Studies of muonium-substituted molecules in propan-2-one and in aqueous solutions of propan-2-one

Abstract

Muonium-substituted molecules, both paramagnetic and diamagnetic, are formed when positive muons are implanted in pure propan-2-one and in binary aqueous systems. These have been studied by the technique known as muon spin rotation, or µ.s.r.

Identification of the paramagnetic fraction as the muonic propan-2-ol radical (CH₃)₂ĊOMu and of the diamagnetic fraction in aqueous solutions as ‘light water’ MuOH is corroborated by the qualitive behaviour of the µ.s.r. parameters frequency, amplitude and linewidth. Comparison of the hyperfine coupling to the muon in (CH₃)₂ĊOMu with that to the hydroxy proton in (CH₃)₂ĊOH reveals an unusually large isotope effect, which is interpreted in terms of the preferred molecular conformation. The significant effect of solvation on the muonic hyperfine constant is interpreted in terms of hydrogen and muonium bonding.

Muonium itself is detectable in dilute aqueous solutions. From measurements of its reaction rate, as well as other considerations, it is concluded that neither thermal nor epithermal muonium can be the major radical precursor. A production mechanism for the observed radical is proposed involving the encounter of radical-anion and muonic-cation states. However, thermal reactions of muonium can account for some loss of polarisation, as well as a corresponding enhancement of the radical and diamagnetic yields measured at sufficiently low magnetic fields.