Theoretical Studies On Mechanisms And Dynamics Property Of Several Important Reactions

Thermodynamic studies mainly focus on the direction and bound of the chemical reactions. Consequently, it resolves the possibility of the reactions. But the practical experience tells us that the reaction maybe not happened in practice when we judge it one reaction is expected to happen in thermodynamics. It shows that the reaction exists a feasibility problem. So, if we want to understand the chemical reaction, we need to know the reaction pathway (reaction mechanism) of the chemical reaction. Also, we need to introduce the time- variable. So, we research the rate constants and the effect impact on it. This is the major content that the chemical dynamics need to discuss. The study on micro-chemical reaction process cannot only deepen the understanding of the nature of the chemical, but also directly relate to the environment of human existence and various applications of daily life. Theoretically, therefore, to explore the process, mechanism and nature of chemical reactions is one of the most fundamental tasks in theoretical chemistry.Owing to the adverse effects of chlorofluorocarbons (CFCs) on stratospheric ozone depletion and global warming, and the environment issue has been one of the important issues of common concern. Control of chemical reactions of atmospheric pollutions is a possible way to solve the problem of atmospheric pollution. In order to effectively control environmental pollution, molecules and free radicals related to atmospheric pollution have been studied as an entry point by researches of reaction mechanism in the atmospheric environment. Since reactions of atmospheric radicals are very quick and their mechanism are very complex, experimental research for mechanism of these reactions has been hampered by difficulties; so in recent years, detailed theoretical studies on the mechanisms of atmospheric radicals reactions become one of the hot topics in the field of theoretical chemistry. On the other hand, as atmospheric pollutants released by combustion process, they have attracted attention for experimental and theoretical chemists. The reactions of these important radical with molecule are one of important topics in planetary atmospheric chemistry as well as in combustion chemistry. In this thesis, we have carried out detailed theoretical investigations on the potential energy surfaces of some important reactions of radical with singlet Oxygen atoms O(1D) and some molecules as well as radical-molecule reactions. Based on the calculated results, some important information of potential energy surfaces including structures and energies of reactant, products, intermediates and transition states is provided, reaction mechanisms are discussed, major products are predicted, and then possible reaction channels. On the basis of the ab initio calculation, the kinetic studies are carried out by VTST simulation and master equation, the rate constant and branching ratios of the products at different temperatures and pressures are obtained. The results obtained in the present thesis may be helpful for further theoretical and experimental studies of these kinds of reactions. The main results are summarized as follows,1. A direct dynamics study was carried out for the multichannel reaction of CH3NHNH2 with OH radical. Two stable Conformers (I, II) of CH3NHNH2 are identified by the rotation of the -CH3 group. For each conformer, five hydrogen-abstraction channels are found. The electronic structure information on the potential energy surface is obtained at the B3LYP/6-311G(d,p) level and the energetics along the reaction path is refined by the BMC-CCSD method. The influence of the basis set superposition error (BSSE) on the energies of all the complexes is discussed by means of the CBS-QB3 method. The rate constants of CH3NHNH2 + OH are calculated using canonical variational transition-state theory with the small-curvature tunneling correction (CVT/SCT) in the temperature range of 200–1000 K. Slightly negative temperature dependence of rate constant is found in the temperature range from 200 to 345 K. The agreement between the theoretical and experimental results is good. It is shown that for Conformer I, hydrogen-abstraction from–NH- position is the primary pathway at low temperature; the hydrogen-abstraction from -NH2 is a competitive pathway as the temperature increases. A similar case can be concluded for Conformer II. The overall rate constant is evaluated by considering the weight factors of each conformer from the Boltzmann distribution function, and the three-term Arrhenius expressions are fitted to be kT = 1.6×10-24T4.03exp (1411.5/T) cm3 molecule-1 s-1 between 200–1000 K. The mechanisms of the two products (CH3NNH2 and CH3NHNH) with OH radical were also investigated theoretically, and the hydrogen-abstraction processes in these two secondary reactions were found to be the major products pathways.2. Theoretical investigations are performed on mechanism of CH3OCF3 and CH3CHCH2 reaction with O(1D). The main results are as follows:(1) For CH3OCF3 with O(1D) reaction, the main results can be summaried as follows: R→a2* CF3OCH2OH→a2 (+M) (1)→P2 H2CO + F2CO + HF (2) R→a1* CF3OCH2OH→a1 (+M) (3)→P3 H2CO + CF3OH (4) R→b* CH3OOCF3→b (+M) (5)→P6 CF3O + CH3O (6)The insertion of the O(1D) on the CH3OCF3 molecule can have three initial routes, i.e., insertion into C-H bond, insertion into C-O bond, and addition to the F atom. The insertion into C-H bond is rather attractive to form CH2(OH)OCF3 a (a1, a2) (-151.5, -152.1 kcal/mol) without any entrance barrier, where a1 and a2 are two isomers of a, corresponding to the position of O-H inside or outside the C1-O2-C3 surface, respectively. Also, O(1D) can insert into C-O bond, barrierlessly leading to intermediate b CH3OOCF3 with the energy of -87.4 kcal/mol. The third entrance pathway is O(1D) addition to the F atom of CH3OCF3 to form c CH3OCF3O, which is only 4.2 kcal/mol lower than that of the reactants. The reaction mechanism of O(1D) with CH3OCF3 has been investigated theoretically by a detailed potential energy surface calculation at the B3LYP/6-311G(d,p) and BMC-CCSD (single-point) levels. There are three initial association ways for the reaction with no barrier, i.e., O(1D) insertion into the C-H and C-O bonds to form an energy-rich intermediates a (a1, a2) CH2(OH)OCF3 (-151.5, -152.1 kcal/mol) and b CH3OOCF3 (-87.4 kcal/mol), respectively, and O(1D) addition to F atom leading to intermediate c CH3OCF3O (-4.2 kcal/mol). Five feasible paths starting from intermediate a are a2 undergoes the 1,2-HF-elimination to give product P2 (H2CO+F2CO+HF), a1 occurs 1,3-H-shift along with the rupture of C-O bond to form product P3 (H2CO + CF3OH). Another competitive pathway is the O-O bond cleavage of b to produce P6 (CF3O + CH3O).We calculate the rate constants of the favorable reaction channels by using master equation methods. In the low pressure limit, three intermediates a1, a2, and b can be completely converted to the products P2, P3, and P6. In these three products, the rate constant is very fast for channel (2), the rate constant for channel (4) is much lower than channel (2), and the rate constant for channel (6) is the lowest one. While in the high pressure (100 atm), the ratio of channel (3) is the largest one, the second one is channel (3). The ratios of channel (1) and (5) can compete with each other. When the temperature is above 1000 K, channel (2) plays an important role as the temperature increase, and exceed all intermediates. The calculated rate constants of the title reaction have a negative temperature effect below 600 K, and a positive temperature effect when the temperature above 600 K.(2) For CH3CHCH2 with O(1D), main results can be indicated as follows: R→a* CH3C(OH)CH2→a (+M) (1)→P1 CH3COCH3 (2)→b1* CH3CHCHOH-cis→b1 (+M) (3)→P5b1 CH3CHCHO-cis+H (4)→b2* CH3CHCHOH-trans→b2 (+M) (5)→P4 CH3CH2CHO (6)→c* CH2(OH)CHCH2→c (+M) (7)→P6 CH2CCH2 + H2O (8)→d* CH3OCHCH2→d (+M) (9)→P10 CH2CHO + CH3 (10)→e* cyclo-O-CH2CHCH3→e (+M) (11)→P11 CH2CHCH2OH (12) O(1D) can insert into the C-H bond from both side of the CC double bond,–CH3 group and C-C bond to form an energy-rich intermediates a CH3CHOCH2, b (b1, b2) CH3CHCHOH, c CH2OHCHCH2, and d CH3OCHCH2, respectively, and O(1D) addition to CC double bond leading to intermediate e cyclo-O-CH2CHCH3. They are all stable in thermodynamics, the energies of all intermediates are -148.2, (-144.8, -145.2), -139.5, -137.7, and -130.3 kcal/mol. They can dissociate and isomerise to the products. In this thesis, by means of quantum chemical and master equation calculations, we calculate the rate constant of each channels and title reaction in certain temperature and pressure. We can conclude that the stability of intermediates are very strong. They can isomerise and dissociate to give products a little. The calculated rate constants of the title reaction have a negative temperature effect below 600 K, and a positive temperature effect when the temperature above 600 K.3. A combined quantum chemical and master equation rate constant calculational study is performed on the mechanism of the CH2CO+3CH2 reaction. The main results are as follows: R→IM1* CH2CH2CO→IM1 (+M) (1)→P1 (CO + 3C2H4) (2)→IM2* CH2COCH2→IM2 (+M) (3)→IM10* (CH3COCH)→P2'(CH3 + HCCO) (4)→P2 (CH3 + HCCO) (5)Two reaction channels are feasible in thermodynamics and kinetics for the reaction CH2CO+3CH2, one occurs the formation of IM1 CH2CH2CO, the isomerize and decomposition to form product P1 CO+3C2H4, the other one is the formation of IM2 CH2COCH2, then give product P2'CH3 + HCCO. The third one is the direct H-abstraction reaction and gives product P2 CH3 + HCCO. The calculated rate constants of the title reaction have a positive temperature effect and little pressure effect.