Theoretical Studies On The Reaction Mechanism And Dynamics Property Of Several HCFCs And HFCs

The aim of this thesis is to study the mechanism and rate constants of several importantHCFCs and HFCs with some free radicals and atoms. The following reactions are studied byab initio and density functional theory direct dynamics method:CF₃CH₂X（X=F, Cl）+Fâ†'productsCF₂ClCClXH（X=F, Cl）+OHâ†'productsCFCl₂CClXH（X=F, Cl）+OHâ†'productsCF₃CHFCF₃+Hâ†'productsCF₃CF₂CHF₂+Hâ†'productsCF₃CHClX（X=F, Cl）+Fâ†'productsCF₃CH₂Cl+Clâ†'products1. The kinetic properties of the hydrogen abstraction reactions of CF₃CH₂F+Fâ†'CF₃CHF+HF and CF₃CH₂Cl+Fâ†'CF₃CHCl+HF are studied by dual-level direct dynamicsmethod. Optimized geometries and frequencies of all the stationary points and extra pointsalong the Minimum-Energy Path （MEP） are obtained at the B3LYP/6-311+G（2d,2p） level.Two complexes with energies less than that of the reactants are located in the reactant sideof each reaction. The energy profiles are further refined with the interpolated single-pointenergies （ISPE） method at the G3（MP2） level of theory. Using canonical variationaltransition state theory （CVT） with the small-curvature tunneling correction （SCT） methodthe rate constants are evaluated over a wide temperature range of200-2000K. Agreementbetween the theoretical and experimental results is good. Our calculations have shownthat C-H bond activity decreases when one hydrogen atom of CF₃CH₃is substituted by afluorine atom, than when substituted with a chlorine atom.2. The reactions of OH （OD） radicals with CF₂ClCClFH, CF₂ClCCl₂H, CFCl₂CClFH, andCFCl₂CCl₂H have been investigated theoretically by a dual-level direct dynamics method.The optimized geometries and frequencies of the stationary points are calculated at theMPW1K/6-311+G（d,p） level. To improve the reaction enthalpy and potential barrier ofeach reaction channel, the single point energy calculation is performed by the MC-QCISDmethod. The enthalpies of formation of the species CF₂ClCClFH, CF₂ClCCl₂H,CFCl₂CClFH, CFCl₂CCl₂H, CF₂ClCClF, CF₂ClCCl₂, CFCl₂CClF, and CFCl₂CCl₂are evaluated by two sets of isodesmic reactions. Using canonical variational transition statetheory （CVT） with the small-curvature tunneling correction （SCT） method, the rateconstants of OH and OD radicals with CF₂ClCClXH （X=F, Cl） and CFCl₂CClXH （X=F,Cl） are evaluated over a wide temperature range of100-2000K at theMC-QCISD//MPW1K/6-311+G（d,p） level. The calculated CVT/SCT rate constants areconsistent with available experimental data. The results show that the tunneling correctionhas an important contribution in the calculation of rate constants at lower temperatures.For the above-mentioned four reactions, the kinetic isotope effects are also calculated.Finally, the effect of fluorine or chlorine substitution on reactivity of the C-H bond isdiscussed.3. The rate constants of the hydrogen abstraction reactions of CF₃CHFCF₃+H andCF₃CF₂CHF₂+H have been calculated by means of the dual-level direct dynamicsmethod. Optimized geometries and frequencies of stationary points and extra points alongthe Minimum-Energy Path （MEP） are obtained at the MPW1K/6-311+G（d,p） level, andthe classical energetic information is further corrected with the Interpolated Single PointEnergy （ISPE） approach by the G3（MP2） level of theory. Using the canonical variationaltransition state theory （CVT） with small-curvature tunneling correction （SCT）, the rateconstants are evaluated over a wide temperature rang of200-2000K. The calculatedCVT/SCT rate constants are in good agreement with available experimental values. It isfound that the variational effect is very small and almost negligible over the wholecalculated temperature region. However, the small-curvature tunneling correction plays animportant role in the lower temperature range. Furthermore, the heats of formation ofspecies CF₃CF₂CHF₂（SC₁or SC₂） and CF₃CF₂CF₂are studied using isodesmic reactionsto further elucidate the thermodynamic properties.4. The hydrogen abstraction reactions of CF₃CHCl₂+F and CF₃CHClF+F are investigatedby dual-level direct dynamics method. The optimized geometries and frequencies of thestationary points are calculated at the B3LYP/6-311G（d,p） and MP2/6-311G（d,p） levels.Higher-level energies are obtained at the G3（MP2） method using the B3LYP andMP2-optimized geometries, respectively. Complexes with energies lower than those of thereactants are located at the entrance of these two reactions at the B3LYP level, respectively.The rate constants are calculated using the variational transition state theory （VTST） withthe small-curvature tunneling correction over a wide temperature range of200-2000K. Theagreement between theoretical and experimental rate constants is good. In addition, the effect of fluorine substitution on reactivity of the C-H bond is discussed. Our calculationsshow that the fluorine substitution deactivates the C-H bond reactivity.5. The hydrogen abstraction reaction of Cl atoms with CF₃CH₂Cl （HCFC-133a） isinvestigated by density function theory （DFT） and ab initio approach, and the rate constantsare calculated by dual-level direct dynamics method. Optimized geometries and frequenciesof reactants, transition state, and products are computed at the B3LYP/6-311+G（2d,2p）level. In order to refine the energetic information along the minimum energy path （MEP）,single-point energy calculations are carried out at the G3（MP2） level of theory. InterpolatedSingle Point Energy （ISPE） method is employed to correct the energy profiles for the titlereaction. The rate constants are evaluated by using the canonical variational transition statetheory （CVT） with a small-curvature tunneling correction （SCT） over a wide temperaturerange of200-2000K. The variational effect for the reaction is moderate at lowtemperatures and very small at high temperatures. However, the tunneling correction has animportant contribution in the lower temperature range. The agreement between calculatedrate constants and available experimental values is seen to be good at lower temperaturesbut diverge significantly in higher temperatures.We hope our calculated results will provide a good estimate for the kinetics of thereactions, which will be useful for the atmospheric modeling calculations and help to assesstheir atmospheric lifetimes of HCFCs and HFCs to find better alternatives to replace CFCs.