| Water, as third most abundant species in our atmosphere behind only N2and O2, has a very significant impact on the processes that occur in the Earth’s atmosphere. It not only as a greenhouse gas exists in our atmosphere in numerous phases, but also can form very stable hydrogen bonded complexes such as HO…H2O, HO2…H2O, O3…H2O, HNO3…H2O,OCIO…H20and H2SO4…H2O. The formations of water-molecule complexes and water-radical complexes have been extensively studied, because they can dramatically affect the chemistry in the atmosphere by changing the reaction mechanism. It has been estimated that up to30%of HO2is complexed with water to form HO2…H2O complex and has been shown to present large errors in atmospheric models if not considered. So the effect of water-molecule on the reactions containing HO2radical is not neglected.In this paper, the catalytic effect of a single water molecule has been investigated by studying the reaction mechanisms of HO2+HO2, CH3O2+HO2, CH2FO2+HO2and HO2+OH reactions. A single water molecule affects each one of these reaction mechanisms in different water-radical complexes and different ways of two hydrogen-atom transfer reactions and double hydrogen atom transfer process. Depending on different water-radical complexes and different ways the water interact, water molecule plays different role in reducing the barrier height and increasing the rate of reaction channels. Beyond the mechanisms presented, another goal of our work is to describe the effect of a single water molecule on these reactions under atmospheric conditions. To meet this goal, the calculated rate constants for different conditions of the reactions containing HO2radical with and without water molecule are calculated. The most valuable results in this thesis can be summarized as follows:(1) The gas-phase hydrogen abstraction reactions of CH3O2+HO2, CH3O2+HO2…H2O, HO2+HO2and HO2+HO2…H2O have been studied at the CCSD(T)/6-311++G(3d,2p)//B3LYP/6-311G(2d,2p) level of theory. The calculated results show that the process of O3formation is much faster than that of1O2and3O2formation in water-catalyzed CH3O2+HO2reaction. This is different from the results for the non-catalytic reaction of CH3O2+HO2, which almost only the process for3O2formation takes palce. Unlike CH3O2+HO2reaction that the preferred process is different in the catalytic and non-catalytic condition, the channel for3O2formation is the dominant in both catalytic and non-catalytic HO2+HO2reaction. Further research shows that the main difference of the water-catalyzed channels between HO2+HO2and CH3O2+HO2reaction is the PES involving the process for O2(1O2and3O2) formation. Such barrier difference is possibly due to that water molecule affects the former catalytic process mainly in double hydrogen transfer process, whereas it affects the latter catalytic process mainly in direct hydrogen abstraction mechanism. Furthermore, the calculated total CVT/SCT rate constants for water-catalyzed and non-catalytic reactions of HO2+HO2and CH3O2+HO2show that the water molecule doesn’t contribute to the rate of CH3O2+HO2reaction though the channel for O3formation in this water-catalyzed reaction is more kinetically favorable than its non-catalytic process. Meanwhile, water molecule plays an important positive role in increasing the rate of HO2+HO2reaction. These results are in good agreement with available experiments.(2) The reaction mechanisms and kinetics of CH2FO2+HO2and CH2FO2+HO2…2O have been studied at the CCSD(T)/6-311++G(3d,2p)//B3LYP/6-311G (2d,2p) level of theory. The calculated results show that the formations of CHFO and3O2are main products in the naked reaction of CH2FO2+HO2. When water molecule is added, the formations of CHFO and O3are main products in CH2FO2+HO2…H2O reaction. Compared with CH3O2+HO2…H2O reaction, F substitution has a significant enhancive effect on water-catalyzed hydrogen abstraction reactivity of CHFO,1O2, O3and3O2in CH2FO2+HO2…H2O reaction. Moreover, the calculated CVT/SCT rate constants for the CH2FO2+HO2reaction without and with a water molecule show that, although the single water molecule plays a positive catalytic effect on enhancing the rate for CHFO and O3formation, in humid conditions the effective rate of CH2FO2+HO2reaction will changes little with respect to dry conditions. Such situation is similar with the effect of water molecule on CH3O2+HO2reaction theoretically at the same level.(3) The catalytic effect of a single water molecule has been investigated by studying the reaction of HO2with OH at the CCSD(T)/aug-cc-pVTZ//CCSD/6-311G(d,p) level of theory. Hydrogen abstraction channels of HO3H,1O2and3O2formation are obtained for the reaction of HO2with OH in the absence of a water molecule. The reaction channel of O2formation is dominant thermodynamically and kinetically. The computed rate constant for the reaction of3O2formation is2.2×10-11cm3·molecule-1·s-1at298K and is in good agreement with previous reported values. In the presence of a water molecule, the same products as the naked reaction are obtained, but the potential energy surface of HO3H,1O2and3O2formation appears to be much more complex and the reactions become more versatile. The calculated results show that3O2formation remains the dominant product and occurs mainly via HO+HO2…H2O reaction channel. Although the channel for O3formation in water-catalyzed reaction channel of HO+HO2…H2O is6-300times larger than its non-catalytic process within the temperature range of216.7-298.2K, its effective rate constants are slower by2-3orders of magnitude than that of the unassisted reaction in the absence of a water molecule. Thus, we can predict that the title reaction rate enhancement by a single-water-molecule does not occur under tropospheric conditions. |
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