| The damage of the ozone layer involves directly the character of the global climate and the quality of the environment of the earth. To find out the mechanism of the damage of the ozone layer is very important so that we can take measures by the mechanism to protect the ozone layer. Besides the well- known freonll, alkene is another kind of substance, which reacts with ozone so that it can result in the damage of the ozone layer. It is reported that hydroxyl radical (OH), which involves air pollution, is formed during the reaction of ozone with alkene. The formation mechanism of OH radical is not clear right now and the reaction mechanisms of ozonelkene still need to be explored although it has been studied for decades. This thesis carried out DFT calculations for the reaction of ozone/i- butene system, which involves the formation of hydroxyl radical (OH). To discuss the selectivity between the two dissociation paths of POZ and the effect of substituents on the C=C double bond of an alkene on this selectivity, we also carried out DFT as well as direct dynamics calculations for the reactions of series of POZs formed from ozone/unsymmetrical alkenes. The geometrical structures of all species were optimized at B3LYP/6-3 11 Glevel and frequencies were obtained at the same level. After the calculation of intrinsic reaction coordinate (IRC) at B3LYP/63llG level, we calculated the reaction rate constants using direct dynamics. The results show that the addition reaction of ozone with 1 -butene conforms to Woodward-Hoffmann rule and can be assorted to [7t 4s+ t 2s] reaction. The concerted addition of ozone to the >C=C< bond of 1 -butene produces an energy-rich ozonide (called POZ) via a five-membered ringtransition state. Then the POZ dissociates along two paths to CH3CH2CHOO+HCHO and CH3CH2CHO+CH200, and the fractions of the two paths are 72.7% and 27.3%, respectively. This result agrees well with the experimental result of 64% and 36%. Our studies on the three hydrogen- migration paths of CH3CH2CHOO to produce OH radical show that along 1,3 hydrogen-migration path CH3CH2CHOO rearranges to form a relaxed complex via a four-membered ring transition state firstly, then the complex dissociates to produce OH radical. Along 1,4 and 1,5 hydrogen-migration paths CH3CH2CHOO rearrange to form hydrogen peroxides firstly via a five- membered ring and a six-membered ring transition states, respectively. With the rupture of the 0-0 bonds the hydrogen peroxides dissociate to produce OH radicals. Among the three hydrogen-migration paths 1,4 hydrogen-migration path is the predominant one to produce OH radical. The calculated potential barriers for 1,3 H, 1,4 H and 1,5 H migrations are 29.82, 15.03 and 37.57 Kcal moV,respectively, and the reaction heats as well as changes of Gibbs free energy are 5.41 and 3. 23, ?9. 75 and ?9. 58, ?. 60 and ?. 62 Kcal ~o1? respectively. Besides the situation of the migrating hydrogen atom, the size and number of the substituents on the C=C bond of an alkene is also very important to the formation of OH radical. For example, the calculated potential barriers for 1,4 H migration of CH200(at CCSD(T)ITZ+2P level), of CH3CI-IO(at B3LYP/6-3 11 G level), and of (CHJ)2C00 (at CCSD(T)/TZ+2P level) are 30.8, 17.2 and 13.6 Kcal mo[, respectively. The abstraction of H atom between Criegec intermediates is another path to produce OH radical, but compared with 1,4 I-I migration its contribution to OH radical formation can be neglected. The recombination of Criegee Intermediate... |
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