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

Published

September 1, 2011

Author(s)

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

Abstract

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.

Citation

Journal of Chemical Physics

Volume

135

Issue

9

Pub Type

Journals

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Keywords

ammonium tunneling, rotational dynamics, QENS, hydrogen storage

Citation

, 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 (NH₄)₂B₁₂H₁₂, Journal of Chemical Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=908657 (Accessed October 20, 2025)