Theoretical Studies On Mechanisms And Dynamics Property Of Several Important Reactions

The estimation of rate constants for a specific chemical reaction is one of the most active subjects in theoretical chemistry. In this thesis, ab initio or density functional theory direct dynamics methods have been used to investigate the reaction mechanism and calculate the rate constants. The following hydrogen abstracted reactions are studied: CHnCl3-nCHO + Cl→products CHClFCHO + Cl→products CHF2CHO + Cl→products CClF2CHO + Cl→products CHnF3-nCH2OH+ Cl→products CF3CH2OH + OH→products C2F5CH2OH + OH→productsOwing to the adverse effects of chlorofluorocarbons (CFCs) on stratospheric ozone depletion and global warming, an international effort has been made to replace them with environmentally acceptable alternatives. Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) have been found widespread industrial use over the past decade. Recently fluorinated alcohols (FAs) are proposed as a new generation of CFCs alternatives in applications such as cleaning of electronic components, refrigeration and carrier compounds for lubricants, since they contain no chlorine or bromine atoms and they don't contribute to the ozone depletion, but may potentially cause the global warming effect. Due to containing the abstracted hydrogen atom, these molecules can be expected to react with atmospheric radical species. Considering the hydroxyl radicals are important reactive intermediate species in the oxidation of organic compounds and combustion chemistry, and the number of them is very abundance in the stratosphere, the studies of the reactions of fluoroalcohols with OH radicals are indispensable for the evaluation of atmospheric lifetime and the environmental impact of these molecules.Although in the troposphere, the fluorinated alcohols will degrade primarily via reaction with OH radicals, the Cl atom is more reactive than OH and it will have a little contribution to the degradation of these species. Also, the reactions of Cl atoms with these species are important in the stratosphere and coastal atmospheric environments. In addition, the reactions of Cl with these species have been exploited to form selected hydroxyalkyl radicals in laboratory kinetic studies due to the similar energies of C-H and Cl-H bonds. Therefore, it is necessary to investigate the site selectivity of the Cl reacted with them and accurate data for the rate constants as well as their temperature dependencies.Some experimental studies have been done on the reactions, 2,2,3,3,3-pentafluoropropyl alcohol (CF3CF2CH2OH), CF3CH2OH, and CF3CHOHCF3 with OH radicals and CH3-nFnCH2OH with Cl atoms, but no reports were made on branching ratios and the rate constants at wide temperature ranges for these reactions. Thus, the reactions of these fluorinated alcohols with OH radicals and Cl atoms are of interest for modeling purpose.Because halogenated aldehydes can be produced as the main oxidation product of several HFCs, HCFCs, and fluorinated alcohols, they are regarded as important reaction intermediates in the atmosphere. Furthermore, these degradation products containing halogens may still transport halogen to the stratosphere and destruct the ozone layer. It is thus necessary to perform the kinetic investigation for these halogenated aldehydes in order to determine their atmospheric lifetime. Here, the focus is on the degradation of the halogenated aldehydes CH3-nClnCHO (n=1-3), CHClFCHO, CHF2CHO, and CClF2CHO initiated by Cl atoms, which is a convenient substitute in laboratory experiments for the attack by OH radials since halogens are more easily generated than OH radials. The experimental rate constants for the reactions Cl atoms with CH3-nClnCHO (n=1-3), CHClFCHO, CHF2CHO, and CClF2CHO have been determined at room temperature, while it is found that there are lack of the other kinetic data, such as the temperature dependence of the rate constants over a wide temperature range, the activation energies and the reaction enthalpies. So it is necessary to perform the theoretical investigations for the above reactions in order to provide further useful information.The main aim of this thesis is to provide accurate results for the reaction path, the relation between temperature and rate constants, and the branching ratios. Firstly, the geometries and frequencies of the stationary points (reactants, complexes, products, and transition states) are calculated at the lower levels, such as, MP2, B3LYP and MPW1K; then, the minimum energy path(MEP) is calculated at the same level by intrinsic reaction coordinate (IRC) theory to confirm that the transition state really connects the minimums along the reaction path. The first and second energy derivatives at geometries along the MEP are obtained to calculate the curvature of the reaction path and the generalized vibrational frequencies. The potential energy profile is refined at the higher levels, such as, CCSD(T), G3(MP2), G2M, and MC-QCISD. All of these calculations are performed by Gaussian 03 program. By means of Polyrate 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 reactions Cl with CH3-nClnCHO (n=1-3) (3.1-3.3) indicates that: A complex exists at the exit of the reactions 3.1a, 3.2a, and 3.3 with the energy lower than the products, respectively. So reactions 3.1a, 3.2a, and 3.3 are the indirect reaction mechanism, while reactions 3.1b and 3.2b proceed a direct H-abstraction reaction without intermediate complex. The variational effect is found for H-abstraction reactions 3.1a and 3.2a, while for reactions 3.1b, 3.2b, and 3.3, the variational effect is very small and almost negligible in the whole temperature range. On the other hand, for reactions 3.1a and 3.2a over the whole temperature range, the small-curvature tunneling correction shoud be negligible, but for reactions 3.1b, 3.2b, and 3.3, in the lower temperature range, the SCT plays an important role in the rate constant calculations. The calculated ICVT/SCT rate constants at the CCSD(T)/6-311+G(d, p)//MP2/cc-pvdz level agree well with the experimental values. The calculated results show that the reactivity decreases from CH2ClCHO through CHCl2CHO to CCl3CHO, which indicates that the chlorine substitutions have a noticeable effect on the reactivity, the more chlorine substitution, the lower reactivity is. And for the title reactions, H-abstraction from the formyl position is the major reaction channel, while H-abstraction from halogenated-methyl position may play a role with the temperature increasing. Furthermore, In order to further reveal the thermodynamics properties, the enthalpies of formation of CH2ClCHO, CHCl2CHO, CCl3CHO, and CH2ClCO, CHClCHO, CHCl2CO, CCl2CHO, CCl3CO are evaluated using the isodesmic reactions. The three-parameter expressions for three reactions within 220-2000 K are k1=9.82×10-16T 1.42exp (484.7/T), k2=5.20×10-18T 2.04 exp (669.0/T), and k3=8.82×10-19T 2.20exp (732.9/T) cm3molecule-1s-1, respectively.2 The PESs of the reaction CHClFCHO + Cl→products (4.1), CHF2CHO + Cl→products (4.2), and CClF2CHO + Cl→products (4.3) have been investigated at the MC-QCISD//MP2/cc-pVDZ level. The H-abstraction channels and the Cl-addition processes are discussed. This theoretical study shows that the H-abstraction reaction is the major reaction pathway, which proceeds via an indirect mechanism. While addition reactions possess higher energy barriers and they can not compete with the H-abstraction reaction channel. The rate constants of five hydrogen abstraction reaction channels (4.1a, 4.1b, 4.2a, 4.2b, and 4.3) have been calculated by canonical variational transition-state theory (CVT) with small-curvature tunneling (SCT) correction over a wide temperature range of 220-2000 K. The theoretical results are in good agreement with the available experimental data and decrease in the order of k1 > k2 > k3, which illustrates that the halogen substitution (F or Cl) in the -CHXY group will decrease the reactivity of the species. The branching ratios show that for the reaction CHClFCHO + Cl→products (4.1), the H-abstraction from -CHO group is the major pathway in the lower temperature, while the H-abstraction from -CHXY group becomes probable with the temperature increasing. While, for the reaction CHF2CHO + Cl→products (4.2) the H-abstraction leading to the products CHF2CO + HCl will prevail over the whole temperature range. In order to further reveal the thermodynamics properties, we have estimated the enthalpies of formation for CHClFCHO, CHF2CHO, CClF2CHO and product radicals CHClFCO, CClFCHO, CHF2CO, CF2CHO, CClF2CO with the values of -86.7, -130.5, -136.8, and -46.8, -50.5, -90.8, -78.5, -97.1 kcal/mol, respectively, via the isodesmic reaction method at the MC-QCISD//MP2/cc-pVDZ level. The CVT/SCT rate constants are fitted and the following three–parameter expressions within 220–2000 K are obtained: k1=5.08×10-16 T1.60 exp (244.6/T), k2=4.80×10-17 T1.86 exp (274.9/T), k3=2.34×10-16 T1.67 exp (37.1/T) cm3molecule-1s-1, respectively.3. The theoretical study on the hydrogen abstraction reaction CF3CF2CH2OH + OH (5.1) identifies two classes of H-abstraction reaction channels. The potential energy surface information is obtained at the B3LYP/6-311G (d, p) and MPW1K/6-311G (d, p) levels and the higher-level energies of the stationary points and a few extra points along the MEP are calculated by MC-QCISD theory. The theoretical rate constants for each reaction channel are calculated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) at the MC-QCISD//B3LYP level. The calculated CVT/SCT rate constants are found to be in good agreement with the available experimental values in the corresponding measured temperature regions. It is shown that hydrogen-abstraction from -CH2- position is the major channel, while H-abstraction from -OH position may be neglected with the temperature increasing. For 5.1a, 5.1b, 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 dissociation energies of bond are calculated by MC-QCISD and G3(MP2) based on B3LYP/6-311G(d, p) and MPW1K/6-311G (d, p) geometries to further confirm that for the title reaction, H-abstraction from the methylene position is the major reaction channel. While H-abstraction from hydroxyl position may play a less role with the temperature increasing. The three-parameter expression for this reaction within 200-2000 K is k = 6.45×10-21 T 2.91exp (74.9/T) cm3molecule-1s-1, respectively.4. For reaction CF3CH2OH + OH (6.1), the potential energy surface information is obtained at the B3LYP/6-311G (d, p) level of theory, and the higher level energies for the stationary points as well as the extra points along the minimum energy path (MEP) are calculated at the MC-QCISD//B3LYP level. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. The rate constants are calculated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) in the temperature range 200-2000 K. The total rate constants are in good agreement with the experimental values. It is shown that for the title reaction, the reaction occurs mainly via the hydroxyl-H-abstraction channel at the lower temperature, while the methylene-H-abstraction channel is preferred when the temperature is higher than 289 K. Furthermore, In order to further reveal the thermodynamics properties, the enthalpies of formation of CF3CH2OH, CF3CHOH, and CF3CH2O are studied using isodesmic reactions. The three-parameter expressions for these reactions within 200-2000 K are ka = 1.40×10-21 T 3.29 exp (-374.1/T), kb = 1.71×10-23 T 3.42 exp (659.0/T), k = 6.06×10-24 T 3.98 exp (284.2/T) k1 =1.05×10-16 T1.46 exp(-468.8/T) and k2 =2.80×10-16 T1.41 exp(1075.7/T) cm3molecule-1s-1, respectively.5. The investigation results of the hydrogen abstraction reactions CH3-nFnCH2OH (n=1-3) + Cl (7.1-7.3) are show as follows: For reations 7.1 and 7.2, three H-abstraction reaction channels are identified, while for 7.3, two reaction pathways are found. The potential energy surface information is obtained at the MP2/6-311G(d, p) level, and the higher level G2M is used to refine the energies of the stationary points and the points selected along the MEP. The theoretical rate constants for each reaction channel are calculated in the temperature range from 200 to 2000 K by canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) at the G2M//MP2 level. The results are consistent with the experimental values for 7.2 and 7.3, but we find the negative temperature dependence for the reaction 7.1. The dissociation energies of bonds are calculated to confirm which is the dominant reaction pathway by G2M and G3(MP2) methods based on MP2/6-311G(d, p) geometries. The calculated results show that for the title reactions, H-abstraction from the methylene positions are the major reaction channels, and the channels of H-abstraction from fluorinated-methyl positions should be taken into account at higher temperatures, while H-abstraction from hydroxyl positions should be neglected. The fluorine substitutions have a noticeable effect on the reactivity, the more fluorine substitution, the lower reactivity is. The three-parameter expressions for these reactions within 200-2000 K are k1=4.35×10-17T 1.78exp (912.9/T), k2=8.87×10-18T 1.95exp (460.0/T), k3=2.25×10-18T 2.03exp (264.9/T) cm3molecule-1s-1, respectively.