Theoretical study of the kinetics of the hydrogen abstraction from methanol.: 2.: Reaction of methanol with chlorine and bromine atoms

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.; Sander, 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 x 10(-12) (T/300)(2.6) exp(-2975/T) cm(3) molecule(-1) s(-1) and k(CH3O) = 2.7 x 10(-12) (T/300)(1.9) exp(-9825/T) cm(3) 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.