I. Gas Phase Proton Affinity of Zwitterionic Betaine. II. High Resolution Spectroscopy of Trapped Ions: Concept and Design

Citation

Lee, Hak-No (1999) I. Gas Phase Proton Affinity of Zwitterionic Betaine. II. High Resolution Spectroscopy of Trapped Ions: Concept and Design. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8gqa-0x72. https://resolver.caltech.edu/CaltechTHESIS:11212019-155237296

Abstract

In an ideal experiment, the system being investigated is isolated from the environment. The only external influences allowed on the system are the parameters that the experimenter chooses to vary, in effort to study their effects on the observables. Moreover, these parameters can be controlled with all the accuracy and precision desired by the experimenter. In chemistry, ion cyclotron resonance (ICR) mass spectrometry may come closer to replicating this ideal condition than any other experimental technique. An ion isolated in an ICR trap is under ultra high vacuum, devoid of physical contact with other atomic and molecular systems, as well as with the apparatus itself. Confined to a small volume and for a practically unlimited length of time, its few connections with the external environment, such as temperature and the electric and magnetic trapping fields, are well under the experimenter's control. And since the motion of a charged particle in electric and magnetic fields is completely known, the ion can be manipulated with an unequaled freedom and certainty. In this dissertation, two experimental methods which utilize these unique capabilities of ICR spectrometry are explored.

In Chapter 1, the kinetic method is applied to determine the gas phase proton affinity of the zwitterion betaine, (CH₃)₃N⁺CH₂CO₂⁻. Protonated dimers of betaine with reference bases of known proton affinities are formed by Cs⁺ bombardment of a glycerol solution in an external ion source FT-ICR. Product distributions resulting from off-resonance collisional activation of isolated adducts are analyzed to yield a value of 242 ± 1 kcal/mol for the gas phase proton affinity of betaine. This is 103 kcal/mol less than that of the isoelectronic tert-butylacetate anion, (CH₃)₃CCH₂CO₂⁻, and the difference can be attributed to the electrostatic dipolar stabilization of the carboxylate anion in betaine. In addition, a general analysis of the kinetic method is presented based on RRKM unimolecular reaction theory. This analysis does not assume a Boltzmann distribution of internal energies in the reactant ion and provides a rationalization for the success of the method even when different experimental techniques are used.

While ICR has proven to be a powerful technique for studying the chemical properties of gas phase ions, it suffers from a serious weakness which prevents full realization of its unique capabilties: The only observable it measures is the mass. This has kept ICR from being widely used to investigate the physical properties of molecular ions. In Chapter 2, a novel method for obtaining high-resolution r.f. and microwave spectra of ions in ICR trap is proposed. Termed internally resonant ion trapping excitation (IRITE), it uses spatially inhomogeneous a.c. electric fields to couple the internal energy states with the ion's translation motion. The resonance absorption of radiation is detected by its effect on the oscillations of the trapped ions, rather than on the radiation. The theoretical concept behind IRITE is introduced, and an experiment designed to demonstrate its feasibility by observing r.f. transitions in HCl⁺ is discussed in details. Combined with ICR's unsurpassed ability to isolate and manipulate chemical systems, this new technique promises to allow chemists to study phenomena previously unobservable even in neutral molecules.

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