"Applications of Multiple Lasers in Mass Spectrometry: Structural Chara" by Thien Tung Khuu

Date of Award

Spring 2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Johnson, Mark

Abstract

The goal of this thesis is to explore the critical role played by water molecules in the proton migration between two spectroscopically distinct protonation sites adopted by 4-amninobenzoic acid (4ABA). This ultimate goal is achieved through several elementary steps. First, structural characterizations of the two protomers (with and without water molecules), one with the protonation site occurs at the acid group (the O-protomer) and one with the protonation site occurring at the amine functionality (the N-protomer), are explored in detail. Then, to capture the slow water-mediated, proton-transfer process at cold temperature, modifications were made to the current hybrid mass-spectrometer and laser-spectroscopic apparatus, enabling time- and temperature-dependent pump-probe measurements. Since the kinetic processes occurring in the cryogenic trap following photoexcitation are complex, protonated water hexamer (H+(H2O)6) was used as a model system to determine the collisional cooling rate of ions in the trap as well as to test the robustness of experimental methods. Finally, the size and temperature thresholds for the water-mediated, long-range proton transfer between the two protomers of H+4ABA were determined by following the spectroscopic behavior of 4ABAH+•(H2O)n=0-4 cluster ions held in a temperature-controlled, radiofrequency ion trap. The two protomers display dramatically different absorption bands in the UV region of the spectrum. This difference was exploited to disentangle their respective vibrational bands in the infrared and subsequently to determine how the rate of spontaneous proton accommodation by the O-protomer depends on the number of water molecules attached to the cation and the cluster temperature. Size-selected 4ABAH+•(H2O)n=0-4 cluster ions were stored in a temperature-controlled radiofrequency ion trap, and most of the O-protomer population was removed by photoexciting its UV band with a laser pulse (~6 ns duration) near 300 nm. Time-dependent isomer populations (on the tens of milliseconds timescale) were observed by monitoring the characteristic vibrational bands of the O-protomer after selectively depleting its population by UV photobleaching. Temperature-dependent recovery of the O-protomer band pattern is observed with an onset at n = 3.

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