title: Low-temperature tunneling and rotational dynamics of the ammonium cations in (NH4)2B12H12.
abstract: Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH(4))(2)B(12)H(12)] reveal two NH(4)(+) rotational tunneling peaks (e.g., 18.5 μeV and 37 μeV at 4 K), consistent with the tetrahedral symmetry and environment of the cations. The tunneling peaks persist between 4 K and 40 K. An estimate was made for the tunnel splitting of the first NH(4)(+) librational state from a fit of the observed ground-state tunnel splitting as a function of temperature. At temperatures of 50 K-70 K, classical neutron quasi-elastic scattering appears to dominate the spectra and is attributed to NH(4)(+) cation jump reorientation about the four C(3) axes defined by the N-H bonds. A reorientational activation energy of 8.1 ± 0.6 meV (0.79 ± 0.06 kJ/mol) is determined from the behavior of the quasi-elastic linewidths in this temperature regime. This activation energy is in accord with a change in NH(4)(+) dynamical behavior above 70 K. A low-temperature inelastic neutron scattering feature at 7.8 meV is assigned to a NH(4)(+) librational mode. At increased temperatures, this feature drops in intensity, having shifted entirely to higher energies by 200 K, suggesting the onset of quasi-free NH(4)(+) rotation. This is consistent with neutron-diffraction-based model refinements, which derive very large thermal ellipsoids for the ammonium-ion hydrogen atoms at room temperature in the direction of reorientation.
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, MS 6102, Gaithersburg, Maryland 20899-6102, USA. nina.verdal@nist.gov
authors.forename: Nina
authors.initials: N
authors.lastname: Verdal
authors.forename: Terrence J
authors.initials: TJ
authors.lastname: Udovic
authors.forename: John J
authors.initials: JJ
authors.lastname: Rush
authors.forename: Vitalie
authors.initials: V
authors.lastname: Stavila
authors.forename: Hui
authors.initials: H
authors.lastname: Wu
authors.forename: Wei
authors.initials: W
authors.lastname: Zhou
authors.forename: Timothy
authors.initials: T
authors.lastname: Jenkins