Investigation of H atom and free radical behavior in gas hydrates

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Muoniated free radicals
Gas hydrates
DFT calculation
NMR of hydrates

Gas hydrates (or clathrate hydrates) are solid crystalline materials composed of a framework of hydrogen-bonded water molecules arranged to form cages which can contain small guest molecules. They have been a subject of research in the oil and gas industry, for carbon dioxide sequestration, gas storage and separation. In order to better understand the applications of hydrates, there is a need to study them at the molecular scale, but there has been relatively little investigation of chemical reactions of the guest molecules. In this thesis project, muon spin spectroscopy was used for the first time to investigate the behavior of muonium (a light isotope of hydrogen) and free radicals in hydrates. Muonium (Mu) and muoniated free radicals were observed in the hydrates of cyclopentene, furan, 2,5- and 2,3-dihydrofuran, pyrrole, thiophene, isoxazole, benzene and acetone. In order to confirm that hydrates were formed, they were characterized by PXRD and solid state 129Xe-NMR and 13C-NMR. The free radicals were formed by addition of Mu to unsaturated organic compounds that reside as isolated guests in the hydrates. Muon and other nuclear hyperfine coupling constants (hfcs) were extracted from μSR spectra of the radicals and compared to liquid-phase data. DFT calculations of hfcs were used to guide the spectral assignments and distinguish between competing radical products where applicable. An extra μ-LCR resonance was seen in the spectra of radicals in the hydrate, indicating that they have restricted motion compared to the liquid state. Muonium and muoniated free radicals were observed simultaneously in the hydrates of acetone and benzene. This was previously only observed in C60 powder and shows that Mu and the radical are in physically separated environments in the hydrates. The Mu amplitude decreases while the radical amplitude increases with temperature. This is consistent with Mu diffusion from the small cage to the large cage in the hydrates, where it can react with the guest. The diffusion occurs at a lower temperature in the acetone hydrate compared to the benzene hydrate.

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Paul Percival
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.