Exploring chemical reaction mechanisms using a light “isotope” of hydrogen

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(Thesis) Ph.D.
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Muonium (Mu = μ+e–) behaves like a light isotope of hydrogen, and adds to molecules with unsaturated bonds to form muoniated radicals. Muoniated radicals were observed by µSR (muon spin rotation, resonance, and relaxation spectroscopy). As an aid to spectral assignments, experimental hyperfine coupling constants (hfcs) were compared to values obtained from density functional theory computations. Detection and characterization of muoniated radicals were used to investigate, evaluate, and elucidate hydrogen atom chemical reaction mechanisms. Instead of the anticipated silyl radicals, beta muoniated disilanyl radicals were detected after reaction of Mu with stable singlet silylenes. A two-step reaction mechanism is proposed to explain the formation of muoniated disilanyl radicals. For the double bond of a silene, it was determined experimentally that Mu adds preferentially to carbon over silicon with the ratio of 2.2 : 1 in THF. This is consistent with thermodynamic calculations and other considerations. Reaction of muonium with germylidenes generated alpha-muoniated germyl radicals. The difference in reactivity of Mu with carbenes, silylenes, and germylidenes was explained in terms of the Lewis basicity of the ylidene (or ylidene analogue) and the Lewis acidity of the radical. The muon hfc in a germyl radical was found to have a linear relationship with the germylidene concentration, consistent with predictions of a dipole dipole reaction field model. Four of six possible radicals were observed from Mu addition to azulene. The ratios of observed products support a competition between two radical reaction mechanisms proposed by Alder et al. for the transformation of azulene into naphthalene. While there are practical limitations to the µSR technique, it has been shown that it is a useful tool for investigation of hydrogen-atom reaction mechanisms. This is the first thesis to explore reaction mechanisms with the aid of µSR.
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