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Studies of the muonium atom in liquid media

University of British Columbia

The μSR (muon spin rotation) and MSR (muonium spin rotation) techniques have been applied to study the reaction of muonium atoms in liquid media. Specifically, the temperature dependence of five types of reactions of muonium atoms in aqueous solution were measured between 1 and 92°C. Results indicated that for an electron-transfer reduction with MnO₄⁻, for an addition to π-bonds in maleic acid and for a spin-conversion reaction with Ni²⁺, the rate constants were all diffusion-limited. An activation energy for diffusion of 4.2 kcal.mole⁻¹ was found, while the A-factors varied somewhat in the range of -10¹³ M⁻¹s⁻¹. For these diffusion-controlled reactions there is no kinetic isotope effect when compared with H-atoms. On the other hand, a large kinetic isotope effect was found for the abstraction reaction of muonium with formate ion (HCO₂⁻) and this rate constant was activation-controlled. Also, the reaction with NO₃⁻ was studied and tends to exhibit a curved Arrhenius plot even over such a short temperature range. Such curvature is consistent with either a contribution from quantum mechanical tunnelling or alternative reaction paths. Other temperature-dependence studies of both the muon and muonium atoms in CS₂, H₂O, and CCℓ₄ are also reported. The muon relaxation rate constant in CS₂ was found to change somewhat from 1 to 40°C. In the case of H₂O, both the muon asymmetry and muonium relaxation rate constant were found to increase slightly from 1 to 92°C. However for CCℓ₄, there was no change found for the muon asymmetry in the temperature range of 2 to 58°C. In addition, the interactions of muonium with O₂ and with porphyrins in water were both studied using the MSR technique. In the case of O₂, the rate constant, which could be due to spin-conversion, was found to be (2.4±0.5)x10¹⁰ M⁻¹s⁻¹ and is compared with the diffusion-controlled limit and the corresponding gas phase reaction. For the reaction of muonium with porphyrins, the rate constants with hemin and the protoporphyrin were found to be 2.7x10⁹ and 6x10⁸ M⁻¹s⁻¹, respectively. The reaction mechanism is probably mainly through the addition to the conjugated double bond for the protoporphyrin and by reduction or partial spin-conversion for the hemin solutions. The theory behind the μSR and MSR techniques has been very briefly discussed but the experimental system for liquid phase studies at TRIUMF is described in some detail with respect to the transverse field technique.

Text

2010-03-26

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http://hdl.handle.net/2429/22646

Chemistry

University of British Columbia

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