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Title: Studies of ions and ion-molecule reactions in the gas phase using mass spectrometry
Author: Parry, A. J.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1991
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Ion-molecule equilibrium and kinetic studies in the gas phase have been performed using pulsed high pressure source mass spectrometry, the home-built source being coupled to an updated MS9 mass spectrometer. Bimolecular proton transfer equilibria involving benzene, ethanol, methanol and acetaldehyde were investigated as a function of temperature and values of ΔH and ΔS were derived. An attempt was made to find evidence for high entropies of protonation and hence a 'dynamic' protonated benzene structure. No such evidence was found. Apparently high entropies involving ethanol, were explained in terms of thermal neutral decomposition. The ionic decomposition rate for protonated ethanol was measured as being close to zero, although measurable rates of decomposition were observed for some protonated halotoluenes. Chloride transfer equilibria were also investigated although far less successfully. Problems with the present inlet system led to inconsistent results, possible resolution of these problems, via inlet redesign, is suggested. The mechanism of proton transfer between CH5+ and fluoro and chlorobenzene at low temperatures was successfully identified using B/E linked scanning and was shown to occur via proton-bound complex formation, although the mechanism was too complex to allow extraction of the rates of individual steps from the experimental data. The proton affinity of the Cℓ atom in chlorobenzene was subsequently bracketed between those of water and methanol. The potential energy surfaces of a number of protonated aromatic species C_6H_5XH^+ , were probed using Mass-analysed Ion Kinetic Energy Spectroscopy (MIKES). All were found to be roughly similar, with the exception of F substituted species, these had a barrier to ring-substituent proton migration which exceeded the minimum dissociation threshold for HF loss, resulting in the HF loss peak in the MIKE spectrum being composite. The kinetic energy release associated with HX loss in these species was observed as having a direct correlation with charge distribution in the substituent protonated molecule. An appendix describes extensive semi-empirical molecular orbital calculations as species encountered in this work.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available