title: Isotope Dependence and Quantum Effects on Atomic Hydrogen Diffusion in Liquid Water.
abstract: Relative diffusion coefficients were determined in water for the D, H, and Mu isotopes of atomic hydrogen by measuring their diffusion-limited spin-exchange rate constants with Ni(2+) as a function of temperature. H and D atoms were generated by pulse radiolysis of water and measured by time-resolved pulsed EPR. Mu atoms are detected by muonium spin resonance. To isolate the atomic mass effect from solvent isotope effect, we measured all three spin-exchange rates in 90% D2O. The diffusion depends on the atomic mass, demonstrating breakdown of Stokes-Einstein behavior. The diffusion can be understood using a combination of water "cavity diffusion" and "hopping" mechanisms, as has been proposed in the literature. The H/D isotope effect agrees with previous modeling using ring polymer molecular dynamics. The "quantum swelling" effect on muonium due to its larger de Broglie wavelength does not seem to slow its "hopping" diffusion as much as predicted in previous work. Quantum effects of both the atom mass and the water librations have been modeled using RPMD and a qTIP4P/f quantized flexible water model. These results suggest that the muonium diffusion is very sensitive to the Mu versus water potential used.
authors.affiliation: Radiation Laboratory and Department of Chemistry & Biochemistry, Notre Dame University , Notre Dame, Indiana 46556, United States.
authors.forename: J A
authors.initials: JA
authors.lastname: Walker
authors.affiliation: Department of Chemistry & Biochemistry, California State University , 1250 Bellflower Boulevard, Long Beach, California 90840, United States.
authors.forename: S P
authors.initials: SP
authors.lastname: Mezyk
authors.affiliation: Institute of Physical Chemistry, University of Stuttgart , Pfaffenwaldring 55, D-70569 Stuttgart, Germany.|Department of Chemistry, University of Pretoria , Pretoria 0002, South Africa.
authors.forename: E
authors.initials: E
authors.lastname: Roduner
authors.affiliation: Radiation Laboratory and Department of Chemistry & Biochemistry, Notre Dame University , Notre Dame, Indiana 46556, United States.|Chemistry Division, Argonne National Laboratory , 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors.forename: D M
authors.initials: DM
authors.lastname: Bartels