A DIRECT INVESTIGATION OF THE REACTION CH3- OVERALL RATE-CONSTANT ANDCH2 FORMATION AT T=298K(OH )

The rate constant and the product distribution of the reaction CH3+OH --> products (1) were studied at room temperature over a range of pres sures (0.7 mbar less than or equal to p less than or equal to 467 mbar ) with M=He as inert bath gas. At pressures of 45 to 467 mbar, towards the high pressure range of the reaction, experimental determinations were performed using the laser flash photolysis technique with transie nt UV absorption spectrometry (LFP/TAS) for direct monitoring of the C H3 and OH radicals. Ln this range. the overall rate constant for the r eaction was found to be practically independent of pressure at k(l) (2 98 K)=(4.4+/-0.8)x10(13) cm(3) mol(-1) s(-1). At pressures of 0.7 mbar to 4.0 mbar; the reaction was studied using thr discharge flow (DF) t echnique with laser magnetic resonance (LMR) for the direct detection of the reactants CH3 and OH and the reaction product CH2. The measured overall rate constants at the low pressures were found to increase sl ightly with increasing pressure and approach the value at ''high'' pre ssure from the LFP/TAS experiments. From simulations of the measured c oncentration-versus-time profiles of the CH3, OH, and CH2 radicals, th e branching ratio for the reaction channel CH3+OH --> CH2+H2O (1.1) wa s determined at p=1.33 mbar, T=298 K to be k(1.1)/k(1)=(0.89+/-0.09). Implications of the results are discussed considering the fall-off cur ve for the reaction, the reaction mechanism and the product distributi on, and the available thermochemical data.