A direct investigation of the reaction CH₃+OH: Overall rate constant and CH₂ formation at T=298 K

The rate constant and the product distribution of the reaction CH 3 +OH r̊ products (1) were studied at room temperature over a range of pressures (0.7 mbar ≤p≤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 transient UV absorption spectrometry (LFP/TAS) for direct monitoring of the CH 3 and OH radicals. In this range, the overall rate constant for the reaction was found to be practically independent of pressure at k 1 (298 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 the discharge flow (DF) technique with laser magnetic resonance (LMR) for the direct detection of the reactants CH 3 and OH and the reaction product CH 2 . The measured overall rate constants at the low pressures were found to increase slightly with increasing pressure and approach the value at “high” pressure from the LFP/TAS experiments. From simulations of the measured concentration‐versus‐time profiles of the CH 3 , OH, and CH 2 radicals, the branching ratio for the reaction channel CH 3 +OHr̊ CH 2 +H 2 O (1.1) was determined at p =1.33 mbar, T =298 K to be k 1.1 lk 1 =(0.89±0.09). Implications of the results are discussed considering the fall‐off curve for the reaction, the reaction mechanism and the product distribution, and the available thermochemical data.