| 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. In this thesis, ab initio and density functional theory combined with the direct dynamics methods have been used to study the following chemical reactions: 1) Reactions H + CHnCl4-n (n=0,1,3), H + CH3Br, and H + C2H5Cl. Incineration is a cleanest method for disposal of halogenated hydrocarbons, which are potential sources of chlorine or bromine in the stratosphere, where they can catalytically destroy ozone. The current studied reactions play an important role in the incineration of halogenated hydrocarbons. To incinerate halogenated hydrocarbons in a more efficient and less hazardous way, it is necessary to further investigate all aspects of this kind of combustion process. It is especially important to understand the incineration mechanism, specific pathways, and rate constants for important elementary reactions.2) Reactions HBr + CHnCl3-n (n=1,2,3) and HBr +C2H5. The reactions of small alkyl radicals and chlorinated alkyl radicals with HBr have been extensively studied during the past decades. This is due to their importance for fundamental chemical kinetics and thermodynamics. The measured rate constants can be used to determine the heats of formation for these radicals. Early measurements showed that these radical reactions were activated process characterized by small positive activation energies. However, since 1988, a number of experimental kinetic studies using flash photolysis techniques have found out the negative activation energy and high rate constants for these reactions.3) Reaction OH+CH3CF3. It is well known that the chlorofluorocarbons (CFCs) are responsible for greenhouse effect and depletion of ozone layer in the atmosphere; HFCs are currently thought to be the prime candidates as CFC replacements. The hydrogen abstraction reaction of hydroxyl radical with hydrofluorocarbons (HFCs) plays an important role in atmospheric chemistry and combustion process.The above-mentioned reactions play an important role in atmosphere and combustion chemistry. So the studies on reaction mechanism and the rate constants of these reactions are of great benefit to environmental and stratospheric ozone protection. There have many experimental investigations concerning rate constants for these reactions, but the theoretical studies are little. The main object of the current thesis is to provide accurate results of the reaction path and the temperature dependence of rate constants and to explore the reaction mechanism of these reactions. The theoretical results may provide useful information for further experimental studies.By means of the Gaussian98 program, at the BH&HLYP or MP2 level, the geometries and frequencies of the stationary points (reactant, transition state, and products) are calculated. The minimum energy path (MEP) is calculated at the same level by intrinsic reaction coordinate (IRC) theory. Furthermore, with the selected points along the MEP, the force constant matrices as well as the harmonic vibrational frequencies are obtained. In order to gain more accurate energy profile, the energies of the selected points on the MEP are refined at QCISD(T) or PMP4(SDTQ) level. Finally, the canonical variational transition state theory with small-curvature tunneling correction is applied to obtain the reaction rate constants by using POLYRATE-Version 8.4.1 program. The main results can be summarized as follows:The studies on the reactions of halogenated hydrocarbons with H atom indicate that (1) in the reactions H + CH3X (X=F, Cl, Br), for the halogen abstraction reactions, the values of classical barrier heights (ΔEf) rapidly decrease in the order CH3F > CH3Cl > CH3Br. For the hydrogen abstraction reactions, however, the change of barrier heights is very slight. This phenomenon may be attributed to the different electronegativity of halogen atom in methyl halide. As a result, thi...
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