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 ((4.1)H), because the negative muon almost perfectly screens one proton charge. We report the reaction rate of (4.1)H with (1)H(2) to produce (4.1)H(1)H + (1)H 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 (0.11)H (where (0.11)H 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.

The adsorption and dynamical behavior of the muonated cyclohexadienyl radical (C 6 H 6 Mu) in NaY zeolite, formed by muonium (Mu) addition on adsorbed benzene, was investigated by the muon spin resonance (µSR) technique, primarily at loadings of 2-3 C 6 H 6 molecules per supercage of NaY. The dynamics of this radical are expected to be the same as its isotopic analogue, C 6 H 7 , for which there are no similar data available. Both TF-µSR and ALC-µSR spectra were recorded, with the most detailed information provided by the positions and line widths of the avoided level crossing resonances. In concert with 2 H NMR, neutron diffraction and molecular dynamics studies of the parent benzene molecule, as well as current theoretical calculations, the dominant adsorption site for the C 6 H 6 Mu radical is believed to be the S II Na cation, within a supercage, which gives rise to three observed ALC lines, corresponding to two different orientations for the muon (proton) of the CHMu methylene group: pointing toward (endo) and away (exo) from the Na cation. The cation interaction gives rise to unprecedentedly large (≈20%) shifts in hyperfine coupling constants, indicative of a strong bond formed with the π electrons of the C 6 H 6 Mu radical. An additional but weaker resonance line is also seen, which is interpreted as being due to adsorption at the window sites between supercages. The ALC lines associated with the C 6 H 6 Mu radical bound to both the Na cation and window sites are all broad, ≈1 kG, change little with temperature and exhibit mainly static line shapes over the whole temperature range studied, from 3 to 322 K. This indicates a much stronger host-guest interaction for C 6 H 6 Mu, particularly with the Na cation, than is known for benzene, to the extent that this site acts as an effective trap for the free radical, over the critical µSR time scale of 50 ns.

We report a new advance in the study of muonium (Mu) reactivity; specifically, we report the rate constant for the Mu + H2(vibrational quantum number n = 1) reaction determined by measurements at 300 K and by converged quantum mechanical calculations. Comparisons are made with earlier results for D + H2 (n = 1) and with the corresponding thermal reaction rates. The measurements are a sensitive probe of the high-curvature region in the entrance valley of the potential energy surface (PES) and thus provide a qualitatively different probe of the PES than that provided by any previous experiment.

Abstraet. Muon spin rotation 0tSR) and avoided level erossing resonanee (ALCR) have been used to determine the hyperf'me eoupling eonstants (hfes) of the muonium-substituted eyclohexadienyl radieals C6H6Mu, C6D6Mu and C6F6Mu in the gas phase, at pressures ~ 1 and 15 atm and temperatures in the range 40-80~ Equivalent studies of polyatomie free radieals in gases, by electron spin resonanee (ESR) speetroscopy, ate generally not possible in this pressure range. The present gas phase results support the findings of earlier studies of eyelohexadienyl radicals in the eondensed phase, by borla ~tSR and ESR. Minor but not insignifieant (~1%) effeets on the hfes ate observed, whieh can be qualitatively understood for sueh nonpolar media in terms of their differing pola¡ This is the first time that eomparisons of this nature have been possible between different phases at the same temperatures. These ~tSR/ALCR gas-phase results provide a valuable benehmark for eomputational studies on radieals, free from possible effeets of solvent or matrix environments.

The neutral muonic helium atom (4)Heμ, in which one of the electrons of He is replaced by a negative muon, may be effectively regarded as the heaviest isotope of the hydrogen atom, with a mass of 4.115 amu. We report details of the first muon spin rotation (μSR) measurements of the chemical reaction rate constant of (4)Heμ with molecular hydrogen, (4)Heμ + H(2) → (4)HeμH + H, at temperatures of 295.5, 405, and 500 K, as well as a μSR measurement of the hyperfine coupling constant of muonic He at high pressures. The experimental rate constants, k(Heμ), are compared with the predictions of accurate quantum mechanical (QM) dynamics calculations carried out on a well converged Born-Huang (BH) potential energy surface, based on complete configuration interaction calculations and including a Born-Oppenheimer diagonal correction. At the two highest measured temperatures the agreement between the quantum theory and experiment is good to excellent, well within experimental uncertainties that include an estimate of possible systematic error, but at 295.5 K the quantum calculations for k(Heμ) are below the experimental value by 2.1 times the experimental uncertainty estimates. Possible reasons for this discrepancy are discussed. Variational transition state theory calculations with multidimensional tunneling have also been carried out for k(Heμ) on the BH surface, and they agree with the accurate QM rate constants to within 30% over a wider temperature range of 200-1000 K. Comparisons between theory and experiment are also presented for the rate constants for both the D + H(2) and Mu + H(2) reactions in a novel study of kinetic isotope effects for the H + H(2) reactions over a factor of 36.1 in isotopic mass of the atomic reactant.

The adsorption and dynamical behavior of the Mu−ethyl radical (MuC2H4) in NaY, HY, and USY faujasites was investigated by the muon spin resonance (μSR) technique, at loadings of one to five ethene molecules per supercage and over a temperature range of ca. 5−500 K (for NaY). The temperature dependences of both the muon and proton hyperfine coupling constants (Hfc's) are reported and compared with similar studies of MuC2H4 in different environments. Both transverse field (TF) μSR and avoided level crossing resonance (ALC) μSR spectra were recorded, with information on molecular motion mainly provided by the ALC line shapes. The muon Hfc's show only a small sensitivity to different frameworks and loadings but exhibit significant (∼10%) shifts at low temperatures, in comparison with bulk values, due to binding of the ethyl radical to cations at S II sites in NaY and to framework hydroxyls in the case of HY(USY). The Δ1 resonances are symmetric and quite broad at the lower temperatures studied, but dramatically further broaden near room temperature, seen also in the TF relaxation rates, suggesting that the Mu−ethyl radical either desorbs from or hops between its binding sites at the higher temperatures. An Arrhenius estimate of the activation energy for desorption gives ∼ 20 kJ/mol, consistent with the dipolar interaction energy between the Mu−ethyl radical and an NaY cluster. The observation of such highly broadened Δ1 ALC lines at the higher temperatures contrasts with the largely static line widths reported previously for the Mu−cyclohexadienyl radical (MuC6H6) in NaY. Sharper Δ0 ALC lines for both the α and β protons of MuC2H4 appear near the same temperatures at which the Δ1 lines overly broaden, and which persist to the highest temperatures (350 K). For NaY, the α proton resonances also broaden further at these temperatures. For both NaY and particularly HY, the temperature dependence of the α proton Hfc's indicates considerable distortion of the Mu−ethyl radical geometry, due to its binding to zeolite sites. Recently published calculations of binding energies and Hfc's for ethyl radicals in NaY and HY suggest a much stronger binding of the MuC2H4 radical than seems warranted by the data and pose as well a conundrum in comparison with earlier results for MuC6H6 in NaY. On the other hand, the temperature dependence of the isotropic muon Hfc's found from the T-atom model for NaY employed in these calculations is in excellent agreement with experiment.

The reaction kinetics for the addition of the muonium (Mu = Jl, + e-) atom to C 2 H 4 and C 2 D 4 have been measured over the temperature range 150-500 K at (N 2 ) moderator pressures near 1 atm. A factor of about 8 variation in moderator pressure was carried out for C 2 H 4 , with no significant change seen in the apparent rate constant k app , which is therefore taken to be at the high pressure limit, yielding the bimolecular rate constant k Mu for the addition step. This is also expected from the nature of the Jl,SR technique employed, which, in favorable cases, gives kapp = k Mu 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 k Mulk H given by its classical value of2.9, from the mean velocity dependence ofthe collision rate but at the lowest temperatures kMulkH ~ 3011 is seen, reflecting the pronounced tunneling ofthe much lighter Mu atom (mIL = 1/9 m p )' The present Mu results should provide accurate tests of reaction theories on currently available ab initio surfaces.

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.