Donald G. Fleming

Kinetic Isotope Effects for the Reactions of Muonic Helium and Muonium with H2

Calculated reaction rates for two hydrogen isotopes, one 36 times heavier than the other, agree with experiments at 500 kelvin. The neutral muonic helium atom may be regarded as the heaviest isotope of the hydrogen atom, with a mass of ~4.1 atomic mass units (4.1H), because the negative muon almost perfectly screens one proton charge. We report the reaction rate of 4.1H with 1H2 to produce 4.1H1H + 1H at 295 to 500 kelvin. The experimental rate constants are compared with the predictions of accurate quantum-mechanical dynamics calculations carried out on an accurate Born-Huang potential energy surface and with previously measured rate constants of 0.11H (where 0.11H is shorthand for muonium). Kinetic isotope effects can be compared for the unprecedentedly large mass ratio of 36. The agreement with accurate quantum dynamics is quantitative at 500 kelvin, and variational transition-state theory is used to interpret the extremely low (large inverse) kinetic isotope effects in the 10−4 to 10−2 range.

Experimental tests of reaction rate theory: Mu+H2 and Mu+D2

Bimolecular rate constants for the thermal chemical reactions of muonium (Mu) with hydrogen and deuterium—Mu+H2→MuH+H and Mu+D2→MuD+D—over the temperature range 473–843 K are reported. The Arrhenius parameters and 1σ uncertainties for the H2 reaction are log A (cm3 molecule−1 s−1)=−9.605±0.074 and Ea =13.29±0.22 kcal mol−1, while for D2 the values are −9.67±0.12 and 14.73±0.40, respectively. These results are significantly more precise than those reported earlier by Garner et al. For the Mu reaction with H2 our results are in excellent agreement with the 3D quantum mechanical calculations of Schatz on the Liu–Siegbahn–Truhlar–Horowitz potential surface, but the data for both reactions compare less favorably with variational transition‐state theory, particularly at the lower temperatures.

Hyperfine constants for the ethyl radical in the gas phase

Abstract Muon spin rotation and level-crossing spectroscopy have been used to measure the muon, proton, deuteron and 13C hyperfine coupling constants for the isotopically substituted ethyl radicals CH2CH2Mu, CD2CD2Mu and 13CH213CH2Mu in the gas phase.

Hyperfine field and diffusion of μ+ in Fe single crystals

Abstract In recent positive-muon spin rotation experiment at TRIUMF on single crystal Fe, a clear temperature dependent change has been observed, for the first time, both in frequency and depolarization rates from 300 K down to 23 K. The μ + depolarization was explained by the μ + diffusion through inhomogeneous dipolar fields and the diffusion constant was found to obey an Arrhenius law (activation energy 17 meV) above 70 K but surprisingly deviated from this at lower temperatures, indicating quantum diffusion. We have also found that the μ + hyperfine field has a temperature dependence slightly stronger than that of the magnetization.

Muonium chemistry in gases: Mu+Br2

We report a precise measurement of the rate of reaction of muonium atoms with bromine molecules in an argon moderator gas at a atm and 23 °C. The bimolecular rate constant is k= (2.4±0.3) ×1011 l/mole‐sec, ten times higher than that for the analogous reaction of hydrogen atoms. Since muonium can properly be treated as a light isotope of hydrogen, this comparison has potential significance for the theory of reaction rates. The technique is described and the results discussed.

Reaction kinetics of muonium with the halogen gases (F2, Cl2, and Br2)

Bimolecular rate constants for the thermal chemical reactions of muonium (Mu) with the halogen gases—Mu+X2→MuX+X—are reported over the temperature ranges from 500 down to 100, 160, and 200 K for X2=F2,Cl2, and Br2, respectively. The Arrhenius plots for both the chlorine and fluorine reactions show positive activation energies Ea over the whole temperature ranges studied, but which decrease to near zero at low temperature, indicative of the dominant role played by quantum tunneling of the ultralight muonium atom. In the case of Mu+F2, the bimolecular rate constant k(T) is essentially independent of temperature below 150 K, likely the first observation of Wigner threshold tunneling in gas phase (H atom) kinetics. A similar trend is seen in the Mu+Cl2 reaction. The Br2 data exhibit an apparent negative activation energy [Ea=(−0.095±0.020) kcal mol−1], constant over the temperature range of ∼200–400 K, but which decreases at higher temperatures, indicative of a highly attractive potential energy surface. This...

Production of thermal muonium in the vacuum between the grains of fine silica powders

Abstract When spin-polarized positive muons are stopped in fine SiO 2 powder under vacuum, muonium precession is observed indicating 45% muonium formation. Depolarization by oxygen indicates that muonium, like positronium, emerges from the powder grains into the voids.

Muonium chemistry: kinetics of the gas phase reaction Mu + F2 → MuF + F from 300 to 400 K

Abstract The MSR (muonium spin rotation) technique was used to measure the chemical reaction rate for Mu + F 2 → MuF + F in N 2 moderator at ≈ 1 atm from 295 to 383 K giving the Arrhenius expression: log 10 k (l/mole s) = (10.83 ± 0.20) - (200 ± 50)/ T , with k = (1.46 ± 0.11) × 10 10 l/mole s at 300 K. This is at least 6.8 times the room temperature rate constant for the analogous H atom reaction. The measured activation energy and enhancement over the H reaction rate are indicative of significant tunnelling in the Mu reaction, in agreement with the recent collinear quantum mechanical calculations of Connor et al.

Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution

Isotope effects are important in the making and breaking of chemical bonds in chemical reactivity. Here we report on a new discovery, that isotopic substitution can fundamentally alter the nature of chemical bonding. This is established by systematic, rigorous quantum chemistry calculations of the isotopomers BrLBr, where L is an isotope of hydrogen. All the heavier isotopomers of BrHBr, BrDBr, BrTBr, and Br(4)HBr, the latter indicating the muonic He atom, the heaviest isotope of H, can only be stabilized as van der Waals bound states. In contrast, the lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding, in accord with its possible observation in a recent experiment on the Mu+Br2 reaction. Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.

Muonium addition reactions in the gas phase: Quantum tunneling in Mu + C2H4 and Mu + C2D4

The reaction kinetics for the addition of the muonium (Mu=μ+e−) atom to C2H4 and C2D4 have been measured over the temperature range 150–500 K at (N2) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C2H4, with no significant change seen in the apparent rate constant kapp, which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant kMu for the addition step. This is also expected from the nature of the μSR technique employed, which, in favorable cases, gives kapp=kMu at any pressure. Comparisons with the H atom data of Lightfoot and Pilling, and Sugawara et al. and the D atom data of Sugawara et al. reveal large isotope effects. Only at the highest temperatures, near 500 K, is kMu/kH given by its classical value of 2.9, from the mean velocity dependence of the collision rate but at the lowest temperatures kMu/kH≳30/1 is seen, reflecting the pronounced tunneling of the much lighter Mu atom (mμ=1/9 mp). The present M...