Theoretical Studies On The Reactions Of Several Important Radicals Systems In Atmosphere And Interstellar Chemistry

Radicals are involved in many chemical processes, including organic chemistry, interstellar chemistry, combustion chemistry, atmospheric chemistry, environmental chemistry, photochemistry, and organic chemistry. Due to the short lives of the radicals and the difficulty to obtain the pure species, the experimental research for their structures and reaction features is very difficult. Therefore, more and more attentions have been focused on their theoretical researches in recent years. The theoretical investigations on the reaction mechanisms of several important radical reactions related to atmosphere chemistry and interstellar chemistry were systematically carried out. Important information of 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 lay a strong foundation for building important radical-molecule model in atmospheric chemistry and interstellar chemistry, and provide theoretical supports and warranty for future experimental study and detection of interstellar molecules in space. The main results are summarized as follows:1. The reaction of atomic radical N (4S, 2D) with NO2 was explored theoretically using density functional, coupled cluster, and M?ller–Plesset perturbation theory. Both singlet and triplet electronic state potential energy surfaces are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311+G(d) and G3B3 levels of theory. On the triplet potential energy surface of this reaction, various possible reaction pathways, including the N-adduct-O-shift and four-center ring formation-decomposition reactions, are considered. The most favorable pathway should be the atomic radical N attacking the N-atom of NO2 firstly to form the adduct 1 NN(OO), followed by one of the NO bonds breaking to give intermediate 2 NNOO, and then leading to the major products P2 (O2 + N2). As efficient routes to the reduction of NO2 to form N2 and O2 are sought, both kinetic and thermodynamic considerations support the viability of this channel. On the singlet potential energy surface of this reaction, all the involved transition states for generation of (2NO) and (O2 + N2) lie much lower than the reactants. Thus, the novel reaction N + NO2 can proceed effectively even at low temperatures and it is expected to play a role in both atmospheric and interstellar processes. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction.2. The reaction of N (4S) radical with NCO (X2Π) radical has been studied theoretically using density functional theory and ab initio quantum chemistry method. The triplet electronic state [N2CO] potential energy surface are calculated at the G3B3 and CCSD(T)/aug-cc-pVDZ//B3LYP/6-311++G(d,p) levels of theory. The basic reaction paths suggested that, initial N2CO formation followed by dissociation to produce N2 and CO molecules. All the energies of the transition states and isomers in the pathway RP1 (R→1→TS1-P1→P1 (N2 + CO)) are lower than that of the reactants; the rate of this pathway should be very fast. Thus, the novel reaction N + NCO can proceed effectively even at low temperatures and it is expected to play a role in both atmospheric and interstellar processes. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for this reaction.3. The reaction of O? anion with N2O has been studied theoretically using density functional theory and ab initio quantum chemistry method. The doublet electronic state potential energy surface are calculated at the G3B3//B3LYP level of theory. Since the isomer and transition state involved in the most favorable pathway are all lower than the reactants in energy, they are expected to be fast, as is confirmed by experiment. So these reactions may be of significance in atmospheric and combustion chemistry.4. The reaction of 3C2 (a3Π) radical with O2 (X3Σ) molecule has been studied theoretically using ab initio Quantum Chemistry method. Both singlet and triplet potential energy surfaces are calculated at the CCSD(T)//B3LYP and G3B3 levels of theory. The singlet potential energy surface shows 5 isomers, 8 transition states, 3 products and 5 pathways, and the triplet potential energy surface displays 5 isomers, 8 transition states, 3 product and 3 pathways. The major pathway is Path RP1 (1): R→1→2→3→P1 (2CO) on the singlet potential energy surface, it is shown that the most feasible pathway should be the O-atom of O2 attacking the C-atom of the 3C2 molecule first to form the adduct 1 CCOO, followed by the O-shift to give intermediate 2 CC(OO), and then to the major products P1 (2CO). Alternatively, 1 can be directly dissociated to P1 via transition state TS1-P1. And R→36→37→38→39→310→P* (CO + 3CO)→P1 (2CO) on the triplet potential energy surface with P1 expected to be the main product. It is an O-adduct-shift mechanism. The efficient triplet OCCO dissociation pathway is a curve crossing channel through the MECP to the singlet surface, and the production is two singlet 1CO molecules. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the 3C2 (a3Π) + O2 (X3Σ) reaction. The reaction heats of formation calculated are in good agreement with that obtained experimentally. The results obtained in the present thesis may lay a strong foundation for building important radical-molecule model in atmospheric chemistry and interstellar chemistry.