Theoretical Studies On Dynamic Properties Of Several Reactions Relevant To Air Pollution

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 and 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: CHF₂CF₂OCH₃ + Cl→products CF₃CHXOCHF₂ (X=H,F and Cl) + Cl→products CF₃CHXOCHF₂ (X=H,F and Cl) + OH→products CHF₂CHXOCF₃ (X=H and F) + OH→products CH₃C(O)OCH₃ + 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 hydrofluoroethers (HFEs) have been 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. In addition, the introduction of ether linkage―O― may lead to an even greater reactivity in the troposphere.Furthermore, the anaesthetics will contribute to the total atmospheric content of chlorine-containing species at a fraction about 5×10^-4, and the use of the halogenated anaesthetics has received much attention because of their potential for stratospheric ozone destruction and their contribution to global warming. Isoflurane (CF3CHClOCHF2) is one of the important anaesthetics and is emitted into the atmosphere at a rate of approximately 750 metric tons year-1. An accurate determination of atmospheric lifetime for isoflurane is necessary to assess its environmental impact.Since 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 hydrofluoroethers 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, CF3CHXOCHF2 (X=H, F and Cl), CHF2CHXOCF3 (X=F and Cl) with OH radicals and CHF₂CF₂OCH₃, CF₃CHXOCHF₂ (X=H, F and Cl) with Cl atoms, but few reports were made on branching ratios and the rate constants at wide temperature ranges for these reactions. Thus, the reactions of these ethers with OH radicals and Cl atoms are of interest for modeling purpose.In addition, volatile organic compounds (VOCs) are emitted to the atmosphere through their use in industry and by biogenic sources. As is well known, the atmospheric oxidation processes of VOCs initiated by OH may contribute to the photochemical formation of ozone and other components of photochemical smog found in urban areas. However, it was found that methyl acetate shows significantly lower photochemical ozone creation potential (POCP) and would appear to offer significant environmental advantages as a substitute solvent. Thus, methyl acetate has considerable potential for substitution in industry. Prior to its large-scale industrial use, it is very necessary to evaluate the atmosphere lifetime and the environmental impact of methyl acetate. Many experimental rate constants for the reactions OH with CH3C(O)OCH3 have been determined, while it is found that there are lack of 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 aim of this thesis is to provide more 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 and B3LYP, and 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 G3(MP2) and MC-QCISD. All of these calculations are performed by Gaussian 98 or Gaussian 03 program. By means of Polyrate 8.4.1 or Polyrate 9.3 program, the rate constants are calculated by conventional transition state theory (TST) and canonical variational transition state theory with small-curvature tunneling correction (CVT/SCT).The main results are summarized as follows,1. The theoretical investigation on the reactions Cl with CHF₂CF₂OCH₃ indicates that: Two pathways, namely H-abstraction from—CH₃ group (R1) and H-abstraction from—CHF2 group (R2), are feasible. The potential energy surface information is obtained at the G3(MP2)//B3LYP/6-311G(d,p) level. The values of enthalpies of formation for CHF₂CF₂OCH₃, CHF2CF2OCH2 and CF₂CF₂OCH₃, are–256.71±0.88,–207.79±0.12 and–233.43±0.88 kcal/mol, respectively, using group balanced isodesmic reactions as working reactions. The theoretical rate constants are calculated by using canonical variational transition state theory (CVT) with the small-curvature tunneling (SCT) correction in the temperature range 200–2000 K. Our calculated results show that H-abstraction from—CH3 group, which have lower C—H dissociation energy and potential barrier, is the dominant channel in the lower temperature range, however the H-abstraction from—CHF₂ group should be taken into account in the higher temperature range. The total rate constants are in good agreement with the experimental values in the measured temperature range 273–363 K. The temperature dependence of the title rate constants within 200–2000K is fitted by the three-parameter expressions, which will be useful for further experiment. 2 Theoretical studies on the reactions of CF₃CH₂OCHF₂+OH and CF₃CH₂OCHF₂+Cl are performed by the dual-level direct dynamics method. The PESs information is obtained at the B3LYP/6-311G(d,p) level, and the higher level energies for the stationary points and extra points along the minimum energy path are refined at the G3(MP2) level. By using group-balanced isodesmic reaction as working reaction, the calculated values of standard enthalpies of formation at the G3(MP2)//B3LYP/6-311G(d,p) level are–315.93±0.33,–260.96±0.12 and–266.72±0.19 kcal/mol for CF₃CH₂OCHF₂, CF3CH2OCF2 and CF3CHOCHF2, respectively. The reaction mechanisms of the two products (CF3CH2OCF2 and CF3CHOCHF2) with OH radical are investigated theoretically. We found that addition-elimination processes may be the major product pathway of CF3CHOCHF2 + OH leading to major products CF3CHO, CF3, CF3COCHF2 and H2O. However, only addition reaction may be important for CF3CH2OCF2 + OH reaction, that is, the adduct CF3CH2OC(OH)F2 may be the major product. The rate constants of the H-abstraction reaction channels are calculated by using canonical variational transition state theory (CVT) with the small-curvature tunneling (SCT) correction. The theoretical results are consistent with the available experimental values in the measured temperature range. For the four H-abstraction reaction channels, variational effect is somewhat important over the whole temperature region, and the SCT correction has large contribution to the rate constants in the lower temperature range. Substitution of hydrogen atoms in CH₃CH₂OCH₃ by fluorine atoms reduces the reactivity of the C―H bond, and as a result, the rate constants for reaction CF₃CH₂OCHF₂ + OH become much smaller than that of CH₃CH₂OCH₃ + OH. To provide good estimation for future laboratory investigation, the fitted three-parameter expressions in the wide temperature range of 200-2000 K for reactions R1, R2 and R5 are given as, k1 = 9.48× 10^-24 T^4.14 exp(108/T), k₂ = 6.96×10^-20 T^2.57 exp(–566/T), k₅ = 3.57×10^-19 T^2.71 exp (303/T) cm3 molecule^-1 s^-1.3. The reactions of OH radical and Cl atom with the molecules CF₃CHClOCHF₂ and CF₃CHFOCHF₂ are investigated theoretically using the dual-level direct dynamics method. The potential energy surface informations are obtained at the B3LYP/6-311G(d,p) level and the higher-level energies of the stationary points and a few extra points along the MEP are calculated by G3(MP2) method. For each reaction, two H-abstraction channels are found. Furthermore, the subsequent reaction mechanisms of the product radicals (CF₃CHFOCF₂, CF₃CFOCHF₂ and CF₃CHClOCF₂, CF₃CClOCHF₂) with OH are also investigated theoretically. We found that only the initial addition processes are feasible for reactions CF₃CHFOCF₂ + OH and CF₃CHClOCF₂ + OH, and thus adducts CF₃CHFOC(OH)F₂ (M5) CF₃CHClOC(OH)F₂ (M7) may be the major products. While for reactions CF₃CFOCHF₂ + OH and CF₃CClOCHF₂ + OH, they can barrierlessly form the initial adducts CF₃C(OH)FOCHF₂ (M6) and CF₃C(OH)ClOCHF₂ (M8) followed by concerted H-shift and the bond rupture processes with lower barrier heights, so the products CF₃CFO, CF₃CClO, CF₂ and H2O may be the major products. The theoretical rate constants for reactions R1-R4 are calculated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT). Our calculated rate constants and the branching ratios are consistent with the experimental values. The variational effect is very small or almost negligible for reaction channels R1, R2 and R3, while it is somewhat large for R4a and R4b over the whole temperature range. The SCT tunneling effect plays an important role in the low temperature range for all reaction channels. The present study shows that the H-abstraction from―CHF― or―CHCl― group is the major channel for reactions of R1, R2 and R3 at the lower temperatures; as the temperature increases, the contribution of both channels should be taken into account. However, for reaction R4, H-abstraction from―CHF₂ and―CHCl― are competitive channels at lower temperatures, and the latter channel becomes more important with the temperature increasing.4. The dual-level direct dynamic method is employed to investigate theoretically the hydrogen abstraction reaction systems CHF₂CHFOCF₃ + OH (R1) and CHF₂CH₂OCF₃ + OH (R2). The PES information is obtained at the B3LYP/6-311G(d,p) level, and the higher level energies for the stationary points and extra points along the minimum energy path are refined at the MC-QCISD theory. Two and three hydrogen abstraction channels are found for reactions R1 and R2, respectively, and four products CF₂CHFOCF₃, CHF₂CF₂OCF₃, CF₂CH₂OCF₃ and CHF₂CHOCF₃ are produced. Using group balance isodesmic reactions as working reactions, theΔHοf ,298 values are -362.89±0.12, -338.21±0.12, -338.10±0.12, -312.43±1.07, -289.82±1.07 and -265.36±0.65 kcal/mol for CHF₂CHFOCF₃, CF₂CHFOCF₃, CHF₂CFOCF₃, CHF₂CH₂OCF₃, CF₂CH₂OCF₃, and CHF₂CHOCF₃, respectively, at the MC-QCISD//B3LYP/6-311G(d,p) level. Furthermore, the subsequent reaction mechanisms of the reactions of the above four product radicals with OH radical are investigated theoretically at the same level in this thesis. We found that all these reactions take the addition-elimination mechanisms. These processes are all thermodynamically and kinetically accessible, and the products CF₂CFOCF₃, CF₂CHOCF₃, CHF₂COCF₃ and H₂O are expected to be observed. The rate constants of the H-abstraction reaction channels are calculated by using canonical variational transition state theory (CVT) with the small-curvature tunneling (SCT) correction. For each H-abstraction reaction channel, variational effect is small over the whole temperature region, and the SCT correction plays an important role in the rate constants calculation in the lower temperature range. For reaction R1, H-abstraction from―CHF2 group (R1a) is more competitive channel over the whole temperature range, whereas the contribution of channel R1b should be considered at the lower temperatures. For reaction R2, the preferred reaction channel is channel R2b. However, the contribution of channels R2a and R2b′should be also taken into account over the whole temperature range. In addition, the rate constant of reaction R2 (k2) is about one order of magnitude larger than that of reaction R6.1 (k1). To provide good estimation for future laboratory investigation, the fitted three-parameter expressions within 250-1200 K for these two reaction are given, k₁ = 2.49×10^-24 T^4.10 exp(-488/T), k₂ = 4.26×10^-23 T^3.80 exp(-404/T) cm³ molecule^-1 s^-1.5. A systematic investigation on the reaction CH3C(O)OCH3 with OH(OD) are performed. A dual-level dynamics method is employed to study the hydrogen abstraction reaction mechanism. The potential energy surface information is obtained at the MP2/6-311G(d,p) level and the energetic information is calculated by the MC-QCISD method. Three H-abstraction channels and one displacement process are identified. There exist the complexes with energies lower than the corresponding reactants or products for each H-abstraction reaction channel. The rate constants calculated by CVT with SCT correction are well consistent with the available experimental values. For channel R1a, R1b and R2, the variational effect is very small over the whole temperature range and the SCT correction plays an important role at the lower temperatures. The calculated branching ratios show that the"out-of-plane hydrogen abstraction"from methoxy end is primary channel at the lower temperatures, while at the higher temperatures the contribution of all the H-abstraction reaction channels should be taken into account. The KIEs for three channels and the total reaction are all"inverse"and increase with the temperature increasing. The three-parameter expression (in units of cm3 molecule-1 s-1) for the OH + CH₃C(O)OCH₃ and OD + CH3C(O)OCH3 reactions at temperature range 200-1200 K are fitted as follows: k = 8.76×10^-24 T^3.75 exp(887/T); k′= 2.38×10^-24 T^3.93 exp(1005/T).