Nuclear Magnetic Resonance Study of Anion and Cation Reorientational Dynamics in (NH4)2B12H12

Skripov, Alexander V.; Skoryunov, Roman V.; Soloninin, Alexei V.; Babanova, Olga A.; Stavila, Vitalie; Udovic, Terrence J.

doi:10.1021/acs.jpcc.7b12046

Title: Nuclear Magnetic Resonance Study of Anion and Cation Reorientational Dynamics in (NH₄)₂B₁₂H₁₂

Journal Article · 29 January 2018 · Journal of Physical Chemistry. C

DOI: https://doi.org/10.1021/acs.jpcc.7b12046 · OSTI ID:1485459

^[1]; Skoryunov, Roman V. ^[1]; Soloninin, Alexei V. ^[1]; Babanova, Olga A. ^[1]; Stavila, Vitalie ^[2]; Udovic, Terrence J. ^[3]

Ural Division of the Russian Academy of Sciences, Ekaterinburg (Russia)
Sandia National Lab. (SNL-CA), Livermore, CA (United States)
National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States)

Here, diammonium dodecahydro-closo-dodecaborate (NH₄)₂B₁₂H₁₂ is the ionic compound combining NH₄⁺ cations and [B₁₂H₁₂]^2– anions, both of which can exhibit high reorientational mobility. To study the dynamical properties of this material, we measured the proton NMR spectra and spin–lattice relaxation rates in (NH₄)₂B₁₂H₁₂ over the temperature range of 6–475 K. Two reorientational processes occurring at different frequency scales have been revealed. In the temperature range of 200–475 K, the proton spin–lattice relaxation data are governed by thermally activated reorientations of the icosahedral [B₁₂H₁₂]^2– anions. This motional process is characterized by the activation energy of 486(8) meV, and the corresponding reorientational jump rate reaches ~10⁸ s^–1 near 410 K. Below 100 K, the relaxation data are governed by the extremely fast process of NH₄⁺ reorientations which are not “frozen out” at the NMR frequency scale down to 6 K. The experimental results in this range are described in terms of a gradual transition from the regime of low-temperature quantum dynamics (rotational tunneling of NH₄ groups) to the regime of classical jump reorientations of NH₄ groups with an activation energy of 26.5 meV. Our study offers physical insights into the rich dynamical behavior of (NH₄)₂B₁₂H₁₂ on an atomic level, providing a link between the microscopic and thermodynamic properties of this compound.