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Exploring main group radicals using an isotope of hydrogen

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Thesis type
(Thesis) Ph.D.
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Muonium, which is considered a light isotope of the H atom, is a radioactive atom with a lifetime of 2.197 µs. Muonium adds to unsaturated molecules to form muoniated radicals. The collection of spectroscopic techniques that are used to observe muoniated radicals are known as µSR. To determine the identity of the muoniated radicals, experimental hyperfine coupling constants (hfcs) of the muoniated radicals obtained from µSR techniques were compared with hfcs of the muoniated radicals calculated using Density Functional Theory (DFT) methods available in the Gaussian 09 software package. µSR studies help us understand the reactivity of molecules towards the H atom and the configuration and conformation of the radicals formed. The polyether ether ketone (PEEK) polymer was tested for suitability in µSR sample cell fabrication. Muoniated radicals formed from monomers of PEEK, 4,4-dihydroxybenzophenone and para-dimethoxybenzene were detected. Since similar radicals expected in PEEK could interfere with sample signals it is concluded that PEEK is unsuitable for µSR sample cells. Phosphaalkene reactions with muonium were studied to understand their behaviour in radical polymerization. The model compound mesPC(Me)2 was studied and two muoniated free radicals, mesP-MuC•(Me)2 and mesP•-C(Mu)Me2 were detected. The mesP•-C(Mu)Me2 radical was compared with its isotopologue mesPH-C•(Me)CH2Mu formed from mesPH(CMe=CH2). A number of phosphaalkenes that differ from each other with respect to electronegativity and the bulkiness of the attached substituent groups were studied. Adamantyl phosphaalkene (AdP=CtBuH) produced only the AdMuP-C•(tBuH) radical while a sample of (CF3)2-mesP=C(Me)2 showed muoniation at both the P and C centers of the P=C bond. Muoniated radicals formed by mesP=CPh2 were identified. This helped to resolve ambiguity in identifying the initiation products of the radical polymerization pathway of mesP=CPh2. The reaction of Mu with 2,4,6-tri-tert-butyl-1,3,5-triphosphosphabenzene (TPB) resulted in two muoniated radicals. Mu addition to the C atoms of the ring resulted in rearrangement to form a bicyclic product. TPB undergoes hydrogenation via a cationic route forming a bicyclic product. In this thesis I propose a radical route for this hydrogenation pathway.In summary we have utilized µSR techniques to broaden the understanding of neutral radical formation from phospha-organic compounds.
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Thesis advisor: Percival, Paul
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