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.

No abstract

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 .

Ab initio calculations at different levels of theory and using several basis sets have been performed for the title two-channel hydrogen-abstraction reactions CH3OH + Cl and Br. These calculations have shown that, similar to CH3OH + F, both reactions proceed via formation of intermediate complexes. Rate constant calculations for this type of reactions have been performed using the equations developed in Jodkowski, J. T.; Rayez, M.-T.; Rayez, J.-C.; Berces, T.; Sandor, D. J. Phys. Chem. 1998, 102, xxxx. The very low energy barrier for the hydroxymethyl channel of the CH3OH + Cl reaction obtained at the G2 level explains the relatively high value of the rate constant for this almost thermoneutral reaction. The very weak negative temperature dependence of the rate constant leading to CH2OH + HCl observed experimentally is also well reproduced by the calculations. For the CH3OH + Br reaction, both channels are endothermic, which explains the low values of the rate constants. A good agreement is obtained with experimental results and the temperature dependence of the rate constants. A temperature fit of the rate constants allows us to express them in a convenient way for chemical modeling studies: k(CH2OH) = 4.8 × 10-12 (T/300)2.6 exp(−2975/T) cm3 molecule-1 s-1 and k(CH3O) = 2.7 × 10-12 (T/300)1.9 exp(−9825/T) cm3 molecule-1 s-1. In chlorine and bromine reactions, the channel leading to CH3O is inactive at least for temperatures below 1000 K. The calculated potential energy surfaces have also allowed the determination of the rate constants for the reverse reactions CH2OH + HCl and HBr which agree well with available experiments.