Theoretical Studies On The Mechanisms And Dynamics Property Of Several Hydrogen Abstraction Reactions

For a specific chemical reaction, the theoretical determination of rate constants has always been one of the main research fields in theoretical chemistry. The aim of this thesis is to study the mechanism and calculate the rate constants of the following hydrogen abstraction reaction by using ab initio and density functional theory direct dynamics methods: CH2ClOH + Cl→products CH3C(O)CH3-nFn (n=1, 3) + OH→products CH3C(O)CH3-nFn (n=1, 3) + Cl→products CHF2CH3-nFn (n=1-3) + OH→products CH3C(O)CH3-nCln (n=1-3) + Cl→products CH3CFCl2+Cl→CH2CFCl2+HCl CH3CF2Cl+Cl→CH2CF2Cl+HClThe massive seasonal reduction of ozone in the Arctic troposphere and in the Antarctic atmosphere during the austral spring is strongly related to catalytic reactions involving chlorine-substituted hydrocarbons in those regions [1, 2]. As a proposed reservoir of chlorine, the compound methylhypochlorite CH3OCl is known to play an important role in the reduction of ozone. Photolytic dissociation has been considered as a main possible route of release of chlorine from CH3OCl. Also, CH3OCl is expected to undergo homogeneous gas-phase photochemical reactions with Cl atom. The rate constant and the mechanism of the CH3OCl + Cl reaction are important to estimate whether CH3OCl may take part in catalytic O3 destroying cycles, or it may act as a relatively stable sink for chlorine, leading to lower O3 depletion rates. Its isomeric chloromethanol ClCH2OH has been predicted to be 43 kcal/mol lower in energy than the competing CH3OCl. But the reaction ClCH2OH + Cl was only determined at 298 K, and little theoretical attention has been made to the reaction mentioned above. In order to understand the atmospheric significance and the dynamic property of ClCH2OH, a theoretical investigation is carried out on the ClCH2OH + Cl reaction to obtain the accurate extrapolation of the rate constants to higher temperatures.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. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are an important class of CFC replacements in many industrial applications. Owing to their containing at least one C–H bond, HCFCs and HFCs are readily attacked by hydroxyl radical (OH) and halogen atom in the atmosphere. Thus, many experimental investigations have been devoted to the determination of the rates and mechanism of the reactions of HCFCs and HFCs with the tropospheric constituents, such as CHF2CH3-nFn (n=1-3) + OH or CH3CFCl2/CH3CF2Cl+Cl. These reactions were studied mainly in the low temperature, and no experimental information was available on the branching ratio of the rate constants of the multichannel reactions. Thus, to gain a deeper insight into the reaction mechanisms and branching ratio of the reactions mentioned above at a wide temperature range, further theoretical studies are very desirable. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been considered to be an important class of potential chlorofluorocarbons (CFCs) substitutes used in a number of industrial applications. When these replacement compounds are oxidized, carbonyl compounds (CCs) are often formed in the atmosphere. Ketones represent an important class of CCs. Acetones is known to be long-lived (months), and its atmospheric degradation in the upper troposphere and lower stratosphere could be the dominant source of OH and HO2 radicals, resulting in increasing ozone production. The most likely fate of the halogenated acetones in the Earth's atmosphere is reacted with OH. Moreover, it has been pointed out that Cl radicals may play an important role for the chemical transformation of organic substances in marine environments. Hence, both mechanism and kinetics of the hydrogen abstraction reactions of fluoroacetones and chloroacetone with OH radical and Cl atom are studied theoretically in this thesis.The main object is to provide accurate results for the reaction path and the temperature dependence relation of rate constants. By means of the Gaussian 03 program, at the lower level (MPWB1K 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 (MC-QCISD/3, BMC-CCSD, and G3(MP2)). By POLYRATE 9.1 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 ClCH2OH + Cl indicates that: The theoretical overall rate constant of the title reaction is in good agreement with the experimental value at (295±2) K. It has been identified that methylene-H-abstraction channel (3.1a), is the dominant reaction pathway for the ClCH2OH + Cl reaction, while hydroxyl-H-abstraction channel (3.1b), is less competitive. The theoretical standard enthalpies of formation for the species, ClCH2OH, ClCHOH, and ClCH2O are calculated at the MC-QCISD/3 //MP2/6-311G(d, p) level with the values of–58.85±0.1,–15.13±0.39, and 3.68±0.39 kcal/mol, respectively. The overall rate constants in the temperature range of 240-2000 K for the title reaction are fitted by the three-parameter expression as (in cm3molecule-1s-1): k1 = 1.36×10-17 T1.87exp (-744.2/T).(2) We employ a density functional theory to study three hydrogen abstraction reactions of CHF2CH3-nFn(n=1–3) with OH radical. The rate constant calculations are carried out using the variational transition state theory (VTST) at the BMC-CCSD//MPWB1K level over a wide temperature range of 220– 1500 K. Theoretical rate constants are in good agreement with the available experimental values. The calculated results show that the reactivity decreases with fluorine substitution increase at the 1-methyl position of fluoroethane, and the order of rate constants is k1 > k2 > k3. The three parameter expressions (in cm3 molecule-1 s-1) for three reactions within 220– 1500 K are k1 = 1.61×10-22 T3.31 exp (215.6/T), k2 = 3.27×10-21 T3.05 exp (747.3/T), and k3 = 3.21×10-22 T3.42 exp (955.8/T), respectively.(3) The hydrogen abstraction reaction systems CH3CCl2F + Cl and CH3CClF2 + Cl are investigated theoretically using the dual-level direct dynamic method. The rate constant calculations are carried out using the variational transition state theory (VTST) at the G3(MP2)//MP2 level over a wide temperature range of 200 ? 3000 K. Theoretical rate constants are in good agreement with the available experimental values. The calculated results show that the reactivity decreases from CH3CFCl2 to CH3CF2Cl, and the H-abstraction from the out-of-plane for (5.1) is the major reaction channel, while the in-plane-H-abstraction is the predominant route of (5.2). Furthermore, in order to further reveal the thermodynamics properties, the enthalpies of formation of CH3CFCl2, CH3CF2Cl, CH2CFCl2, and CH2CF2Cl are studied using the isodesmic reactions and hydrogenation reactions. The variational effect is small and the small-curvature tunneling effect is only important in the lower temperature range on the rate constants for all reaction pathways. The three-parameter expressions for reactions (5.1) and (5.2) within 200– 3000 K are obtained as follows: (in cm3 molecule-1 s-1) k1 = 2.11×10-19 T2.64 exp (1686.97/T) and k2 = 2.47×10-20 T2.31 exp (1923.15/T).(4) The theoretical study on the hydrogen abstraction reaction systems CH3C(O)CH3-nFn (n=1, 3) + OH/Cl is performed. The rate constant calculations are carried out using the variational transition state theory (VTST) at the BMC-CCSD//MP2 level over a wide temperature range of 200 ? 2000 K. Theoretical rate constants show good agreement with the available experimental values. The calculated results show that the reactivity decreases from CH3C(O)CH2F to CH3C(O)CF3, and for the multi-channel reactions (6.1) and (6.2), the H-abstraction from the fluoromethyl position is the major reaction channel. Furthermore, in order to further reveal the thermodynamics properties, the enthalpies of formation of CH3C(O)CH2F, CH3C(O)CF3, CH3C(O)CHF, CH2C(O)CH2F, and CH2C(O)CF3 are studied using the isodesmic reactions. the three parameter expressions for the CVT/SCT rate constants for the title reactions within 200-2000 K are obtained as follows [cm3 molecule-1 s-1]: k1 = 1.87×10-21 T3.24 exp (–714.76/T), k2 = 6.96×10-18 T2.31 exp (–84.96/T), k3 = 7.25×10-22 T3.26 exp (141.05/T) and k4 = 2.78×10-18 T2.54 exp (1308.04/T).(5) The mechanisms for the title reaction CH3C(O)CH3-nCln (n=1-3) + Cl→products have been studied using ab initio method. The theoretical study shows that the major addition-elimination channel is chloromethyl-elimination channel, and as it possesses a higher energy barrier, it can not compete with the H-abstraction reaction channel. Thus the title reactions are dominated by abstraction channels. The rate constant of the hydrogen abstract reaction channels are obtained using the variational transition state theory (VTST) at the BMC-CCSD//MP2 level over the temperature range of 200?360 K. Theoretical rate constants show good agreement with the available experimental values and decrease in the order of k1 > k2 > k3. For the multi-channel reactions R1 and R2, the H-abstraction reactions prefer to abstract from the chloromethyl position rather than to abstract from the methyl position. Furthermore, in order to reveal the thermodynamics properties, the enthalpies of formation of CH3C(O)CH2Cl, CH3C(O)CHCl2, CH3C(O)CCl3 and the corresponding products are studied using the isodesmic reactions at the BMC-CCSD//MP2/cc-pVDZ level. The three parameter fits for the CVT/SCT rate constants for the reactions CH3C(O)CH3-nCln (n=1-3) + Cl within 200–360 K is made and gives the following expressions [cm3 molecule-1 s-1]: k1 = 1.17×10-18 T2.41 exp (–211.86/T), k2 = 2.92×10-22 T3.45 exp (–269.21/T), and k3 = 1.03×10-19 T2.73 exp (911.00/T).