| Atmospheric pollution has become increasingly important as a social issue which was widely concerned in the world. The combustion process will produce a large number of free radicals and oxygen and nitrogen compounds, which would have caused serious air pollution problem. In order to control pollution more effectively, a comprehensive understanding of the chemical process of combustion reaction mechanism is necessary. Nitrogen oxides and small molecule radical reaction is active topics on the international at present. In this thesis, the theoretical investigations on the potential energy surfaces for some important gas phase reactions of nitrogen oxides with small molecule and radicals are carried out. Important information from potential energy surfaces such as structures and energies of intermediate isomers and transition states, possible reaction channels, reaction mechanisms and major products are obtained. The results obtained in the present thesis may be helpful for further theoretical and experimental studies for this kind of reactions. In this thesis some reactions about HFCs compounds are also carried out. And the Multi-configuration Self-consistent Field program which was performed initially by Liu et al. is improved. The energy of CO molecule is calculated using this program.The main results are summarized as follows:1. The doublet potential-energy surface for the reaction of C2H with O, including 3 minimum isomers and 3 transition states, is explored theoretically using the coupled cluster and density functional theory. The initial association between C2H and O is confirmed to be a barrierless process forming a low-lying adduct named as 1 (HCCO), followed by C-C bond rupture leading to product P1(CH + CO), which might be the most abundant considering form both energetic and entropic factors. Less competitively, 1 can lead to P2 (CCO + H) directly via C-H bond cleavage or undergo H-shift and ring-closure to 2(c-COC-H), and then take H-shift and ring-opening to 3(HOCC) followed by dissociation to P2 (CCO + H). Because the intermediates, transition states and products involved in the feasible pathways all lie below the reactants, the C2H + O reaction is expected to be rapid, as is confirmed by experiment. All the reaction pathways are listed as below: Path 1: C2H + O (R) HCCO (1) CO + CH (P1) Path 2: C2H+O (R) HCCO (1) CCO+H (P2) Path 3: C2H+O (R) HCCO (1) c-COC-H (2) HOCC (3) CCO+H (P2) Path 4: C2H+O (R) HCCO (1) c-COC-H (2) HOCC (3) C2+OH (P3)2. The PES of reaction CHF2/CF3+NO2 are studied. The main results are summarized as follows:(1) These reactions proceed mostly through singlet pathways and less through triplet pathways.(2) For CHF2+NO2 reaction,the following reaction pathways are obtained: Path 1: R→a1→b11→P11 (CHFO + FNO)→P16 (CHFO + F + NO) Path 2: R→a1→b11→b12 (b13)→b14→P12 (CHFO + FON)→P16 (CHFO + F + NO) Path 3: R→a1→b11→P13 (CF2O + HNO) Path 4: R→a1→b11→b13→P14 (CF2O + HON). Path5: R→a1→P15 (CF2 + HONO) Five kinds of primary products P11 (CHFO + FNO), P12 (CHFO + FON), P13 (CF2O + HNO), P14 (CF2O + HON) and P15 (CF2 + HONO) should be observed. Among these products, P11, P13 and P15 are most competitive products in high temperature. P15 is kinetically the most feasible product in low temperature, and P11, P13 are thermodynamically feasible. In addition, P16 (CHFO + F + NO) is thermodynamically unfavorable product and may have a small amount involved in products. (3) For CF3 + NO2 reaction, the dominant decomposition pathway is Path 1 leading to product P21 (CF2O + FNO), which can further dissociate to give P23 (CF2O + F + NO). The second pathway is Path 2 leading to product P22 (CF2O + FON). Our results agree well with the experimental observations. Because the rate-determining transition state involved in the feasible pathways lie above the reactants R, the two reactions are expected to play an important part at high temperatures. (4) The PES for the CH3-nFn + NO2 (n = 1–3) reactions are similar. By comparison, it is readily found that the potential energy surfaces of the three CH3-nFn + NO2 (n = 1–3) reactions are almost in parallel. (1) For the three reactions, isomer b H3-nFnCONO (n = 1–3) can undergo 1,3-fluorin migration from C to N atom associated with N—O1 weak bond cleavage leading to species CH2-nFnO(n = 1–3) + FNO as the most favorable product. (2) For the three reactions, the fragment FNO in the most favorable product can further dissociate to form F atom and NO. (3) All reactions involve the same middle-N association process CH3-nFn + NO2→H3-nFnCNO2 (n = 1–3). It is worthwhile to note that for rate-determining process from H3-nFnCNO2 to H3-nFnCONO, the barrier heights (by taking the energy of reactants R as reference) of the three reactions are different, that is, 0.3-barrier for CH2F + NO2, 3.6-barrier for CHF2 + NO2, and 5.9-barrier for CF3 + NO2. We can discuss the reactivity of the three reactions in terms of these barrier heights. The increasing barrier height may decrease the fluoromethyl reactivity from CH2F + NO2, CHF2 + NO2 to CF3 + NO2. But because of the lack of experimental data of CH2F + NO2 and CHF2 + NO2 reactions, the conclusion has no experimental evidence. So it is necessary to do some experimental study on the two reactions. Furthermore, based on the substitution effect, the reactivity sequence of CH3-nFn towards NO2 can be interpreted. Because the F atom with higher electronegativity strongly attracts a nonbonded single electron located at the C atom, the electron density on the carbon atom is reduced, which leads to a decrease of the reactivity of C atom in CH3-nFn attacking on the N atom in NO2 along with the increasing of fluorine substitution.3. The reactions of NCO+N/O are studied. For reaction NCO+O, firstly adducts a (ONCO) was formed barrierlessly. And then passed a low barrier transition state to get the main product P1 (CO + NO). This is the main reaction channel. The rate constant for producing P1,P2,P3 should meet kP1 > kP2 > kP3. For reaction NCO+N, the product Pa (N2+CO) has high thermodynamic stability.4. The dynamical properties of the hydrogen abstraction reactions of CHF2CHF2/CHF2CF3 + X(X=F, Cl) in the temperature range 200– 1500 K are investigated theoretically. The minimum energy paths (MEPs) of these reactions are calculated at the MP2/6-311G(d,p) level, and the energies along the MEPs are further refined at the G3(MP2) (single-point) level. With the aid of canonical variational transition state theory including the small-curvature tunneling correction, the rate constants of the title reactions were calculated over a wide temperature range. Agreement between the CVT/SCT rate constants and the experimental values is good. The rate constants may relate with the hardnessηof the haloethane molecules.5. Improved the performance of the Multi-configuration Self-consistent Field program which was firstly written by Liu et al. This program consists of three components: the combination coefficients configuration optimization, geminal parameters optimization and the molecular orbital optimization. And the geminal parameters optimization is a new one compared with the conventional Multi-configuration Self-consistent Field program. Using the Fortran90 language to optimize the code of the program can make it more efficient and convenient to use and maintain. And then, we use three symplectic bases and their combination as the wave function to calculate the ground state energy of the CO molecule in four different internuclear distances. The results are compared with that of HF and CCSD (T) method. The results show that only select small amount of symplectic bases can obtain good results. |
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