John M. Stadlbauer
University of British Columbia
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Chemical Physics Letters | 1988
Krishnan Venkateswaran; Robert F. Kiefl; Mary V. Barnabas; John M. Stadlbauer; B. W. Ng; Zhennan Wu; David C. Walker
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 In micelles this addition reaction appears to be superceded by enhancement of the abstraction reaction because the radical is not observed.
Chemical Physics | 1989
Krishnan Venkateswaran; Mary V. Barnabas; Zhennan Wu; John M. Stadlbauer; B. W. Ng; David C. Walker
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.
Chemical Physics Letters | 1988
Krishnan Venkateswaran; Mary V. Barnabas; Zhennan Wu; John M. Stadlbauer; B. W. Ng; David C. Walker
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.
Hyperfine Interactions | 1984
B. W. Ng; John M. Stadlbauer; Yasuo Ito; Yasuhiro Miyake; David C. Walker
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.
Hyperfine Interactions | 1986
John M. Stadlbauer; B. W. Ng; David C. Walker
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.
Journal of Chemical Physics | 1981
Y. C. Jean; B. W. Ng; John M. Stadlbauer; David C. Walker
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...
Radiation Physics and Chemistry | 1997
John M. Stadlbauer; Krishnan Venkateswaran; David C. Walker
Abstract Muonium atoms react with chloroacetic acid and chloroacetate ions in dilute aqueous solution with rate constants of 2.3 × 106 and 9.1 × 105 dm3 mol−1 s−1 respectively. These are compared with the reactions of 1H atoms (and eaq−) and discussed in terms of a pair of competing kinetic isotope effects. Muonium reacts at least eight times faster than H overall, and probably 28 times faster in forming Cl−. It behaves as a nucleophile, thus resembling eaq− more than H, in reacting faster with the acid than the anion. Muoniums reactions must be governed to a considerable extent by quantum-mechanical effects arising from its very small mass.
Hyperfine Interactions | 1994
John M. Stadlbauer; Krishnan Venkateswaran; Gerald B. Porter; David C. Walker
The muon level-crossing-resonance technique has been used to resolve major discrepancies that exist in muon-spin-resonance studies (both free-radical formation and muonium decay rates) in the competition between benzene and styrene. The results, obtained for ∼30 mM solutions in ethanol and for 2.5 mM aqueous micelles solutions, show that muonium atoms (Mu) react 8 (±2) times faster with styrene than with benzene. In the above cases thermalized Mu is unquestionably the reactive species, which is known to show nucleophilic intra-molecular selectivity in the case of styrene. But a similar value, 9 (±2), was also obtained for undiluted mixtures of liquid benzene and styrene (neat mixture) — where the precursor might have been ‘hot Mu’ (which should display weaker selectivity than Mu) or cations derived fromμ+ (which should show higher selectivity). These results support the view that thermalized Mu is the predominant reactive species in liquid benzene and styrene.
Hyperfine Interactions | 1991
John M. Stadlbauer; Mary V. Barnabas; Zhennan Wu; David C. Walker
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.
Hyperfine Interactions | 1984
Yasuhiro Miyake; Yoneho Tabata; Yasuo Ito; B. W. Ng; John M. Stadlbauer; David C. Walker
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.