Theoretical Studies On The Mechanisms Of The Reactions Of CH₃OCF₂CF₂OCH₃ With Oh Radicals And Cl Atoms

Recognition of the adverse environmental impact of chlorofluorocarbon(CFC) release into the atmosphere has led to an international effort to replace CFCs with environmentally acceptable alternatives. A new class of hydrofluoropolyethers(HFPEs) having the structure CH₃O(CF₂CF₂O)n(CF₂O)mCH₃ have been developed as potential CFC replacements. Compared with other fluorinated compounds, HFPEs have high thermal and chemical stability and excellent heat exchange characteristics. HFPEs contain no chlorine and hence do not contribute to stratospheric ozone. However, due to the presence of the C-F bond, HFPEs may still contribute to the global warming. Thus, prior to large-scale industrial use an assessment of the atmospheric chemistry, and hence environmental impact, of these compounds is needed.CH₃OCF₂CF₂OCH₃ is one of the simplest hydrofluoropolyethers (HFPEs) having the structure CH₃O(CF₂CF₂O)n(CF₂O)mCH₃. As far as we know, only one kinetic study focusing on CH₃OCF₂CF₂OCH₃ has been reported. The major atomspheric degradation processes of CH₃OCF₂CF₂OCH₃ are the hydrogen abstraction reactions with OH radicals or Cl atoms under both atmospheric and combustion conditions. To ascertain the environmental impact of them released into the troposhere, further theoretical studies of the reactions CH₃OCF₂CF₂OCH₃ with OH radicals or Cl atoms on the mechanisms and predicting the rate constants over a wide temperature range are very necessary.In our work, direct density functional theory dynamics methods have been employed to investigate the kinetics properties. Direct ab initio dynamics allows studies of detailed dynamics of chemical reactions from first principles. This method is based on a full variational transition state theory plus multidimensional semiclassical adiabatic ground-state tunneling approximations with the potential energy information to be calculated directly from a sufficiently accurate level of ab initio molecular orbital theory. Here, the newly developed BB1K method (Becke88-Becke95 one-parameter model for kinetics methods) is applied in the potential energy surface calculation. BB1K in conjunction with the 6-31+G(d,p) basis set was stated to be a very powerful and accurate method for radical hydrogen abstraction and hydrogen transfer reactions, to provide good saddle point geometries, barrier heights, and reaction path information (the first and second derivatives of PES). Each stationary point on the PES is characterized by harmonic vibrational frequency analysis, that is, the local minima possess all real frequencies, while the transition states (TSs) have only one imaginary frequency. Specially, the reactant CH₃OCF₂CF₂OCH₃ has two stable conformers SC1 (C2h) and SC2 (C₂), and SC2 is 0.14 kcal/mol more stable than SC1. For SC1 with C2h symmetry, two kinds of channels are feasible for the H-abstraction from the -CH₃ group, namely,"out-of-plane hydrogen abstraction"and"in-plane hydrogen abstraction". However, in our calculations, it was clearly observed that the transiton-state optimization for the in-plane hydrogen abstraction eventually led to the same TS structure corresponding to the abstraction from out-of-plane hydrogen. So only one distinct H-abstraction TS for the reactions of SC1+OH and SC1+Cl has been located. With respect to conformer SC2 with C2 symmetry, the H atoms in each–CH₃ group are not equivalent, and as a result, we have located two distinct hydrogen abstraction channels. Similar as the case of SC1, the transition state for the abstraction from the H atom (H1 in SC2) in the C-O-C plane can not be found. Therefore, there are two distinct H-abstraction channels for the reaction of SC2 + OH or SC2 + Cl. Meanwhile, the possible displacement processes are also considered for the above reactions. Theoretical calculations suggest that the title reactions proceed exclusively via hydrogen abstraction channels, while the contributions of the displacement processes to the overall reaction are negligible due to the higher barriers.Based on the above the potential energy surface, the theoretical rate constants for all above reactions are calculated by improved canonical variational transition state theory (ICVT) with the small-curvature tunneling correction (SCT) within 200-2000 K. The overall rate constants for CH₃OCF₂CF₂OCH₃ (SC1 or SC2) + OH/Cl reactions are evaluated by considering the weight factor of each conformer from the Boltzman distribution function. The calculated ICVT/SCT rate constants are found to be in good agreement with the only available experimental value at 296 K. It is shown that for reactions CH₃OCF₂CF₂OCH₃ (SC1 or SC2) + OH/Cl, the hydrogen abstraction from conformer SC2 dominates the reaction over the whole temperature range. Since there is no data available at other temperatures, for convenience of future experimental measurements, the three-parameter fits based on the ICVT/SCT rate constants for the title reactions within 200-2000K are given as follows (in cm³molecule^-1s^-1): kOH = 1.56×10^-20T^2.47exp(-124.64/T) and k_Cl = 0.40×10^-14T^1.05exp(-206.16/T). In addition, it is well known that the knowledge of the enthalpy of formation of the molecule or radicals is important in evaluating their thermodynamic and kinetic properties in the atmospheric processes. However, most of the species involved in the above reactions have been little studied experimentally. Here, the enthalpies of formation of the reactant CH₃OCF₂CF₂OCH₃ and product radicals CH₃OCF₂CF₂OCH₂ and CH₃OCF₂CF₂O were evaluated via isodesmic reactions. We hope that the theoretical results may be helpful for further experimental study.