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



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


Diammonium dodecahydro-closedidecabirate (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 unusual material, we have measured the proton NMR spectra and spin-lattice relaxation rates in (NH4)2B12H12 over the temperature range of 6 K to 475 K. Two reorientational processes occurring at different frequency scales have been revealed. In the temperature range of 200 K to 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 rates reaches approximately equal} 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 the activation energy of 26.5 meV. Our study offers physical insight into the rich dynamical behavior of (NH4)2B12H12 on a microscopic level, providing a link between the microscopic and bulk properties of this compound.
Journal of Physical Chemistry C


Ammonium ion, Complex hydride, Diammonium dodecahydro-closo-dodecaborate, Dodecahydro-closo-dodecaborate ion, NMR, Nuclear magnetic resonance, Nuclear spin relaxation, Reorientation, Rotational tunneling


Skripov, A. , Skoryunov, R. , Soloninin, A. , Babanova, O. , Stavila, V. and Udovic, T. (2018), Nuclear Magnetic Resonance Study of Anion and Cation Reorientational Dynamics in (NH<sub>4</sub>)<sub>2</sub>B<sub>12</sub>H<sub>12</sub>, Journal of Physical Chemistry C, [online], (Accessed April 22, 2024)
Created January 28, 2018, Updated October 12, 2021