Direct Dynamics Study On The Hydrogen Abstraction Reactions Of Several Hydrofluorocarbons And Certain Radicals

Hydrofluorocarbons (HFCs) are new potential fire suppressions due to the character of effective, nontoxic and low global environmental impact and widely used in industry as replacements of the ozone-destroying chlorofluorocarbon (CFCs) and halon fire suppression agents. Moreover, HFCs can result in greenhouse effect because of their strong absorption of the infrared radiation. Then, from the point of view of protecting environment effectively, many investigators pay considerable attention to the reaction mechanism and the kinetic property of HFCs. In this paper, the direct dynamics method is employed to study the hydrogen abstraction reaction of CH₃CH₂F with O(³P) CH_4-nF_n(n=1-3) with CH₃, CH₃F with C₂H₃, respectively. The reactions are as follows:The optimized geometries and harmonic vibrational frequencies of the reactants, products and transition states are calculated at the MP2, BHandHLYP and B3LYP levels, and the minimum energy paths are constructed by the intrinsic reaction coordinate (IRC) method. To gain more accurate potential energy surface, single point energies of all the species and the selected points along the minimum energy path are refined at the QCISD(T) level. We further employ the Polyrate 8.2 program to calculate the rate constants by the canonical variational transition state theory with small curvature tunneling correction. Our work will focus on three aspects:1. For the hydrogen abstraction reaction of CH₃CH₂2F and O(³P), the potential barriers of the three reaction channels R1 R2a and R2b are 46.3 60.2 and 58.9 kJ/mol at the QCISD(T)/6-311G(d,p)//MP2(full)/6-311G(d,p) level, respectively. The variational effect is small for all the channels, and the tunneling effect is considerable for the calculation of the rate constants in the lower temperature range. At 298 K, the rate constants of α-H and β-H abstraction reaction are 5.22x10¹⁷, 1.30x10¹⁹cm3molecule"1-s"1, respectively. When the temperature arrives at about 1 250 K, the values are 5.10><10'13, 5.22* 10"13 cm3-molecule"1-s'1, respectively. It shows that at the lower temperature the main process occurring is a-H abstraction, while 0-H abstraction will compete kinetically with the former as the temperature increases.2. For the hydrogen abstraction reaction of CH4-nFn(n=l ³) and CH3, the reaction energies Aï¿¡ of Rla. R2a and R3 are -12.7. -9.5 and 11.8 kJ/mol, and the corresponding potential barriers AE? are 67.0, 62.2 and 67.5 kJ/mol at the QCISD(T)/6-311+G(d,p)// BHandHLYP/6-311G(d,p) level, respectively. Compared with the influence of the fluorine atom number on the rate constants at the higher temperature, it is relatively large at the lower temperature. However, the rate constants still do not change regularly with the increasing number of the fluorine atom. At 437 K, the CVT/SCT rate constants of the three reactions are 6.72xlO"1!\ 8.01xl0"18 and 8.82x10"20cm3molecule"1s"1, which are in agreement with the available experimental values 3.31xlO"19, 1.05xl0"18 and 8.33xl0"20 cm3-molecule'1-s"1. The results indicate that the tunneling effect is considerable in the lower temperatures, while the variational effect is almost negligible over the entire process.3. Comparing the optimized geometries of the structures in the hydrogen abstraction reaction of CH3F and C2H3 at several different levels of theory, we found that the calculated results from B3LYP method are closer to the experimental and the high level QCISD/6-311G(d,p) results than those from MP2 method. In addition, the geometries of the species studied have no significant influence if enlarging the basis set 6-311G(d,p). At the QCISD(T)/6-311++G(d,p)//B3LYP/6-311G(d,p) level, the reaction energy is -38.2 kJ/mol, and the corresponding potential barriers AE* of the reaction Rl -. R2 and R3 are 43.2, 43.9 and 44.1 kJ/mol, respectively, which are in excellent agreement with the previous result of 43.1 kJ/mol. The tunneling effect is significant in the lower temperatures, while the variational effect is negligible over the entire process.The present study not only enriches the dynamics information of the above three reaction systems, but also provides a theoretical prediction for the further experimental measurements.