Ab initio direct dynamics studies on the reactions of chlorine atom with CH 3 − nCl nCHO ( n = 1–3)

Abstract

The H-abstraction reactions CH 3 − n Cl n CHO + 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 CH 2ClCHO, CHCl 2CHO, i.e., there are two probable channels for reactions (R1) and (R2). The enthalpies of formation for the species CH 2ClCHO, CHCl 2CHO, CCl 3CHO, and the radicals CH 2ClCO, CHClCHO, CHCl 2CO, CCl 2CHO, CCl 3CO 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.