Dann, Jonathan Roger; (1996) Matrix isolation study of ozone with some halogen containing alkanes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The main aim of this research is to study, using Fourier-transform infrared spectroscopy, the photochemical reaction of ozone with some halogen-containing alkanes in low temperature matrices. The reactions between halogenated alkanes and ozone, studied in this thesis, can be applied to gas phase atmospheric research with regard to ozone depletion. One example, not expanded on in this thesis, is the search for ozone-friendly species (refrigerants, propellants etc.), especially since one of the provisions of the Montreal protocol is to phase-out such species. Matrix reactions are carried out at low temperatures and, this means that the reactants are often effectively unable to react and, thus many 'reactive' or otherwise 'difficult to study' compounds can be stabilized and studied spectroscopically. In these experiments, the matrices must be photolysed in order to initiate a reaction; and we have used infrared, visible and ultraviolet irradiation to initiate reactions. By careful selection of the photolysis wavelength range used to irradiate the matrix it is possible to form different products and, thus reveal the photochemical reaction path. The matrix environment also enables us to detect reactions that would not occur in the gas phase; in a matrix the species are held in close proximity to one another, allowing a variety of secondary reactions to occur, whilst, in the gas phase the primary products usually separate rapidly. This facet of a matrix reaction - by which the products are held closely together - has enabled us to study a range of nearest-neighbour complexes that were generated in situ by careful selection of the precursors. Using matrix techniques, the reaction of ozone with halogen-containing compounds leads to the observations below. In the cases of the single iodine-containing precursors ozone binds weakly with the iodine atom, and this modifies the photochemistry of ozone, allowing the effective dissociation of ozone. The transfer of an oxygen atom to the precursor leads to the formation of several new species. In addition to detecting these new species, it was possible to determine wavelength-dependent photolysis pathways for these reactions. The reaction of ozone with the halogen-containing precursors, studied in this thesis, leads invariably to the production of carbonyl complexes. The rigid nature of the matrix means that the spectra of these perturbed carbonyl complexes can be recorded, and the wavenumbers of specific bands compared between similar species. Similar comparisons are made between the carbon monoxide...Lewis acid complexes which tend to be produced after further photolysis of the carbonyl complexes. Trends are observed for these complexes in which the bands of the complex are shifted from those of the isolated species; this shift can be related to the Lewis acid strength of the perturber. Finally, the carbonyl (COBrF), formed in the reaction of tribromofluoromethane with ozone, dissociates via an alternative mechanism to produce the radical-atom pair ECO and Br. The study of the subsequent reactions of these two might possibly have important implications with regard to processes occurring in the atmosphere.

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