Low-temperature tunneling and rotational dynamics of the ammonium cations in (NH4)2B12H12

Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH4)(2)B12H12] reveal two NH4+ rotational tunneling peaks (e. g., 18.5 mu eV and 37 mu eV 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 NH4+ 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 NH4+ 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 NH4+ dynamical behavior above 70 K. A low-temperature inelastic neutron scattering feature at 7.8 meV is assigned to a NH4+ 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 NH4+ 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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3624495]