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

Diammonium dodecahydro-closo-dodecaborate (NH4)(2)B12H12 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)(2)B12H12 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 similar to 10(8) 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)(2)B12H12 on an atomic level, providing a link between the microscopic and thermodynamic properties of this compound.