Theoretical Study of the Kinetics of the Hydrogen Abstraction from Methanol. 2. Reaction of Methanol with Chlorine and Bromine Atoms

Abstract

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.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II – gas phase reactions of organic species

“…The inclusion of the data of on Cl+CH 3 OD is justified considering the strong body of evidence showing that the H-abstraction proceeds solely at the CH 2 -group at room temperature (see , Radford et al, 1981and Jodkowski et al, 1998. The relative rate study of Wallington et al (1988) …”

On-the-fly ab initio trajectory calculations of the dynamics of Cl atom reactions with methane, ethane and methanol

“…The van der Waals wells associated with the encounter complexes are so shallow in the case of the methane and ethane surfaces that they are expected to have no impact upon the dynamics, which should therefore be direct. Calculations of stationary point energies and geometries for the reaction of Cl atoms with methanol demonstrate that reaction proceeds along a minimum energy pathway that incorporates lower energy preand post-transition state intermediate complexes, 1,23 bound by Ϫ4.64 and Ϫ9.64 kcal mol Ϫ1 , respectively, relative to the reagent energies. The differential cross sections reported by Ahmed et al 24 from crossed molecular beams and velocity map imaging experiments showed scattering that is predominantly in the backward/sideways direction, suggesting direct ͑despite the potential wells͒ rebound dynamics, but recent experiments from Kitsopoulos and co-workers demonstrate that there is also a strong forward scattered component.…”

Theoretical study of the kinetics of chlorine atom abstraction from chloromethanes by atomic chlorine

“…All computational effort is then related to calculating the sum of the states, W ( E , J ) This calculation depends on the level at which the conservation of angular momentum is considered and is discussed in detail in Refs. [21, 22]. …”

“…The results of direct calculations [13, 20–22] show that the dominant contribution to the rate constant is given by states with energy E not higher than V TSx  + 3RT. In the case of a sizable (compared with RT) energy barrier V TSx , the value of the product of the microcanonical branching fractions at an energy slightly higher than V TSx becomes close to unity and the TST rate constant k TST is then a good approximation of the exact rate coefficient, especially at ambient temperatures [13, 20–22].…”

“…In the case of a sizable (compared with RT) energy barrier V TSx , the value of the product of the microcanonical branching fractions at an energy slightly higher than V TSx becomes close to unity and the TST rate constant k TST is then a good approximation of the exact rate coefficient, especially at ambient temperatures [13, 20–22]. …”