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Reactions of muonium and positronium in solution Barnabas, Mary Vijayarani


Positive muons are produced at TRIUMF as pion decay products and form muo-nium atoms in media such as water during their two microsecond lifetime. Muo-nium is a hydrogen-like atom with virtually the same Bohr radius and ionization energy as ¹H, ²H and ³H, but with a mass one-ninth that of ¹H. Its reactions toward solutes are studied by μSR (the muon spin rotation technique) and the muonated free radicals it forms are studied by μLCR (muon level crossing resonance spectroscopy). In this thesis, rate constants for muonium reactions were determined for a range of solutes from simple amides to DNA bases. Their values ranged from <10⁵ M⁻¹s⁻¹ to >10¹⁰M⁻¹s⁻¹. Kinetic isotope effects (relative to-¹H) varied from 100 to 0.01 depending on the reaction-type. In the presence of added micelles the rates of some abstraction reactions were very greatly enhanced, whereas most addition reactions were unaffected by the micellar microenvironment. Muonium was seen by LCR spectroscopy to add at a diffusion-limited rate across thiocarbonyl groups, with muonium attached to the carbon and thus yielding thiyl radicals. Uracil and thymine were studied by both μSR and LCR and their relative addition probabilities at C(5) and C(6) were determined. Ortho-positronium (the atom consisting of an electron-positron pair with parallel spins) was studied by the positron annihilation lifetime technique for comparison with muonium in most of these same solutions. Its reactions are different, not 'adding' or 'abstracting' for instance, and it shows none of the properties of a hydrogen isotope.

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