Mary V. Barnabas

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 In micelles this addition reaction appears to be superceded by enhancement of the abstraction reaction because the radical is not observed.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.

Thiyl radicals studied by muon level-crossing spectroscopy

Abstract Addition of muonium atoms to thiocarbonyl compounds is shown to result in the formation of sulfur-centered radicals (R-S·) which are observed by level-crossing-resonance spectroscopy. This technique enables one to determine the proton and nitrogen hyperfine coupling constants in thiyl radicals: a task which cannot be accomplished directly be ESR. The radicals are formed here in dilute aqueous solution from muonium atoms produced by irradiation with positive muons.

Ortho-m meta-, para-directional effects in muonium addition to benzoic acid in water

Isomeric free radicals formed by Mu adding to o/m/p positions of benzoic acid in water were detected by LCR and the corresponding resonance positions and proton hyperfine coupling constants were obtained. There was an isotope effect of the ‘second kind’ for these Mu-radicals in the range of 1.26–1.32. The ‘fractional’ formation rate constants per site are 2.7×109, 0.45×109 and 0.85×109 M−1s−1 respectively. Ortho-addition dominates by a large factor, which is in accord with the electron-withdrawing character of the −COOH group.

Relaxation of thiyl radicals as a measure of the probable timescale over which the missing fraction of muonium in water is lost

Muonium atoms produced in water react with thioacetamide (CH3CSNH2) to form a sulphur-centred (thiyl) free radical whose level-crossing-resonance from the CH3-protons has been detected and analysed. Its amplitude increases with concentration below ∼10−2 M but decreases at higher concentrations with a concommitant increase in linewidth. This is interpreted as indicating overlap with paramagnetic species from the muon track. It starts when the track is a few nanoseconds old.

Muonium and micelles

A pseudo first-order term has been added to the kinetic analysis of micelle-mediated muonium reactions. It is discussed with respect to enhancements of several orders of magnitude for reactions such as that of 2-propanol. It covers the possibility of ‘trapping’ of muonium followed by tunneling.

Primary reactivity of muons in radiation tracks

Abstract When using 4 MeV positive muons as a radiation source, the 4 MeV of radiation chemistry that occurs is ignored and attention is focused on the fate of the muon at the very end of its track. On measuring the yields of the various muon-containing products there are seen to be missing fractions, which are compared with geminate yields of radiolysis. Muon studies were made by muon-spin-rotation and level-crossing-resonance techniques. The former gives information about muonium atoms only if they form in less than a nanosecond and live for more than a microsecond, whereas the latter technique allows any muonated free radical present as the muon decays to be studied. The latter is used here to determine the formation rate of a product—specifically the radical formed from addition of muonium to acetone.

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.

Muonated cyclohexadienyl radicals observed by level crossing resonance in dilute solutions of benzene in hexane subjected to muon-irradiation

Abstract Benzene is used here as a scavenger of muonium to produce the muonated cyclohexadienyl radical in dilute solutions in n-hexane. The radical was identified by level crossing resonance spectroscopy (LCR) by observing the proton resonance of the —CHMu group occuring at 2.059T. Its yield is found to equal the sum of the muonium atom yield and the “missing” muon yield in hexane (total 35% of the incident muons). Consequently, the complete dispersement of muons in different chemical associations is now accounted for in a saturated hydrocarbon liquid, and is seen to be similar to that in water.