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Abstract: The dynamic properties of the hydrogen abstraction reactions of CF2H2 and CF3H with F atom are investigated in the temperature range of 182–2000K. The minimum-energy path (MEP) is optimized at MP2/6-311G(d, p) level, then the energy profiles are refined at the CCSD(T)/6-311++G(3df, 2pd) level (single-point). The theoretical rate constants, which are calculated by the variational transition state theory (VTST) including the small curvature tunneling (SCT) correction, are in good agreement with the experimental ones. It is found that the rate constant of the CF2H2 +F reaction are larger than that of the CF3H+F reaction and the activation energies exhibit in the just opposite order. This phenomenon can be rationalized by the hardness η of the halomethane molecules. The comparison of the two reactions with the CFH3 +F reaction is made. It is found that the rate constants decrease in the order of CFH3 +F>CF2H2 +F>CF3H+F. The effect of fluorine substitution leads to a dramatic increase in the activation energy and a decrease in the preexponential factor. We hope that present theoretical studies for these compounds can give further information concerning how fluorine substitution affects the rate constants of hydrogen abstraction reactions. [Copyright &y& Elsevier]

Abstract: Dual-level direct dynamics method is employed to investigate the H-abstraction reaction CF3CHOHCF3 with OH radical. Two hydrogen-abstraction reaction channels are possible: one from the methylene (–CH–) position and the other from the hydroxyl (–OH) position. The minimum energy path is calculated at the B3LYP/6-311G(d,p) level, and the energetic information is further refined by a new powerful and inexpensive BMC-CCSD method. To testify the accuracy of the structures and the energies, the recently developed hybrid density functional theory BB1K and higher level MC-QCISD are applied to this system. Hydrogen-bonded complexes are presented at both reactants and products sides of these two channels, which indicating that the reaction may proceed via an indirect mechanism. The rate constants for each reaction channel are evaluated by canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide range of temperatures from 200 to 2000K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values in the temperature region 250–430K. The present results indicate that the two channels are competitive. At lower temperature, the reaction occurs mainly via the hydroxyl-H-abstraction channel, while the methylene-H-abstraction channel is preferred when the temperature is higher than 273K. [Copyright &y& Elsevier]

Abstract: The theoretical investigations are performed on the reaction mechanisms, including hydrogen abstraction and addition reaction channels for the three reactions CHClFCHO+Cl→products (R1), CHF2CHO+Cl→products (R2), and CClF2CHO+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 2000K, are in good agreement with the available experimental values. The Arrhenius expressions are fitted to be (in cm3 molecule−1 s−1) k 1 =5.08×10−160 T 1.60 exp(244.6/T), k 2 =4.80×10−17 T 1.86 exp(274.9/T), and k 3 =2.34×10−16 T 1.67 exp(37.1/T), respectively. Our conclusions show that for reaction CHClFCHO+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. [Copyright &y& Elsevier]

Abstract: The H-abstraction reactions CH3− 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 (–CH3− n Cl n ) positions for CH2ClCHO, CHCl2CHO, i.e., there are two probable channels for reactions (R1) and (R2). The enthalpies of formation for the species CH2ClCHO, CHCl2CHO, 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 2000K. 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. [Copyright &y& Elsevier]

We present dual-level direct dynamics calculations for the CH3CHF2 + O(3P) hydrogen abstraction reaction in a wide temperature range, based on canonical variational transition-state theory including small curvature tunneling corrections. For this reaction, three distinct transition states, one for α-abstraction and two for β-abstraction, have been located. The potential energy surface information is obtained at the MP2(full)/6-311G(d,p) level of theory, and higher-level single-point calculations for the stationary points are preformed at several levels, namely QCISD(T)/6-311+G(3df,3pd), G2, and G3 using the MP2 geometries, as well as at the G3//MP4SDQ/6-311G(d,p) level. The energy profiles are further refined with the interpolated single-point energies method at the G3//MP2(full)/6-311G(d,p) level. The total rate constants match the experimental data reasonable well in the measured temperature range 1110–1340 K. It is shown that at low temperature α-abstraction may be the major reaction channel, while β-abstraction will have more contribution to the whole reaction rate as the temperature increases. [Copyright &y& Elsevier]

A dual-level direct dynamics method is employed for the dynamics of the hydrogen abstraction reaction CF3CHFCF3+O(3P) using density functional theory with the 6-311G(d, p) and 6-311++G(3df, 2p) (single-point) basis sets. Two DFT methods, B3LYP and B3PW91, produce similar results for the geometries, frequencies, and energetics of the stationary points, in good agreement with the corresponding experimental values. The classical energy profile is corrected with the interpolated single-point energy (ISPE) approach. The variational transition-state calculations are carried out with small curvature tunneling (SCT) at 300–2000 K. Agreement between the theoretical and experimental values is good for the rate constants and activation energy calculated at the B3LYP/6-311++G(3df, 2p)//B3LYP/6-311G(d, p) level in the measured temperature range 586–726 K. It is shown that the variational effect is significant for this reaction, and the tunneling effect is small. [Copyright &y& Elsevier]

The title reactions are investigated by ab initio direct dynamics method. The geometries for the reactants, products and transition states are optimized at the BH&H-LYP/6-311+G(d, p) level. With the aid of intrinsic reaction coordinate theory, the minimum energy path (MEP) is obtained at the same level, and the energies along the MEP are further refined by performing the single-point calculations at the QCISD(T)/6-311+G(3df, 2p) level. For both reactions, the calculated ICVT/SCT rate constants are in good agreement with the experimental results, and the variational effect is small and small-curvature tunneling effect is important only in the lower temperature range. [Copyright &y& Elsevier]

Abstract: By means of direct dynamics methods, the theoretical investigations are carried out on the multiple-channel reaction Br+CH2BrCl. The optimized geometries, frequencies, and minimum energy path (MEP) are all obtained at the MP2/6-311+G(d,p) level, and energetic information is further refined at the QCISD(T)/6-311+G(3df,2p) (single-point) level. The rate constants for three reaction channels, H-abstraction, Br-abstraction, and Cl-abstraction, are calculated by the improved canonical variational transition state theory (ICVT) with small-curvature tunneling (SCT) contributions over the wide temperature region 200–4000K. The theoretical three-parameter formula of overall rate constants is given as k =9.93×10−19 T 2.48 exp(−3890.89/T) (in cm3 molecule−1 s−1). Furthermore, the rate constants of reverse reaction CHBrCl+HBr→Br+CH2BrCl (R−1) are in good agreement with the available experimental data. For the title reaction, H-abstraction reaction channel is the predominant channel in the lower temperatures while as the temperature increases, the contributions of Br- and Cl-abstraction channels should be taken into account. [Copyright &y& Elsevier]

Abstract: By means of direct dynamics methods theoretical investigations are carried out on the multiple-channel reaction system Cl+CH3S(O)CH3. The optimized geometries, frequencies and minimum energy path (MEP) are all obtained at the BH&H-LYP/6-311G(2d,2p) level, and energetic information is further refined by the MC-QCISD (single-point) theory. The rate constants for two main reaction channels, to yield CH3S(O)CH2 +HCl and to yield CH3S(O)Cl+CH3, are calculated by canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contributions over the wide temperature region 200–2000K. The theoretical overall rate constants and the branching ratio are in good agreement with the available experimental data; and the three-parameter formula k =1.32×10−16 T 2.18exp(−48.12/T) cm3 molecule−1 s−1 over the temperature region 200–2000K is given. For the title reaction, CH3S(O)CH2 +HCl is the major product, and CH3S(O)Cl+CH3 is a minor product over the whole temperature region. [Copyright &y& Elsevier]