Ionisation studies of chlorinated and fluorinated compounds

Parkes, Michael Anthony (2007). Ionisation studies of chlorinated and fluorinated compounds. University of Birmingham. Ph.D.

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Abstract

Ionisation processes in gas-phase polyatomic molecules have been studied using threshold photoelectron photoion coincidence and selected ion flow tube techniques for cations and an electron attachment mass spectrometer for anions. Cation formation has been studied for fluoroform (CHF\(_3\)), octafluorocyclobutane (\(c\)-C\(_4\)F\(_8\)), octafluorocyclopentene (\(c\)-C\(_5\)F\(_8\)), monochloroethene (C\(_2\)H\(_3\)Cl), the three isomers of dichloroethene (C\(_2\)H\(_2\)Cl\(_2\)) (i.e. 1,1-dichloroethene, (\(Z\))-1,2-dichloroethene and (\(E\))-1,2-dichloroethene), trichloroethene (C\(_2\)HCl\(_3\)) and tetrachloroethene (C\(_2\)Cl\(_4\)). Comparison between the data from the photoionisation and the ion-molecule reactions show that the dominant charge-transfer mechanism is long-range in nature. Detailed studies of the reactions of cations with the three dichloroethenes have been performed to look for evidence of isomeric effects. Major differences are seen when the reactant ion is CF\(_3\)\(^+\); when 1,1-dichloroethene is the neutral reactant the only ionic product is C\(_2\)H\(_2\)Cl\(^+\), but if the neutral is 1,2-dichloroethene then the only product is CHCl\(_2\)\(^+\). Only minor differences are seen with other reagent ions. For the reactions of all six chloroethenes with CF\(_3\)\(^+\), product ions are observed which can only be formed by extensive rearrangement across the carbon-carbon double bond. Mechanisms are suggested involving bridged trigonal intermediates to explain the production of the different channels. Studies have been performed on several perfluorocarbons, CHF\(_3\), \(c\)-C\(_4\)F\(_8\) and \(c\)-C\(_5\)F\(_8\), due to their potential for use in industry. They are all excellent at etching substrates and have a lower global warming potential than the currently used gases. CHF\(_3\) shows signs of non-statistical dissociation following photoionisation, and is shown to react via a largely long-range charge transfer mechanism in the ion-molecule reactions studied. For \(c\)-C\(_5\)F\(_8\) the ground electronic state of the ion is found to be very weak under threshold conditions whilst being much more intense when He(I) photons are used. The existing electron attachment mass spectrometer is described, and also extensive modifications which have been made to this apparatus. The use of new electronics and acquisition system give excellent results in the new equipment. The new arrangement has been extensively characterised by studying attachment to seven molecules, including several perfluorocarbon molecules (e.g. the isomers of C\(_4\)F\(_8\)). Some isomeric effects on the measured rate coefficients are observed in the data.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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