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Low-Temperature Tunneling and Rotational Dynamics of the Ammonium Cations in (NH4)2B12H12



Kristina Verdal, Terrence J. Udovic, John J. Rush, Vitalie Stavila, Hui Wu, Wei Zhou, Timothy Jenkins


Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH4)2B12H12] reveal two NH4+ rotational tunneling peaks (e.g., 18.5 υeV and 37 υeV 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 to 70 K, classical neutron quasielastic scattering dominates the spectra, attributed to NH4+ cation jump reorientation about the four C3 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 quasielastic linewidths in this temperature regime. This activation energy is in accord with a change in NH4+ dynamical behavior above 70 K. A low-temperature neutron inelastic scattering feature at 7.8 meV is assigned to an 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 confirmed by neutron-diffraction-based model refinements, which indicate overly large thermal ellipsoids for the ammonium-ion hydrogen atoms at room temperature.
Journal of Chemical Physics


ammonium tunneling, rotational dynamics, QENS, hydrogen storage


, K. , Udovic, T. , Kasianowicz, J. , Stavila, V. , Wu, H. , Zhou, W. and Jenkins, T. (2011), Low-Temperature Tunneling and Rotational Dynamics of the Ammonium Cations in (NH<sub>4</sub>)<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, Journal of Chemical Physics, [online], (Accessed May 22, 2024)


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Created September 1, 2011, Updated February 19, 2017