Theoretical Study On The Mechanism And Degradation Of Several Fluorinated And Chlorofluorinated Esters In The Atmosphere

The environmental pollution has received extensive attention all over the world; therefore, how to effectively control and manage air pollution is an important issue in the research of atmospheric science. The speeding up of industrialization and rapid economic growth have caused the growing environmental pollution. In some areas, the phenomenon of air pollution such as acid rain, photochemical smog and greenhouse effect occurrs frequently. Studying of the chemical reaction of the air pollutants in the atmosphere and understanding their degradation mechanism may provide a possible way to solve the air pollution problem. Chlorofluorocarbons（CFCs） are widely used as industrial refrigerants, foaming agent and detergent due to their excellent physical and chemical properties. However, with the increasing concentration of CFCs, the study shows CFCs can lead to ozone depletion in the stratosphere and cause the greenhouse effect after entering the atomosphere. Therefore, chlorofluorocarbons have been banned all over the word, and searching new alternatives of CFCs has been become the global focus in recent years.Both HFEs and HCFEs have been seen as the third generation of CFCs’ s alternatives because they not only have similar physical and chemical properties to CFCs, but also are expected to have greater reactivity and shorter atmospheric lifetimes in the troposphere with the introduction of ether linkage-O-. The main degradation products of HFEs and HCFEs in the atmosphere are the corresponding hydroflurinated esters（FESs） and chlorofluorinated esters（CFESs）, respectively. With the strong infrared radiation（IR） activity of the C-F bonds, FESs and CFESs may contribute to the global warming potentials, and the chlorine atoms in CFESs can cause ozone depletion in the stratosphere. The reaction with atmospheric OH radicals is the dominant degradation process of these compounds. Therefore, in order to effectively assess the impact on air pollution and the global warming of FESs and CFESs, it is necessary to obtain accurate datas of the rate constants and a full understanding of their degradation reaction mechanism with the OH radicals.In this thesis, we mainly focus on the degradation mechanism of CF₂HC（O）OCH₃, CF₃C（O）OCH₂CH₃, CF₂ Cl C（O）OCH₂CH₃ and CF₂ Cl C（O）OCH₃ with OH radicals in the atmosphere. The quantum chemistry conbined with theoretical calculation method of molecular reaction dynamics is employed. We obtain the potential energy surface of the four reactions above at the level of MCG3-MPWB//M06-2X/aug-cc-p VDZ and determine the reaction mechanism. The dual-level potential surfaces of the reactions are obtained by using the interpolated single-point energy（ISPE） method. The rate constants are calculated by the improved canonical variational transition state theory with small-curvature tunneling correction（ICVT/SCT） in the temperature range of 200-1000 K. We predict the reaction channels and the branching ratios over a wide temperature range. The main results are summarized as follows:（1） The theoretical study on the reaction of CF₂HC（O）OCH₃ + OH. At the level of M06-2X/aug-cc-p VDZ, two stable conformers are identified at the M06-2X/aug-cc-p VDZ level, CF₂HC（O）OCH₃（?） with Cs symmetry and CF₂HC（O）OCH₃（II） with C1 symmetry. The CF₂HC（O）OCH₃（?） is found to be more stable than the CF₂HC（O）OCH₃（II） by about 0.39 kcal mol-1. For the accuracy of the verification of the transition state and steady structure information we use the B3 LYP method to further optimize the geometric structure of all stationary points. The calculation results of the two methods are very close. For each conformer, the possible H-abstraction channels from-CH₃ and-CF₂ H groups are taken into account. At the level of MCG3-MPWB//M06-2X/aug-cc-p VDZ, the dynamic calculation results show that for CF₂HC（O）OCH₂（?）, the H-abstraction reactions mainly take place at the-CH₃ group over the whole temperature range, and for CF₂HC（O）OCH₃（II）, the H-abstractions from the-CH₃ group are the major channels at low temperature, while the H-abstractions from the-CF₂ H group become more important than those from-CH₃ group with the increasing of temperature. Theoretical results show reasonable agreement between the calculated rate constant and the available experimental data at 298 K. The three parameter expressions for each conformer and the overall reaction within 200-1000 K are k1 = 4.58×10-25T4.01 exp（-720/T） cm3 molecule-1 s-1, k2 = 7.04×10-23T3.41 exp（-489/T） cm3 molecule-1 s-1, koverall = 7.31×10-24T3.68 exp（-551/T） cm3 molecule-1 s-1, respectively. In addition, the contribution of CF₂HC（O）OCH₃（?） to the overall reaction is more important at low temperature, while the contribution of CF₂HC（O）OCH₃（II） is more important at high temperature.（2） The mechanism and the rate constant of the hydrogen abstraction reaction CF₃C（O）OCH₂CH₃ + OH has been studied. At the MCG3-MPWB//M06-2X/aug-cc-p VDZ level, Two conformers of CF₃C（O）OCH₂CH₃（? and II） are located, in which CF₃C（O）OCH₂CH₃（?） is about 0.47 kcal mol-1 more stable than CF₃C（O）OCH₂CH₃（II）. Three distinct hydrogen abstraction channels are identified for the reaction of conformer ? + OH（R1）, and five hydrogen abstraction channels are ascertained for the reaction of conformer II + OH（R2）. The rate constants for each reaction channel are calculated using ICVT/SCT over a temperature range of 200-1000 K. For reactions of R1 and R2, the channels of H-abstraction from-CH₃ groups are the major reaction channels at low temperatures, while the H-abstraction form-CH₂- sites become competitive channels with the temperature increasing. The two conformers have competitive contribution to the overall reaction in the low temperature range（T<250 K）; while at T>250 K, CF₃C（O）OCH₂CH₃（II） becomes the major one. The total reaction rate constant is in good agreement with the experimental value at room temperature. In order to provide reference for the experimental study, the fitted three-parameter expression over a temperature range of 200-1000 K for these two reactions are given as（in units of cm3 molecule-1 s-1）: k1 = 1.92×10-20T2.69 exp（-592/T）; k2 = 1.20×10-20T2.90 exp（-550/T）; koverall = 8.43×10-21T2.89 exp（-573/T）.（3） The mechanism of the hydrogen abstraction reaction of CF₂Cl（O）OCH₂CH₃ + OH has been studied theoretically by a dual-level direct dynamics method at the MCG3-MPWB//M06-2X/aug-cc-p VDZ level. Five stable conformers（RC1-RC5） of the reactant CF₂Cl（O）OCH₂CH₃ are located, and for each conformer, the possible H-abstraction channels from-CH₃ and-CH₂- groups are taken into account. The rate constants are calculated using the ICVT/SCT and the selectivity of the reaction sites of CF₂Cl（O）OCH₂CH₃ is evaluated. Our results show that for the conformers RC1 and RC2, the H-abstraction reactions mainly take place at the-CH₂- group at low temperature, while for the conformers RC3, RC4 and RC5, the hydrogen abstraction from the-CH₃ group is the major channel in the whole considered temperature range. The overall rate constant is obtained by considering the weight factors of the five conformers calculated from the Boltzmannn distribution function. It is found that the calculated koverall at 298 K is in good agreement with the available experimental data. The three parameter expression for the overall reaction within 200-1000 K is fitted to koverall = 5.45×10-25T4.54 exp（-685/T） cm3 molecule-1 s-1.（4） The mechanism of the CF₂Cl（O）OCH₃ + OH reaction has been investigated at the MCG3-MPWB//M06-2X/aug-cc-p VDZ and MCG3-MPWB//M06-2X/aug-cc-p VDZ level. There are two distinguishable stable conformers（RC1 and RC2） for the reactant CF₂Cl（O）OCH₃, and four H-abstraction channels as well as one substitution channel associated with them. It is shown that the latter one should be negligible. The rate constants for each of the H-abstraction channels are evaluated by the improved canonical variational transition theory（ICVT） with the small-curvature tunneling（SCT） approximation at the M06-2X/ aug-cc-p VDZ level. The overall rate constant（k T） is obtained by considering the weight factor of each conformer from the Boltzmann distribution function, and the calculated value agrees well with the available experimental value. A three-parameter rate constant-temperature expression for the total reaction within 200-1000 K is fitted to: k T=1.60×10-23T3.01 exp（-798/T）. The dynamic calculation results also show that the conformer RC1 has greater contrubution to the total reaction in the whole temperature, while the conformer RC2 has less contrubution to the total reaction. In addition, the atmospheric lifetimes of CF₂HC（O）OCH₃, CF₃C（O）OCH₂CH₃, CF₂Cl（O）OCH₂CH₃ and CF₂Cl（O）OCH₃ are estimated for 144 days, 87 days, 10 days and 7days, respectively.