Investigation of Kinetics and Thermochemistry of Ion-Molecule Reactions Using Photoionization Mass Spectrometry

Citation

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. https://resolver.caltech.edu/CaltechTHESIS:01232017-162540529

Abstract

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.

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