Ab initio calculations at different levels of theory have been performed for the title H-abstraction reactions. Total energies at stationary points of the potential energy surfaces for the reaction systems were obtained at MP2 and MP4 levels and improved by using Gaussian-2 (G2) methodology. The calculated G2 heats of reaction agree well with the experimental ones for both methoxy (product resulting from hydroxyl-side attack) and hydroxymethyl (product resulting from methyl-side attack) reaction channels. Calculations of the potential energy surfaces for the reaction systems show that H-abstraction from methanol by H, CH 3 , and OH (for methoxy reaction channel) proceeds by simple metathesis. The mechanism of the hydroxymethyl channel of reaction CH 3 OH + OH appears to be more complex, and it may consist of two consecutive processes. The reaction rate is determined by the energy barrier of the first process. Differences in the heights of the calculated energy barriers explain the differences in the reactivity of H, CH 3 , and OH toward methanol. The calculated barriers indicate a significant dominance of the hydroxymethyl formation channel for the CH 3 OH + H and CH 3 OH + OH reaction systems. Rationalization of the derived energy barriers has been made in terms of the polar effect. The calculated rate constants are in very good agreement 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 methanol combustion.

Kinetics and mechanism of the reaction of OH with have been studied by discharge-Ñow CH 3 C(O)CH

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Ab initio calculations at different levels of theory and using several basis sets were performed for the title two-channel hydrogen-abstraction reaction. Conclusions are drawn from G2 energies. These calculations have shown that this reaction, which can give two products (namely, CH 3 O and CH 2 OH), proceeds through the formation of intermediate complexes followed by transition states with quite negligible activation energy. We propose a method for the calculation of the rate constant of a bimolecular reaction proceeding through the formation of two intermediate complexes. General equations, taking into account the rotational energy, are derived from RRKM theory, using the simplified version of the SACM theory. The resulting calculated overall rate constant as well as the yield of the methoxy branching ratio are in very good agreement with experimental findings. The expressions for the site-specific rate constants k(CH 3 O) and k(CH 2 OH) allow the description of the reaction kinetics over a wide range of temperatures. A temperature rate constant fit, convenient for chemical modeling studies, is k(CH 3 O) ) 1.0 × 10 -10 (T/300) 0.5 cm 3 molecule -1 s -1 and k(CH 2 OH) ) 6.9 × 10 -11 (T/300) 0.27 cm 3 molecule -1 s -1 .

The mechanism of the reaction of OH with acetone has been studied by quantum chemical computations. 21 stationary points (among them reactant complexes, reaction transition states, intermediate complexes and product complexes) have been characterised on the potential energy surface of the reaction. The MP2 method with 6-31G(d,p) basis set was employed for geometry optimisation. Electronic energies were obtained at the CCSD(T)/6-311G(d,p) level of theory. Hydrogen abstraction was found to occur through two complex mechanisms; no transition state for direct abstraction could be located. Minimum energy path analyses have revealed two distinct pathways which lead to CH 3 (+CH 3 COOH) formation. One of them sets out the abstraction channel and proceeds via intermolecular complexes and the other one involves addition of OH to the carbonyl double bond and subsequent decomposition of the adduct hydroxy-alkoxy radical. The rate limiting steps involve large energy barriers and, consequently, these pathways do not explain the high methyl yields observed experimentally at and below room temperature. Characteristic for the reaction of OH with acetone is the existence of numerous hydrogen-bridged complexes on the potential energy surface that are stabilised by as much as 3.2-26.6 kJ mol À1 binding energy. Some properties of these complexes and their possible role in the molecular mechanism of the reaction are discussed.y Electronic supplementary information (ESI) available: Harmonic vibrational frequences and MP2 structural parameters for intermolecular complexes, reactants, transition states and reaction products. See