title: Reorientational dynamics of the dodecahydro-closo-dodecaborate anion in Cs2B12H12.
abstract: Rapid reorientational motions of the B(12)H(12)(2-) icosahedral anion, a key intermediate in borohydride dehydrogenation, are revealed by quasielastic neutron scattering (QENS) measurements of Cs(2)B(12)H(12) between 430 and 530 K. At 430 K, over the range of momentum transfers collected, the elastic incoherent structure factor (EISF) is consistent with a model for reorientational jumps about a single molecular axis. At temperatures of 480 K and higher, however, the EISF suggests the emergence of multiaxis reorientation by dynamically similar, independent jumps about two axes, on average, preserving crystallographic order. Alternatively, if one assumes that the anions are undergoing temperature-dependent rotational trapping, then the EISF is also consistent with a jump model involving a temperature-dependent mobile fraction of anions statistically tumbling between discrete crystallographic sites. Although neutron vibrational spectra demonstrate that the anion torsional modes soften dramatically with increasing temperature, the QENS-derived activation energy of 333 meV for reorientation clearly shows that the anions are not undergoing isotropic rotational diffusion.
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States. 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: Ronald L
authors.initials: RL
authors.lastname: Cappelletti
authors.forename: Wei
authors.initials: W
authors.lastname: Zhou