Theoretical studies on dynamics and thermochemistry of the reactions CHClFCHO, CHF2CHO and CClF2CHO with the Cl atom

The theoretical investigations are performed on the reaction mechanisms, including hydrogen abstraction and addition reaction channels for the three reactions CHClFCHO + Cl -> products (R1), CClF2CHO + Cl -> products (R2), and CCIF2CHO + Cl -> products (R3) by ab initio direct dynamics approach. The electronic structure information for the stationary points is obtained at the MP2 level of theory using the cc-pVDZ and aug-cc-pVDZ basis sets. The classical energy profile is refined by multi-coefficient correlation method based on quadratic configuration interaction with single and double excitation (MC-QCISD) using the MP2 optimized geometries. The enthalpies of formation for the reactants and product radicals involved in the three reactions are estimated at the MC-QCISD//MP2 level via isodesmic reactions. We find that each addition reaction pathway has a much higher potential energy barrier, and therefore its contribution to the total rate constants can be neglected. The rate constants for the H-abstraction reactions, which are evaluated by canonical variational transition state theory with the small-curvature tunneling correction over a range of temperatures from 220 to 2000 K, are in good agreement with the available experimental values. The Arrhenius expressions are fitted to be (in cm(3) molecule(-1) s(-1)) k(1) = 5.08 x 10(-160) T-1.60 exp (244.6/T), k(2) = 4.80 x 10(-17) T-1.86 exp (274.9/T), and k(3) = 2.34 x 10(-16) T-1.67 exp(37.1/T), respectively. Our conclusions show that for reaction CHCIFCHO + Cl -> products (R1), the channel of hydrogen abstraction from the formyl (-CHO) position is the primary pathway at low temperature, but as the temperature increases the hydrogen abstraction from the -CHClF group is more probable. While for reaction CHF2CHO + Cl -> products (R2), the pathway of hydrogen abstraction from the formyl position is always the primary channel over the whole temperature range. We also find that the halogen substitute (F or Cl-substitution) reduces the reactivity of the corresponding species. (c) 2005 Elsevier B.V. All rights reserved.