Foley, Loraine Johanna; (2001) Photochemically induced reactions between ozone and halogenated species: A matrix isolation study. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The main objective of this research was to study the photochemically induced reactions of ozone with some halogenated species in low temperature matrices, using Fourier-transform infrared spectroscopy, to observe and characterise novel intermediates and to determine pathways for such reactions. Ozone plays a significant role in the chemistry of both the stratosphere and troposphere and thus information concerning the photochemistry of halogenated species in the presence of ozone is of considerable atmospheric importance and is therefore relevant to gas phase atmospheric research. The FT-IR matrix isolation technique is well suited to providing background information on gas phase atmospheric reactions since most atmospheric species exhibit infrared spectra, and the ability to isolate low concentrations of reactive chemical species in the matrix mimics the conditions of dilution in the atmosphere. Thus the results obtained from matrix isolation experiments can complement those obtained from gas phase studies. Since matrix-isolated species are held at very low temperatures, any thermal reactivity is quenched and reactions have to be initiated either photochemically or thermally. In these experiments the matrices are photolysed in order to initiate a reaction by using a range of wavelengths in the visible and UV regions. Any reactive or unstable species produced by irradiation at a particular wavelength are stabilised indefinitely for spectroscopic analysis, thus allowing the photochemical pathway to be revealed. In the matrix the species are held in close proximity to one another allowing secondary reactions to occur that would not have occurred in the gas phase; this has enabled a range of nearest-neighbour complexes to be generated in situ and detected in this study. Of the halogenated species studied, the halogen cyanides, ICN and BrCN, were separately co-deposited with ozone and the photo-induced reactions are reported. Ozone is shown to form a complex with ICN but not with BrCN and so the photochemistry of ozone is altered in the O3/ICN reaction, allowing the formation of several new species to be observed. Harsh UV irradiation is required to initiate a reaction in the O3/BrCN matrix, as well as in the reactions between ozone and the trihalomethanes (CHBr2Cl and CHBrCl2) where several novel carbonyl-Lewis acid and carbon monoxide-Lewis acid complexes were detected. The co-deposition of ozone with each of the halogenated ethenes, BrCH=CHBr and C1CH=CHC1, led to the formation of a charge transfer complex which dissociates to form several new species including carbonyl and carbon monoxide species. Finally, the photo-induced reaction of ozone with some diiodo-species highlights the different photochemical behaviour of a molecule containing two iodine atoms as distinct from one in the presence of ozone. In each case the photoproducts were identified by FT-IR spectroscopy and photochemical pathways proposed. In some instances nitrogen dioxide and solid oxygen matrices were used to provide alternative sources of O atoms for reaction.

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