Muonium chemistry in condensed media

• Main Author: Ng, Chi Biu William
• Language: English
• Published: University of British Columbia 2010
• Subjects: Muons
• Online Access: http://hdl.handle.net/2429/24335

Muonium (u⁺e⁻, chemical symbol Mu) consists of an orbital electron associated with a positive muon as nucleus. It can be regarded as a very light 'isotope' of the hydrogen atom because it has essentially the same Bohr radius and ionization energy. Thus it can be used as a sensitive probe of isotope effects and of H-atom reactions which cannot be studied by conventional techniques. Due to the unique nuclear spin properties of the muon, there are several techniques available for investigation. These include muon spin rotation (μSR), muonium spin rotation (MSR) and muonium radical spin rotation (MRSR) in transverse magnetic fields, as used in this study. Various fundamental aspects of muonium formation and of chemical reaction kinetics have been explored by the experiments presented in this thesis. These are summarized below. (i) From the magnetic field dependence, it was verified that Mu does not react chemically with water to any significant extent. Its observed spontaneous slow spin relaxation arises from experimental artifacts such as magnetic field inhomogeneities and/or Mu-frequency beating. (ii) The MRSR technique was used to observe and identify muonium-substituted free radicals via their pair of precession frequencies in high transverse magnetic fields in pure benzene, pure styrene, and their mixtures. The results have implications regarding the mechanism of radical formation and selectivity, (iii) Both μSR and MSR experiments were performed on neopentane (liquid & solid) and concentrated KOH solutions. The μ⁺ and Mu yields in these systems indicated that a spur model of Mu formation is neither appropriate nor adequate to explain the results, (iv) In muonium solution kinetic studies, the reaction Mu + OH[sub=aq; sub=-] was found to be relatively slow, with a substantial activation energy (E ) and no kinetic isotope effect compared to H at room temperature. The reaction shows Mu behaving as a "muonic" acid, (v) Kinetic studies of the abstraction of D by Mu from DCO₂⁻ as a solute in water gave a large E[sub=a]. Upon comparison with HCO₂⁻, the isotope effects (k[sub=M]/k[sub=H] and k[sup=I; sub=D]/k[sub=I; sub=D]) imply that quantum mechanical tunnelling does not dominate the abstraction of H and D atoms in HC0₂⁻ and DC0₂⁻ by either H or Mu at room temperature, (vi) The MSR technique was used to initiate a study of model biological systems (various solutes incorporated in cyclodextrins and micelles). The results demonstrated the sensitive and non-destructive nature of the MSR technique, (vii) Hydrocarbons were also investigated: including measuring their muon yields, their temperature dependence, the effect of an external electric field, and yields in solvent mixtures. Almost all the data obtained seem to be at variance with the expectations of significant intra-spur processes in Mu formation, but are consistent with that of a 'hot atom' mechanism. === Science, Faculty of === Chemistry, Department of === Graduate