Theoretical study of the kinetics of chlorine atom abstraction from chloromethanes by atomic chlorine
Ab initio calculations at the G3 level were used in a theoretical description of the kinetics and mechanism of the chlorine abstraction reactions from mono-, di-, tri- and tetra-chloromethane by chlorine atoms. The calculated profiles of the potential energy surface of the reaction systems show that the mechanism of the studied reactions is complex and the Cl-abstraction proceeds via the formation of intermediate complexes. The multi-step reaction mechanism consists of two elementary steps in the case of CCl4 + Cl, and three for the other reactions. Rate constants were calculated using the theoretical method based on the RRKM theory and the simplified version of the statistical adiabatic channel model. The temperature dependencies of the calculated rate constants can be expressed, in temperature range of 200-3,000 K as k(CH3Cl + Cl) = 2.08 x 10 (11) x (T/300)(1.63) x exp (-12780/T) cm(3)molecule (1)s (1) k(CH2Cl2 + Cl) = 2.36 x 10 (11) x (T/300)(1.23) x exp (-1960/T) cm(3)molecule (1)s (1) k(CHCl3 + Cl) = 5.28 x 10 (11) x (T/300)(0.97) x exp (-9200/T) cm(3)molecule (1)s (1) k(CCl4 + Cl) = 1.51 x 10 (11) x (T/300)(0.58) x exp (-7790/T) cm(3)molecule (1)s (1) The rate constants for the reverse reactions CH3/CH2Cl/CHCl2/CCl3 + Cl-2 were calculated via the equilibrium constants derived theoretically. The kinetic equations k(CH3 + Cl-2) = 6.70 x 10(-13) x (T/300)(1.51) x exp(270/T) cm(3)molecule(-1)s(-1) k(CH2Cl + Cl-2) = 7.64 x 10(-14) x (T/300)(1.43) x exp(390/T) cm(3)molecule(-1)s(-1) k(CHCl2 + Cl-2) = 6.81 x 10(-14) x (T/300)(1.60) x exp(-370/T) cm(3)molecule(-1)s(-1) k(CCl3 + Cl-2) = 6.81 x 10(-13) x (T/300)(1.52) x exp(-550/T) cm(3)molecule(-1)s(-1) allow a very good description of the reaction kinetics. The derived expressions are a substantial supplement to the kinetic data necessary to describe and model the complex gas-phase reactions of importance in combustion and atmospheric chemistry.
chloromethanes chlorine abstraction reaction mechanism gas-phase reactions chemical kinetics