Theoretical Studies On The Reaction Mechanism And Dynamic Properties Of Several Kinds Of Atmospheric Pollutants

The study and determination of reaction rate constants are one of the main research fields inexperiments. However, due to the limitation of the existing experimental equipment, laboratory chemistscan only obtain the rate constants in a limitated temperatures. So it is important to study the rate constantsby using the theoretical method.In this thesis, the dual-level direct dynamics methods have been employed to study themicrocosmic mechanism and rate constants of the H-abstraction reactions. We hope our calculated resultswill provide a useful information for the further experiments. These reactions are as follows:CH3NHNH2+Oâ†'product(CH3)2NNH2+Oâ†'productCF3CHFCF3/CF3CF2CHF2+X(X=F，Cl, and Br)â†'productCH3OCF2CHFCl/CH2FOCF2CHFCl/CHF2CF2OCHFCl+OHâ†'product(CF3)2CHOCH2F/(CF3)2CHOCHF2+OHâ†'product1. The mechanism of multi-channel reaction CH3NHNH2(SC1and SC2)+Oâ†'products isinvestigated theoretically and density functional theory (DFT), and dynamics properties are explored by adual-level direct dynamics method. The information of potential energy surface is carried out at theBMC-CCSD//MPW1K/6-311G(d,p) level. Using the canonical variational transition state theory (CVT)with a small-curvature tunneling correction (SCT), the rate constants of each channel are evaluated over awide temperature range of200-2000K on the basis of obtained electronic structures and energyinformation. The total rate constants are calculated from the sum of the individual rate constants taking intoaccount the Boltzmann distribution of two conformers. The reactivity of H atom located in different groupsis compared.2. A dual-level direct dynamics method is employed to study the reaction mechanism of (CH3)2NNH2(Unsymmetrical dimethylhydrazine) with O atom. The geometries and frequencies of all the stationarypoints are optimized at the MPW1K/6-311G(d,p) level, and the energy profiles are further refined byinterpolated single-point energies (ISPE) method at the BMC-CCSD level of theory. The rate constants of O atom with (CH3)2NNH2are evaluated over a wide temperature range of200-2000K by using thecanonical variational transition-state theory (CVT) with the small curvature tunneling correction (SCT).The agreement between the theoretical and experimental rate constants is good around room temperature.The channels of H abstraction from-NH2position favor below1200K. With temperature increasing,contributions from other channels should be taken into account. The reactivity of N2H4, CH3NHNH2, and(CH3)2NNH2toward atomic O is compared to explore the methylation effect.3. The reaction mechanisms of reactions CF3CHFCF3and CF3CF2CHF2with X Atom (X=F, Cl, andBr) are studied theoretically. Moreover, the dynamics properties of reactions CF3CHFCF3/CF3CF2CHF2+X (X=F and Cl) are calculated by dual-level (X//Y) direct dynamics methods. The electronic structureinformation is obtained at the lower level, i.e., MPW1K/6-311+G(d,p). Then, single-point energies arecalculated by G3(MP2) method based on the lower level DFT geometries. Subsequently, the rate constantsare evaluated using the variational transition-state theory (VTST) proposed by Truhlar and co-workers overa temperature range of200-2000K. The reactivity of reactions CF3CHFCF3/CF3CF2CHF2with threehalogen atoms (F, Cl, and Br) is compared. Our calculations show that the barrier heights of reactionsCF3CHFCF3/CF3CF2CHF2with F atom are much less than other reactions. So the rate constants ofreactions CF3CHFCF3/CF3CF2CHF2with F atom are greatly larger than those of other reactions. Finally,the non-linear Arrhenius expressions of reactions CF3CHFCF3/CF3CF2CHF2+X (X=F and Cl) arepresented to assistant further experiments.4. The mechanism and dynamic properties for the multi-channel reactions of CH3OCF2CHFCl+OH(R5-1), CH2FOCF2CHFCl+OH (R5-2), and CHF2OCF2CHFCl+OH (R5-3) were carried out theoretically.The geometric parameters were optimized at the BMK/6-311+G(d,p) level. Subsequently, the energieswere refined at the BMC-CCSD level. Based on the information of partial potential energy surface, the rateconstants were evaluated by using the canonical variational transition state theory (CVT) with asmall-curvature tunneling correction (SCT) method. For every reaction, there are two possibleH-abstraction positions, i.e.,-CH3-nFn(n=0,1, and2) group and-CHFCl group. The major reaction channeland the effect of fluorine substitution on the reactivity are explored. Both questions are elucidated byanalyzing the reaction energy, barrier height, bond dissociation energy, and rate constants. 5. The information related with the mechanism of reactions (CF3)2CHOCH2F+OH (R6-1) and(CF3)2CHOCHF2+OH (R6-2) was explored theoretically at the BMC-CCSD//BMK/6-311+G(d,p) level.Based on the optimized structures, energies, and other information, the rate constants were evaluated by thecanonical variational transition state theory with small curvature tunneling contributions (CVT/SCT) in atemperature range of220-2000K. For each reaction, there are both hydrogen-abstraction and displacementchannels. What is more, more than one hydrogen atom can be abstracted. The relationship betweenhydrogen-abstraction and displacement, between different hydrogen-abstraction channels, and betweenreactions R6-1and R6-2are elucidated.