Theoretical Studies On The Mechanisms And Dynamics Property Of Several Important Reactions

The theoretical determination of rate constants for a specific chemical reaction has always been one of the main research fields in chemistry. It is one of the most active subjects to predict the rate constants in different conditions theoretically. In this thesis, ab initio and density functional theory direct dynamics methods have been used to take a systematic theoretical study on the following hydrogen abstraction reactions: CH₃OCH₃+H →CH₃OCH₂+H₂ CH₃OCH₃+CH₃ →CH₃OCH₂+CH₄ CH₃OCH₃+OH →CH3OCH)2+H₂O CF₃OCH₃+OH →CF₃OCH₂+H₂O CF₃OCHF₂ + OH →CF₃OCF₂+H₂O CF₃OCHF₂ + Cl →CF₃OCF₂+HCl CHFO + F →CFO+HF CHFO + Cl →CFO + HCl Due to the beneficial effects such as a high octa number, a reduction of hydrocarbon emissions, and low soot formation, DME (CH3OCH3) has been proposed as an alternative diesel fuel and a fuel antiknock agent. An understanding of the low-and high-temperature oxidative behavior of DME is therefore of practical interest. The propagation steps of decomposition include the hydrogen abstraction reactions from DME by hydrogen atom and methyl radical. Some experimental and theoretical studies on the oxidation and decomposition of DME have been carried out. But there is discrepancy between their results. Chlorofluorocarbons (CFCs) are being phased out under the Montreal protocol because of recognition of the adverse impact of them on stratospheric ozone and greenhouse warming. An international effort has been undertaken to replace CFCs with environmentally acceptable alternatives. Hydrofluorocarbons (HFCs) are an important class of CFC replacements in many industrial applications. Since HFCs do not contain Cl atoms, they have a zero ozone depletion potential (ODP), but they must still be evaluated as possible greenhouse gases. Many experimental investigations have been devoted to the determination of the rates and mechanism of the reactions of HFCs with the tropospheric constituents, such as CHFO + F/Cl. On the other hand, the partially fluorinated ethers (hydrofluoroethers; HFEs), which have been promoted as third generation candidates of alternative compounds for CFCs because the second generation alternative, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), maybe contribute to global warming. HFEs have almost zero impact on ozone and low greenhouse gas emissions, and they have been applied in the cleaning of electronic equipment, heat transfer agents in refrigeration systems, and carrier fluids for lubricant deposition. The 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. By means of the Gaussian 98 program, at the lower level (B3LYP and MP2), the geometries and frequencies of the stationary points(reactant, transition state, complex and product) 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 (QCISD(T) and G3 and G3(MP2)). By POLYRATE 8.4.1 program, conventional transition state theory(TST) and canonical variational transition state theory with small-curvature tunneling correction(CVT/SCT) are applied to obtain the reaction rate constants. The main results can be summarized as follows: (1) The theoretical study on the reaction CH3OCH3 + H CH3OCH2 +H2 (3.1) and CH3OCH3 + CH3 CH3OCH2 + CH4 (3.2) indicates that: The barrier height of CH3OCH3 with H is lower than that of CH3OCH3 with CH3 by about 6.0 kcal/mol. The changes of the geometry and generalized normal-mode vibrational frequencies along the minimum energy reaction path mainly take place in the ranges of s = –1.0―0.5 (amu)1/2 bohr for reaction (3.1) and –0.5―0.75 (amu)1/2 bohr for reaction (3.2). The calculations of the reaction enthalpies and bond dissociation energy at the QCISD(T)/6-311+G(3df,3pd)//MP2/6-311+G(d,p) level show good agreement with experiments. The rate constants and activation energies of the two reactions calculated by canonical variational theory with small-curvature tunneling are consistent with the experimental values in the temperature regions where they have been measured. The variational effect is small, while tunneling makes an important contribution to the rate constants in the lower temperature range. The rate constants within 207―2100 K of the two reactions are fitted by the three-parameter expressions: k1 = 1.16×10-19 T3exp(–1922/T) and k2 = 1.66×10-28 T5exp(–3086/T) cm3 molecule-1 s-1. (2) The theoretical study on the reaction CH3OCH3 + OH CH3OCH2 + H2O (4.1) and CF3OCH3 + OH CF3OCH2 + H2O (4.2) indicates that: For each reaction two H-abstraction reaction channels are identified at the G3//MP2/6-311G(d, p) level. The abstraction from the out-of-plane hydrogen (channel a) for both reactions is the dominant pathway, which proceeds via an indirect mechanism, while the direct abstraction (channel b) from the in-plane hydrogen is less competitive. The two barrier heights of reaction (4.1) are lower than those of reaction (4.2) by about 2.7 kcal/mol for channel a and 2.1 kcal/mol for channel b, which is in accordance with the fact that the C―H dissociation energy in CH3OCH3 is about 6 kcal/mol less than that in CF3OCH3. The overall rate constants and activation energies of the two reactions calculated by canonical variational transition-state theory with the small-curvature tunneling (CVT/SCT) correction are well consistent with the experimental values in the corresponding measured temperature regions. The variational effect has some contribution for channel (4.1a) at lower temperatures, and the SCTcorrection plays an important role to the rate constants in the lower temperature range for all reaction pathways. The rate constants within 230―2000 K of the two reactions are fitted by the three-parameter expressions: k1 = 3.33 ×10-20 T2.91 exp(–409.7/T) and k2 = 1.23 ×10-24 T3.93 exp(–188.2/T) cm3 molecule-1 s-1. (3) The theoretical study on the reaction CF3OCHF2 + OH CF3OCF2 + H2O (5.1) and CF3OCHF2 + Cl CF3OCF2 + HCl (5.2) indicates that: The potential energy surface information is obtained at the B3LYP/6-311G(d, p) level, and higher-level energies for the stationary points and a few extra points along the minimum energy path are calculated by the G3(MP2) theory. The rate constants of the hydrogen abstraction reactions (5.1) and (5.2) calculated by canonical variational transition-state theory with the small-curvature tunneling (CVT/SCT) correction are well consistent with the available experimental values (with a maximum factor of 2.0 at 393 K) in the corresponding measured temperature regions. For reactions (5.1) and (5.2), the variational effect has a minor contribution in the whole temperature range, while the SCT correction plays an important role in the lower temperature region. The rate constants within 200―2000 K of the two reactions are fitted by the three-parameter expressions: k1 = 4.95 ×10-30 T5.40 exp(–347/T) and k2 = 1.86 ×10-23 T3.43 exp(–1579/T) cm3molecule-1s-1. At the same time, standard enthalpies of formation of CF3OCHF2 and CF3OCF2 are theoretically estimated using group balance isodesmic reactions. The calculated values of –312.3 ±1.0 kcal mol-1 for CF3OCHF2 and –257.3 ±1.0 kcal mol-1 for CF3OCF2 are obtained by high-level single-point energy calculations at the G3(MP2) levels based on the B3LYP/6-311G(d, p) geometries. (4) The potential energy surface (PES) information of the F + CHFO reaction is built up at the MP2/6-311+G(d, p), MP4SDQ/6-311G(d, p), and QCISD(T)/6-311+G(3df, 2p) (single point) levels. It is shown that, in terms of the PES, the dominant reaction pathway is the hydrogen abstraction reaction (6.1) that forms the products HF and CFO, while addition route of the fluorine to CHFO leading to H + CF2O is less competitive. The energy profile of reaction (6.1) is refined with the ISPE method at the...