Theoretical study of the kinetics of reactions of the monohalogenated methanes with atomic chlorine
Ab initio calculations at the G2 level were used in a theoretical description of the kinetics and mechanism of the hydrogen abstraction reactions from fluoro-, chloro- and bromomethane by chlorine atoms. The profiles of the potential energy surfaces show that mechanism of the reactions under investigation is complex and consists of two - in the case of CH3F+Cl - and of three elementary steps for CH3Cl+Cl and CH3Br+Cl. The heights of the energy barrier related to the H-abstraction are of 8-10 kJ mol(-1), the lowest value corresponds to CH3Cl+Cl and the highest one to CH3F+Cl. The rate constants were calculated using the theoretical method based on the RRKM theory and the simplified version of the statistical adiabatic channel model. The kinetic equations derived in this study k(CH3F + Cl) = 6.75 x 10(-12) x (T/300)(2.12) x exp(-900/T) cm(3)molecule(-1)s(-1) k(CH3Cl + Cl) = 6.97 x 10(-12) x (T/300)(1.73) x exp(-795/T) cm(3)molecule(-1)s(-1) k(CH3Br + Cl) = 6.26 x 10(-12) x (T/300)(1.82) x exp(-795/T) cm(3)molecule(-1)s(-1) allow a description of the kinetics of the reactions under investigation in the temperature range of 200-3000 K. The kinetics of reactions of the entirely deuterated reactants were also included in the kinetic analysis. Results of ab initio calculations show that D-abstraction process is related with the energy barrier of 5 kJ mol(-1) higher than the H-abstraction from the corresponding non-deuterated reactant molecule. The derived analytical equations for the reactions, CD3X+Cl, CH2X+HCl and CD2X+DCl (X = F, Cl and Br) are a substantial supplement of the kinetic data necessary for the description and modeling of the processes of importance in the atmospheric chemistry.
hydrogen abstraction chemical kinetics gas-phase reactions halomethanes kinetic isotope effect