A Experimental and Theoretical Study of Degradation Pathways for Atmospheric Halocarbons

The issue of ozone depletion by chlorofluorocarbons (CFCs) is of great current concern. UV photodissociation of CFCs in the stratosphere produces a chlorine atom and a trihalomethyl radical. While the catalytic role of atomic chlorine in ozone depletion processes has been well studied, the part played by the other fragment of the dissociation remains largely unknown. Two possible catalytic cycles initiated by CX_3 (X = F, Cl) have been proposed, in which CX_3O and CX_3O_2 are the key species to understand these cycles. The dissociation pathway of CX_3 O radicals have been studied using Hartree-Fock theory, Moller-Plesset and spin projection correlation methods. The dissociation barrier heights and the heat of reaction for these processes have been determined. CF _3O has received extensive study both theoretically (using ab initio theory) and experimentally (using laser-induced fluorescence technique). The experimental observations for CF_3O are consistent with the theoretical predictions. A detailed theoretical study has been carried out on dissociation pathways of carbonyl halides resulting from the degradation of CX_3O radicals, and on the molecular properties of some electronic excited states of CF_2O using ab initio theory. It was found that the first electronic excited state of CF_2O is due to the ^1(n topi*) transition, and possesses a non-planar configuration. The study on the dissociation barrier heights for carbonyl halides suggests that chlorofluoromethane with less than two fluorine atoms degrades to carbon monoxide; those with at least two fluorine atoms yield carbon dioxide in the stratosphere. The laser photolysis/infrared chemiluminescence technique has been applied to the kinetic study of the reaction of CF_3 with NO _2. The absolute reaction rate constant was measured at 300 +/- 4 K to be (2.5 +/- 0.4) times 10^{-11} cm ^3 molecule^{-1} s^{-1} from monitoring the rise time of CF_2O time-resolved vibrational fluorescence. C_2 in triplet state has been produced by infrared multiphoton dissociation of CF _3OOCF_3. Both high resolution excitation spectrum and dispersed spectra of C_2 Swan band were collected and analyzed. The newly observed 2-0 transition of the Swan band has been identified and assigned.