The neutral muonic helium atom 4Heμ, 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 4Heμ with molecular hydrogen, 4Heμ + H2 → 4HeμH + 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, kHeμ, 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 kHeμ 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 kHeμ 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 + H2 and Mu + H2 reactions in a novel study of kinetic isotope effects for the H + H2 reactions over a factor of 36.1 in isotopic mass of the atomic reactant.
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We thank Dr. Syd Kreitzman for his management of the CMMS Facility and for the technical support it provides. A special thanks is given to Neil Aucoin for his engineering input and design of the heater for the target cell used in these experiments. We gratefully acknowledge NSERC for its financial support of this research. The work at Pacific Northwest National Laboratory (PNNL) was supported by the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Science, U.S. Department of Energy (DOE). Battelle operates PNNL for DOE. The work at the University of Minnesota was supported in part by the DOE under Grant No. DE-FG02-86ER13579. The work at Northwestern was supported by AFOSR grant FA9550-10-1-0205.