Theoretical study for the reaction of CH₃OCl with Cl atom

A direct dynamics method is employed to study the kinetics of the multiple channel reaction CH₃OCl + Cl. The potential energy surface (PES) information is explored from ab initio calculations. Two reaction channels, Cl-and H-abstractions, have been identified. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) are calculated at the MP2 level of theory using the 6-311G(d, p) and cc-pVTZ basis sets, respectively. The single-point energies along the MEPs are further refined at the G3(MP2)//MP2/6-311G(d, p), G3//MP2/6-311G(d, p), as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MP2/cc-pVTZ geometries. The enthalpies of formation for the species CH ₃OCl and CH₂OCl are calculated via isodesmic reactions. The rate constants of the two reaction channels are evaluated by using the variational transition-state theory over a wide range of temperature, 220-2200 K. The calculated rate constants exhibit the slightly negative temperature dependence and show good agreement with the available experimental data at room temperature at the G3(MP2)//MP2/6-311G(d, p) level. The present calculations indicate that the two channels are competitive at low temperatures while H-abstraction plays a more important role with the increase of temperature. The calculated k_1a/k₁ ratio of 0.5 at 298 K is in general agreement with the experimental one, 0.8 ± 0.2. The high rate constant for CH₃OCl + Cl shows that removal by reaction with Cl atom is a potentially important loss process for CH₃OCl in the polar stratosphere.