Zhennan Wu

A level-crossing-resonance study of muonated free-radical formation in solutions of acetone in hexane, water and dilute micelles

The (CH3)2COMu radical forms when positive muons are stopped in pure acetone and dilute mixtures of acetone in n-hexane or water. Muonium is the precursor of the radical in dilute solution and evidently differs from hydrogen in adding readily to the carbonyl group. In micelles this addition reaction appears to be superceded by enhancement of the abstraction reaction because the radical is not observed.

Muon level crossing resonance study of radical formation in allylbenzene, styrene and toluene

Abstract All chemical states of the muons in a μSR experiment have now been determined in toluene, allylbenzene and styrene. There are no “missing fractions” because the sum of the various muon-containing free-radicals equals 1- P D , where P D is the directly formed diamagnetic fraction. Use of the new technique of level crossing resonance spectroscopy has enabled yields to be determined and identification of individual isomeric radicals. For toluene, there is a total radical fraction of 0.77 and a distribution of 2.5:2:1 for ortho: meta: para addition within the ring. For allylbenzene, ≈ 70% of the muonated radicals are side chain addition products and of these nearly 40% have Mu on the second C; and, for the 30% adding to the ring, there is virtually no selectivity of site as the o : m : p ratio is the statistical ratio 2:2:1. Toluene and allylbenzene, however, differ dramatically from styrene. In styrene, 82% of the muons form radicals and 85% of these arise from formal addition of muonium to the end C of the side chain to give muonated phenylethyl radicals. The remaining 15% are seen to be distributed (2:1) between the ortho and para positions of the ring, with no addition at the meta position. The high degree of preference shown by styrene indicates strong selectivity in achieving the most stable radical. Proton hyperfine couplings for all of these radicals have also been determined.

Micelle-induced enhancement of the reactivity of muonium atoms in dilute aqueous solution

Abstract Enhancements in rate constants from 10 6 M −1 s −1 to more than 10 10 M −1 s −1 have been found for the reaction of muonium atoms with 2-propanol in water when micelles are added.

Comparison of muonated free radicals formed in pure liquids with those in dilute solutions: Origins of radicals

LCR spectroscopy was used to explore the mechanism by which muonated free radicals form in unsaturated organic liquids. This was achieved by comparing the relative yields of the different radicals formed from the compound as a pure liquid with those found when it was studied as a solute in dilute aqueous micelle solutions. In the latter medium, thermalized muonium atoms are the only precursors. Two types of examples were evaluated: first, styrene and allylbenzene since they give a full range of side-chain and ring addition products: and second, methyl acrylate and acrylonitrile since the former alone shows conformational s-cis-trans isomerism. Both types of process were expected to be particularly sensitive to the nature and charge of the reactive species involved. The results suggest that neutral muonium is the dominant precursor in all these liquids, with ‘hot’ muonium perhaps contributing about 30% of the radicals.

Muonium atoms compared to hydrogen atoms and hydrated electrons through reactions with nitrous oxide and 2-propanol

Abstract N2O and 2-propanol constitute a pair of solutes with completely opposite relative reactivities towards H and e-aq. It is observed in the present study that muonium atoms (Mu) fall directly between H and e-aq for both of these reactions. Aspects of mass-dependent processes and kinetic isotope effects are discussed with regard to these findings.