Theoretical study of the kinetics of the hydrogen abstraction from methanol. 3. Reaction of methanol with hydrogen atom, methyl, and hydroxyl radicals

Ab initio calculations at different levels of theory have been performed fo r the title H-abstraction reactions. Total energies at stationary points of the potential energy surfaces for the reaction systems were obtained at MP 2 and MP4 levels and improved by using Gaussian-2 (G2) methodology. The cal culated G2 heats of reaction agree well with the experimental ones for both methoxy (product resulting from hydroxyl-side attack) and hydroxymethyl (p roduct resulting from methyl-side attack) reaction channels. Calculations o f the potential energy surfaces for the reaction systems show that H-abstra ction from methanol by H, CH3, and OH (for methoxy reaction channel) procee ds by simple metathesis. The mechanism of the hydroxymethyl channel of reac tion CH3OH + OH appears to be more complex, and it may consist of two conse cutive processes. The reaction rate is determined by the energy barrier of the first process. Differences in the heights of the calculated energy barr iers explain the differences in the reactivity of H, CH3, and OH toward met hanol. The calculated barriers indicate a significant dominance of the hydr oxymethyl formation channel for the CH3OH + H and CH3OH + OH reaction syste ms. Rationalization of the derived energy barriers has been made in terms o f the polar effect. The calculated rate constants are in very good agreemen t with experiment and allow a description of the kinetics of the reactions under investigation in a wide temperature range with the precision that is required by practical applications such as modeling of the chemistry of met hanol combustion.