Ab initio direct dynamics studies on the reactions of chlorine atom with CH_{3 - N}Cl_nCHO (n = 1-3)

The H-abstraction reactions CH_{3 - n}Cl_nCHO + Cl (n = 1-3) ((R1)-(R3)) have been studied by the ab initio direct dynamics method. The potential energy surface (PES) information required for the rate constant calculation for each reaction is obtained at the MP2/cc-pVDZ and CCSD(T)/6-311+G(d, p) (single-point) levels of theory. The hydrogen atom can be abstracted from both the formyl (-CHO) positions and the chlorinated methyl (-CH_{3 - n}Cl_n) positions for CH₂ClCHO, CHCl ₂CHO, i.e., there are two probable channels for reactions (R1) and (R2). The enthalpies of formation for the species CH₂ClCHO, CHCl ₂CHO, CCl₃CHO, and the radicals CH₂ClCO, CHClCHO, CHCl₂CO, CCl₂CHO, CCl₃CO are computed at the CCSD(T)/6-311+G(d, p)//MP2/cc-pVDZ level. The rate constants for each reaction channel are evaluated by using improved canonical variational transition state theory (ICVT) with a small-curvature tunneling correction (SCT) over a wide range of temperatures from 220 to 2000 K. The calculated ICVT/SCT rate constants are found to be in good agreement with the available experimental values. It is shown that in the low temperature range, the reactions proceed predominantly via H-abstraction from formyl position, while the other H-abstraction channel from methyl should be taken into account with the increase of the temperature. Also, the reactivity decreases substantially with chloride substitution at the methyl position of acetaldehyde.