Microcosmic Dynamic Studies On The Several Important Hydrogen And Halogen Abstraction Reactions

The study and determination of reaction rate constants has always been one of the important 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 microcosmic mechanism and rate constants of the following several important hydrogen and halogen abstraction reactions: Cl + CHBrF2 →CBrF2 + HCl Cl + CHBr2Cl →products Br + SiH4 →products OH + SiH4 →SiH3 + H2O OH + SiH3CH3 →products Br + CH3SOCH3 →products It plays an important role that the study of the microcosmic mechanism of above-mentioned reactions in dealing with environment pollution. The reactions of chlorine atoms with halogenated hydrocarbons play an important role in the industrial chlorination processes and in the incineration of hazardous halogenated wastes. Halogenated hydrocarbons are known to be important atmospheric species and are very efficient in destroying ozone layer in the stratosphere and for the greenhouse effects. The studies on reaction microcosmic mechanism and investigation on the dynamics property and rate constants of these reactions play significant role for environmental and stratospheric ozone protection. Because the temperature used in the experiment is far from covering the whole temperature range of practical interest, the accurate extrapolation of rate constants to higher temperatures requires the theoretical study. Since no experimental information is available on the branching ratio of the rate constants of the multiple-channel reactions, theoretical investigation is desirable to give a further understanding of the reaction mechanism. To our best knowledge, no previous theoretical work has addressed Cl + CHBr2Cl and Cl + CHBrF2 reactions. Silane and its methyl-substituted homolog are considered as important reagents in plasma chemical vapor deposition (CVD) and in the semiconductor manufacturing process. The use of volatile silicon compounds may lead to their emission into the atmosphere, where they can be removed by reactions with a variety of reactive species, such as Cl, Br, I, Si, O(3P), O(1D), H and OH as reported. The reactions of Si, O(3P), O(1D) and H with SiH4 have been investigated theoretically. For most hydrocarbons, hydrogen abstraction by OH radicals is one of the major channels for their removal in the atmosphere. The flammability of silanes makes them hazardous, and reaction with OH radicals is a likely step in silane combustion, which will help us to determine the flame speed and explosion limits. Br atom is known to be an important atmospheric species and is very efficient in destroying ozone layer in the stratosphere and for the greenhouse effects. Furthermore, the reaction Br + SiH4 has been used as a source of SiH3 radicals in kinetic experiments. To our best knowledge, little theoretical attention have been paid to the reactions of Br + SiH4, OH + SiH4, OH + SiH3CH3, they can proceed through H-abstraction from the SiH3, CH3, or SiH4 groups and Br-atom displacement. Largely emitted by oceans, dimethyl sulfide (DMS) has been postulated to be involved in global climate system through the formation of aerosols and clouds influencing in this way the earth radiation budget. Dimethyl sulfoxide (DMSO) is an intermediate species of the addition route of the OH-initiated oxidation of dimethyl sulfide. Atmospheric DMSO is mainly produced by the addition pathway of the OH reaction with DMS. Laboratory studies have shown that DMSO is also formed in reactions of DMS with BrO. DMSO has been observed in the marine atmosphere. Despite the importance of DMSO in the atmospheric sulfur chemistry, the gas-phase reactions of this species have not been studied deeply so far. Although theOH reaction with DMSO is the major gas phase oxidation process for DMSO in the atmosphere, reactions with halogen atoms and radicals (Cl, Br, ClO, BrO), for which tropospheric concentrations (except Cl) are much higher than those of OH, may also play some important role. 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 Gaussian03 program, at the BH&HLYP or MP2 level, the geometries and frequencies of the stationary points (reactant, hydrogen bond complex, 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), MP2, CCSD(T), G3MP2 or MC-QCISD level. Finally, the canonical variational transition state theory or improved canonical variational transition state theory with small-curvature tunneling correction are applied to obtain the reaction rate constants by using POLYRATE-Version 9.1 program. The main results can be summarized as follows: 1. For the reactions mentioned above, the potential surface information is obtained, including geometries, energies, gradients, and force constnats matrices of the stationary points (reactants, transition states and products), some extra points along the minimum energy path (MEP), the reaction enthalpies(? H0298 ) and potential barriers(?E) with zero-point energy correction(ZPE). Subsequently, the rate constants of the above-mentioned reactions are calculated using the variational transition state theory (VTST). The theoretical results obtained are in good agreement with the experimental values. It indicates that the present calculations can provide reliable prediction of the rate constants for the above-mentioned reactions at higher temperatures. 2. For the multiple channel reactions of Cl + CHBr2Cl,Br + SiH4,OH+ SiH3CH3 and Br + CH3SOCH3, there are no experimental information on the branching ratio available up to now. Therefore, theoretical investigations are necessary to obtain the reaction branching ratio and give a deeper understanding of reaction microcosmic dynamic mechanisms of these multiple channel reactions. The theoretical results may be helpful for future experimental measurements. 3. For reactions Cl + CHBrF2 →CBrF2 + HCl and Cl + CHBr2Cl →CBr2Cl + HCl, the ground-state vibrational adiabatic potential curve (VaG) has two barriers, and the nonregular VaG shape might be yielded by the combination of two different factors: the low energy barrier and the relatively large early drop of the zero point energies(ZPE) prior to the saddle point zone. 4. For reactions CHF3 + Cl, CHClF2 + Cl and CHBrF2 + Cl, with the electronic layer increase (F < Cl < Br), C—H bond reactivity increases also in the order of CHF3 < CHClF2 < CHBrF2, this results in the increase of the rate constants in the order of CHF3 < CHClF2 < CHBrF2. It can conclude that the increase in rate constant mainly results from the decrease in the activation energy. 5. By means of direct dynamic studies of silane and monosilane with OH radicals, the following rule of has been found: methyl substitution on hydrogen atom of silane activates Si―H bond toward abstraction, rate constants is increase, i.e., k(SiH3CH3) > k(SiH4). The rate constants of SiH3CH3 + OH reaction are larger than those of SiH4 + OH reaction over the whole temperature range. For the reaction SiH3CH3 + OH, H-abstraction channel from the SiH3 group R1a is the major channel, H-abstraction from the CH3 group R1b is the minor channel. 6. The reaction Br + SiH4 has two alternative reaction pathways, H-abstraction and Br-displacement channels. The potential barrier height of H-abstraction reaction R1 is much lower than that of Br-displacement reaction R2 by more than 10 kcal/mol, and this shows that H-abstraction channel R1 is the absolute dominant over the displacement channel R2 for the Br + SiH4 reactions. The variational effect is important in the lower temperature range and small-curvature tunneling (SCT) correction is small.