Theoretical Studies On Mechanisms And Dynamic Properties Of Several Fluoroalkane Reactions

The study and determination of reaction rate constants has always been one of the main research fields in chemistry. It is one of the most active subjects to predict the rate constants theoretically. Due to the adverse effects of chlorofluorocarbons (CFCs) on stratospheric ozone depletion and global warming, an international research effort is underway to investigate the environmental impact of the release of CFCs in the atmosphere. Hydrofluorocarbons (HFCs) are one class of CFCs replacements used in refrigeration and foam blowing and cleaning applications. Since they contain no chlorine or bromine, HFCs have essentially no ozone depletion potential. They do, of course, contain numerous C-F bonds which results in infrared absorption around 1100 cm-1, the middle of the 8 to 12μm atmospheric transmission window. Thus, HFCs have the potential to be potent greenhouse gases if their atmospheric concentrations become large enough. Most atmospheric species HFCs have at least one C-H bond and are susceptible to be attacked by active atoms or radicals in the atmosphere, in other words, the atmospheric lifetimes and tropospheric chemistry of such compounds are controlled primarily by their reactions with the tropospheric radicals or active atoms. To estimate the atmospheric lifetimes of such species, accurate data for the rate constants and their temperature dependence are needed. In this thesis, we have conducted theoretical calculations on the kinetics of the reactions of OH, F and Cl with fluoroalkane. Ab initio and density functional theory combined with the direct dynamics methods have been used to study the following chemical reactions: CF₂HCF₂CFH₂ + OH→products CF₂HCFHCF₂H + OH→products CF₃CFHCFH₂ + OH→products CF₃CF₂CFH₂ + OH→products CF₃CFHCF₂H + OH→products CF₃CF₂CFH₂ + Cl→products CF₃CFHCF₂H + Cl→products CF₃CH₂CF₃ + OH→products CF₃CH₂CF₃ + F→products CF₃CH₂CF₃ + Cl→productsThe main object is to provide accurate results for the reaction path and the temperature dependence relation of rate constants, in order to explore the reaction mechanism of these reactions. At the lower level (BB1K and BH&HLYP), the geometries and frequencies of the stationary points are calculated. The minimum energy path (MEP) is calculated at the same level by intrinsic reaction coordinate (IRC) theory. Furthermore, selected points along the MEP, the force constant matrices as well as the harmonic vibrational frequencies are obtained. In order to gain more accurate information of energy, the energies of the selected points on the MEP are refined at the higher level (G3(MP2), BMC-CCSD, and MC-QCISD/3). All of these calculations are performed by Gaussian 03 program. By means of Polyrate 9.3(8.4.1) program, the rate constants are calculated by conventional transition state theory (TST), canonical variational transition state theory with small-curvature tunneling correction (CVT/SCT), and improved canonical variational transition state theory with small-curvature tunneling correction (ICVT/SCT). The main results are summarized as follows,(1) The theoretical investigation on the reaction CF₂HCF₂CFH₂ + OH indicates that: Three low-energy conformers of CHF₂HCF₂CFH₂, one with Cs and two with C1 symmetries, are located, by the rotations of the -CF₂H and -CFH₂ groups, the relative population of three conformers (denoted conformer I, II, and III) are considered in the overall rate constant calculation. The rate constant calculations are carried out using the improved canonical variational transition state theory (ICVT) with the small-curvature tunneling correction (SCT) at the G3(MP2)//BB1K/6-31+G(d,p) level over a wide temperature range of 200–2000 K. It is shown that the theoretical rate constants agree well with the available experimental values with the maximum deviation within a factor of 2 and the temperature dependence of the overall rate constant is fitted by three parameter expression within 200-2000 K. The present calculation suggests that conformer III dominates the title reaction over the whole temperature range.(2) Theoretical studies on the mechanisms of the CF₂HCFHCF₂H + OH (R1) and CF₃CFHCFH₂ + OH (R2) reactions have been done at the G3(MP2)//BB1K/6-31+G(d, p) level. The rate constants of the two reactions calculated by the improved canonical variational transition-state theory (ICVT) with a small curvature tunneling correction (SCT) are in reasonable agreement with available experimental data. It is shown that H-abstractions form both -CFH and -CF₂H/CFH₂ sites are competitive and should be taken into account for R1 and R2 over the temperature range of 200-1000 K. For R1, the major channel is the H-abstraction from the -CF₂H group at low temperature, however, the H-abstraction from the -CFH group becomes more important as temperature rises. On the contrary, R2 occurs mainly via H-abstraction from the–CFH group at low temperature and H-abstraction from the -CFH₂ group at high temperature. The calculated branching ratios of kCFH/k1 and kCFH/k2 are 0.50 and 0.55 at 298 K, respectively, in quite good agreement with the SAR estimates. The three-parameter fits based on the ICVT/SCT rate constants for the title reactions within 200-1000 K are k1 = 2.13×10–20 T 3.09 exp(–855.38/T) and k2 = 5.23×10–22 T 3.64 exp(–683.25/T). Furthermore, due to lack of the experimental data, we calculated the enthalpies of formation of reactants CF₂HCFHCF₂H, and CF₃CFHCFH₂, and the product radicals CF₂HCFCF₂H, CF₂HCFHCF₂, CF₃CFCFH₂, and CF₃CFHCFH by using isodesmic reactions at G3(MP2)//BB1K/6-31+G(d, p) level.(3) For reactions CF₃CF₂CFH₂ and CF₃CFHCF₂H + X (X = OH and Cl), the potential energy surface information is obtained at the BB1K/6-31+G(d, p) level and higher level energies of stationary points are calculated at the BMC-CCSD//BB1K level along with the points selected on the MEP. The theoretical rate constants are calculated in the temperature range from 200 to 1000 K by the canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) at the BMC-CCSD//BB1K/6-31+G(d, p) level. Theoretical rate constants show good agreement with the available experimental data. The three parameter expressions for above reactions within 200?1000 K are k1T = 4.08×10?22 T 3.24 exp(?631.48/T), k2T = 6.46×10?21 T 2.90 exp(?940.60/T), k3T = 5.53×10?22 T 3.15 exp(?882.21/T), and k4T = 3.30×10?18 T 2.10 exp(?1669.11/T) cm3 molecule-1s-1, respectively. Specially, the reactant CF₃CF₂CFH₂ has two stable conformers, SC1 with Cs symmetry and SC2 with C1 symmetry, so the total rate constants are obtained by the Boltzmann distribution function. The present calculations suggest that SC2 has predominant contribution to the reaction of CF₃CF₂CFH₂ + OH over the whole temperature range, while for CF₃CF₂CFH₂ + Cl, SC1 and SC2 are competitive with each other. Furthermore, the enthalpies of CF₃CF₂CFH₂, CF₃CFHCF₂H, CF₃CF₂CFH, CF₃CFCF₂H, and CF₃CFHCF₂ are studied by using isodesmic reactions at BMC-CCSD//BB1K/6-31+G(d, p) and MC-QCISD// BB1K/6-31+G(d, p) levels. And the good agreement is obtained between two high levels, with the largest deviation within 1.0 kcal/mol.(4) For reactions CF₃CH₂CF₃ + OH/F/Cl, the potential energy surface information is obtained at the BB1K/6-31+G (d, p) level, and the higher-level energies of the stationary points and a few extra points along the MEP are calculated by powerful and cheap BMC-CCSD theory. The reactions of CF₃CH₂CF₃ + OH and CF₃CH₂CF₃ + F possess the"early"transition states as expected for an exothermic reaction, while reaction CF₃CH₂CF₃ + Cl proceeds through a"late"transition state as expected for an endothermic reaction. The theoretical rate constants for each reaction channel are calculated by the canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) at the BMC-CCSD//BB1K level. The theoretical results are in good agreement with the available experimental data and decrease in the order of k2 > k1 > k3, which is just opposite of the orders of the theoretical activation energies. The three-parameter fits within 200–1000 K are k1 = 1.18×10–19 T2.51 exp(–1761.16/T), k2 = 2.47×10–15 T 1.36 exp(–689.34/T), and k3 = 4.24×10–21 T 3.22 exp(–2819.55/T) cm3 molecule–1 s–1, respectively. Furthermore, the estimated enthalpies of formation of CF₃CH₂CF₃ and CF₃CHCF₃ using isodesmic reactions are–336.16 and–278.13 kcal/mol at the BMC-CCSD//BB1K level.