Theoretical Study On The Mechanism And Kinetics Of HFCs Degradation Reactions In Atmosphere

On the basis of the quantum chemistry theory and the transition state theory, the degradation reactions of 1,1,1,2-tetrafluoroethane (HFC-134a) and 1,1-difluoroethane (HFC-152a) in the atmosphere are investigated theoretically to reveal their mechanism and kinetics properties.â… Theoretical study on the degradation reaction mechanisms of HFC-134a in atmosphere.1. The hydrogen abstraction reactions of HFC-134a with hydroxyl radical and Cl atom are studied by theoretical calculations, respectively. Equilibrium molecular geometries and harmonic frequencies of the reactants ,transition state and products are calculated using high level ab initio method, the energy barriers of the two reactions are 7.63 and 10.40 kcal/mol, respectively. Rate constants over the temperature range of 200-1000K are calculated using transition state theory with Eckart tunneling corrections, fitted in the following expressions: HFC-134a + OH:k(T) =1.57×10^-27×T4.88e^(246/T) cm³ molecule^-1 s^-1 HFC-134a + Cl: kf (T)=4.15×10^-24T^4.17e ^(-970/T) cm³ molecule^-1 s^-1 The calculated results are reasonable agree with the available experimental values.2 The mechanism of CHFCF₃ degradation reactions in atmosphere.(1) By using quantum chemistry method, the oxidation reaction of CHFCF₃ with O₂ have been investigated. There are two reaction pathways ,one is CHFCF₃ with O₂(³Î£_g^-) leading to CHF(O)CF₃ and O(³P), anther is reaction with O₂(¹Î”_g) leading to CHF(OO)CF₃.The reaction barrier of the former is about 49.37kcal/mol higher than the latter .Thus the latter is the mainly pathway ,and thermodynamically favorable to form the products.(2) The reaction of CHF(OO)CF₃ with NO has been studied theoretically. All the geometries ,vibrational frequencies and energies of different stationary points are calculated. By vibrational mode analysis and population analysis ,two mainly channels are found. CHF(O)CF₃+NO2 are formed either through breaking the peroxy bond of the tran-conformer, or via rearrangement of the trans-conformer to cis-conformer and its subsequent dissociation.(3) The dissociation and oxidation reaction mechanisms of CHF(O)CF₃ are investigated by quantum chemistry method. Through the analysis the information attained in calculation ,the mechanism is explained. The reaction barriers of two pathways are 8.49 and -31.85kcal/mol,repectively .The mainly reaction pathways is oxidation reaction of CHF(O)CF₃with O)2(¹Î”_g) , leading to CF(O)CF₃ and HO₂ ,which may increasing the oxidation ability of atmosphere.(4)The mechanism of CF(O)CF₃ hydrolysis reaction is theoretical study ,by analyzing the calculated geometries ,vibrational frequencies and energies information. The reaction need pass through two higher barriers (37.12and 28.94kcal/mol). It is hard to form CF₃COOH in thermodynamical.â…¡Theoretical study on the degradation reaction mechanisms of HFC-152a in atmosphere.1. Quantum chemistry method is used to investigated the hydrogen abstraction reaction of HFC-152a by hydroxyl radical and Cl atom. Equilibrium molecular geometries and harmonic frequencies of the reactants ,transition state and products are calculated .One transition state is located forα-H abstraction, and two are identified forβ-H abstraction. Furthermore, the rate constants of each reaction channels are calculated by using transition state theory with Eckart tunneling corrections over temperature 200-1000K.The results show that theα-H abstraction is mainly channel, forming CH₃CF₂. The rate constants of mainly channel can be fitted in the following expressions respectively. HFC-152a + OH: k1=3.64×10^-23T^3.56exp^(139/T) cm³ molecule^-1 s^-1 HFC-152a + Cl: k1=2.54×10^-18T^2.31exp^(121/T) cm³ molecule^-1 s^-1 The mainly channels account for 96.0-77.5% and 100.0%-74.4% over the all reaction , respectively.2 The mechanism of CH₃CF₂ degradation reactions in atmosphere.(1)The oxidation reaction of CH₃CF₂with O₂(1Δg) have been theoretically investigated. This is a barrierless reaction, and thermodynamically favorable to form CH₃CF₂(OO).(2) The reaction of CH₃CF₂(OO) with NO has been studied using quantum chemistry method. The optimized geometry , energy and frequencies of all points along the reaction channels are calculated. Two channels are confirmed ,leading to CH₃CF₂(O) and NO2.The mainly channel via rearrangement of the trans-conformer to cis-conformer and subsequent dissociation. This channel should passed two barriers , with energy of -10.02 and -8.18kcal/mol,repectively.(3) The dissociation reaction mechanism of CHF(O)CF₃ are investigated by quantum chemistry method. This reaction is exothermic with barrier of 9.94kcal/mol,lead to CHF(O) and CF₃.Our theoretical study on the mechanism and kinetic of HFCs atmospheric reactions can provide some bases, which limited in experiments ,for further research on the HFCs environmental effects...