Theoretical Study of H-AbstractionReactions fromCH3Cl and CH3Br Molecules by ClO and BrO Radicals.

The rate constants of the H-abstraction reactions fromCH3Cl and CH3Br molecules by ClO and BrO radicalshave beenestimated over the temperature range of 300–2500 K using fourdifferent levels of theory. Calculations of optimized geometricalparameters and vibrational frequencies are performed using B3LYP andMP2 methods combined with the cc-pVTZ basis set. Single-point energycalculations have been carried out with the highly correlated ab initiocoupled cluster method in the space of single, double, and triple(perturbatively) electron excitations CCSD(T) using the cc-pVTZ andcc-pVQZ basis sets. Canonical transition-state theory combined withan Eckart tunneling correction has been used to predict the rate constantsas a function of temperature. In order to choose the appropriate levelsof theory with chlorine- and bromine-containing species, the referencereaction Cl (2P3/2) + CH3Cl →HCl + CH2Cl (Rref) was first theoretically studiedbecause its kinetic parameters are well-established from numerousexperiments, evaluation data, and theoretical studies. The kineticparameters of the reaction Rrefhave been determined accuratelyusing the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory. This levelof theory has been used for the rate constant estimation of the reactionsClO + CH3Cl (R1), ClO + CH3Br (R2), BrO + CH3Cl (R3), and BrO + CH3Br (R4). Six-parameter Arrhenius expressions havebeen obtained by fitting to the computed rate constants of these fourreactions (including cis and trans pathways) over the temperaturerange of 300–2500 K. [ABSTRACT FROM AUTHOR]