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Investigation of kinetics and thermochemistry of ion-molecule reactions using photoionization mass spectrometry


Williamson, Ashley Deas (1976) Investigation of kinetics and thermochemistry of ion-molecule reactions using photoionization mass spectrometry. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/mgxv-k439.


Chapter I introduces the methodology of photoionization mass spectrometry and lists common applications, including study of ionmolecule reactions. A major advantage of photoionization in the study of ion chemistry lies in the favorable photoionization threshold laws, which frequently permit accurate knowledge of the internal energy distribution of reactant ions. Study of reactions as this distribution is varied allows measurement of the effects of reactant ion internal energy on the reaction kinetics. The photoionization mass spectrometer consists of a discharge lamp, a one-meter normal incidence vacuum monochromator, and a medium pressure quadrupole mass spectrometer. The instrument and its operating conditions are detailed.

Chapter II contains a photoionization study of the reactions of the molecular ion in vinyl fluoride to yield the ionic products C3H3F2+, C3H4F+, and C3H5+. Quantitative measurements are reported of the effect of the vibrational state of the reactant ion on the product distribution and overall reaction cross section. Reaction cross sections for all three channels decrease with reactant internal energy. The effect on the reaction pathway producing C3H3F2+ is especially pronounced, with 0.19 eV of vibrational excitation being sufficient to reduce the reaction probability by 80%. Deactivation of vibrationally excited reactant ions competes with the reaction and is shown to be an efficient process.

Chapter III details a study of the major ion-molecule reaction pathways in ketene and ketene-d2 by photoionization mass spectrometry and ion cyclotron resonance spectroscopy. For processes involving the molecular ion, the variation of reaction cross section with ion vibrational state is pronounced. The threshold determined for the endothermic process CH2CO+ + CH2CO → C2H4+ + 2CO provides a novel confirmation of the recent redetermination of the heat of formation of ketene.

In Chapter IV photoionization efficiency data are presented for the parent and major fragment ions in 2,2-difluoropropane and 2-fluoropropane. Appearance potentials for CH3 and CH4 loss may be used to relate the heats of formation of the olefin radical cations and fluorinated ethyl carbonium ions to the parent neutral and to one another. A thermochemical cycle allows determination of the proton affinities of vinyl fluoride and 1,1-difluoroethylene. The fragmentation thresholds in 2-fluoropropane appear to be too high by 7-9 kcal/mole. standard heats of formation determined by this study are: (CH3)2CF2, -129.8 ± 3.0 kcal/mole; CH3CF2+, 108.5 ± 3.2 kcal/mole; (CH3)2CF+, 138.0 ± 1.6 kcal/ mole; CH3CHF+, 162.6 ± 1.1 kcal/mole.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Beauchamp, Jesse L.
Thesis Committee:
  • Unknown, Unknown
Defense Date:17 September 1975
Funding AgencyGrant Number
Veterans AdministrationUNSPECIFIED
State of CaliforniaUNSPECIFIED
Record Number:CaltechTHESIS:01232017-162540529
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:10022
Deposited By: Benjamin Perez
Deposited On:24 Jan 2017 15:37
Last Modified:09 Nov 2022 19:20

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