nm photolysis of acetone containing traces of water, with CH 3 in large excess. The course of reaction was followed by monitoring the concentration of OH using laser-induced fluorescence (LIF), while CH 3 was monitored by absorption at 216.4 nm. The experimental data were analyzed in the first instance by a simple model based on the decay of OH and CH 3, which was verified by more comprehensive numerical simulations. Further analysis of the data, using a combined master equation/inverse Laplace transform/RRKM procedure, yielded estimates of the association rate coefficient of CH 3 and OH ( = (8.0 ± 0.3) × 10 -11 ( T /300 K -0.79±0.09 cm 3 molecule -1 s -1 ) and for the enthalpy change for the CH 3 + OH → 1 CH 2 + H 2 O channel (Δ = 1.6 ± 2.0 kJ mol -1 ). Pressure-dependent rate coefficients for the various channels are also calculated and parameters for a modified Troe representation determined.