B. W. Ng

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 In micelles this addition reaction appears to be superceded by enhancement of the abstraction reaction because the radical is not observed.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

Muonium radicals in benzene-styrene mixtures

Muonium radicals were observed through theirμSR precession frequencies in high transverse magnetic fields in pure benzene, pure styrene and their mixtures, all as liquids at room temperature. In benzene-styrene mixtures, the radicals obtained in each pure liquid are both present, so no slow (10−9−10−5 s) intermolecular exchange occurs; but strong selectivity was found with the formation of the radical from styrene being about eight-times more probable than the radical from benzene.

Reaction of muonium with O2 in aqueous solution

Abstract The interaction between Mu and O 2 in water, which could be a spin-conversion process, has a rate constant of (2.4 ± 0.5) × 10 10 M −1 s −1 and is compared with the diffusion-controlled limit and the corresponding gas phase reaction.

The role of substituents on the addition of muonium to aromatics: As seen through muonium-radicals in allyl benzene compared to styrene, and from hammett parameters in aqueous solution

Muonium-radicals resulting from insertion into the benzene ring are found to be much more prevalent in allyl benzene (C6H5CH2CH=CH2) than in styrene (C6H5CH=CH2). The salient implication of this comparison is that intramolecular rearrangements preceeded the μSR observation for the case of styrene. In turn, this suggests that muonium-containing free radicals, as seen directly by kilogauss transverse field μSR, are not necessarily theprimary radicals. Therefore, the elucidation of mechanism (and identification of the precursor) of Mu-radical formation is further complicated by the fact that the observations may refer to thermodynamically more stable secondary radicals-those resulting from a variety of intra-or inter-molecular relaxations or exchanges. Primary kinetic selectivities of thermalized muonium atom addition reactions can be determined, however, through the substituent effect on the Hammett linear free energy parameter in dilute solution. Results have been obtained for substituted benzenes and benzoic acids. Muonium apparently has a mild nucleophilic character. And, most interestingly, this is opposite to that of its heavy isotope hydrogen.

Muonium reactions in micelles

The chemical rate constants for muonium (Mu) atoms reacting with iodine, phenol, Ni++, Fe(CN)−36, and naphthalene in three aqueous anionic micellar systems (sodium hexyl sulfate, sodium octyl sulfate, and sodium dodecyl sulfate), were measured by MSR (the muonium spin rotation technique). The results show that the Mu reactivities toward I2, phenol, and naphthalene were significantly increased when these solutes were located inside micelles as compared to homogeneous aqueous solutions. A large increase in the rate constant occurred at the critical micelle concentration, showing that MSR can be used to probe such properties of micelles. The observed rate constants did not change much with the size of the micelles, but the rate was less than that found in a pure organic solvent, supporting the view that the core of a micelle is relatively viscous due to its ordered structure. Since Mu is a light isotope of hydrogen, it may be inferred that H atoms would have analogous behavior in micellar systems—including t...

Electron scavenger effects on diamagnetic muon polarization in cyclohexane

The effect of electron scavengers on diamagnetic polarization PD in cyclohexane were examined and compared with the experimental yield of Ps formation (inhibition and anti-inhibition effects) for the same solutions. The effect of C6F6 on PD has been shown to sharply contrast, as we call “non anti-inhibition”, to the anti-inhibition effect in Ps formation. These results suggest that Muonium formation is different from Ps formation, and does not agree with the simple spur reaction model.

Application of the Hammett free energy relationship to muonium addition reactions with benzoic acid derivatives in aqueous solutions

Abstract Kinetic isotope effects and Hammett reaction parameters ϱ have been determined for the interaction of muonium atoms (Mu) in water with substituted benzoic acids as reactive solutes. The ϱ parameter for muonium has a value of +0.27 ± 0.05, which indicates a weakly nucleophilic attack by Mu to give free-radical intermediates. In contrast, the analogous H-atom reactions show slight electrophilic character and are slower than Mu by more than the mean thermal velocity effect. These results were obtained in “muon spin rotation” studies.

Recent results in muonium solution kinetics

Using Muonium Spin Rotation (MSR) techniques the aqueous solution kinetics of several muonium addition reactions and spin conversion interactions have been studied. The addition reactions show both diffusion and activation-controlled reaction rates with isotope effects between 1 and 3 for diffusion-control and between 7 and 31 for activation-control reactions. Barrier energies are typically ≈15 kJ/mole and ≈30 kJ/mole, respectively, for these processes in water. Spin-conversion interactions involving Ni(aq+2 and Ni(cyclam)+2 complexes showed that spin-conversion of “triplet” Mu by a paramagnetic solute occurs at or near the diffusion-controlled limit while the chemical reaction with the diamagnetic configuration of Ni(cyclam)+2 occured some 100 times slower at kM ⩽ 5×108M−1s−1.