Nuclear Magnetic Resonance Study of Molecular Dynamics in Ammine Metal Borohydride Sr(BH4)2(NH3)2

We investigated molecular dynamics in Sr­(BH4)2(NH3)2, which is a representative compound for a series of ammine metal borohydrides, a novel family of systems exhibiting promising hydrogen storage properties with high hydrogen content and low decomposition temperature. Two types of hydrogen-containing groups present, namely, BH4 and NH3, prompt the investigation whether these materials differ from other borohydrides from a molecular dynamics point of view. The investigation was performed using 1H and 11B NMR spectroscopy and spin–lattice relaxation techniques. Different thermally activated reorientational processes of BH4 tetrahedra about their 2- and 3-fold symmetry axes were identified from temperature-dependent spin–lattice relaxation rates for both nuclei, and the corresponding activation energies were obtained. In addition, a selectively deuterated compound was studied to further distinguish the dynamic processes between both hydrogen-containing groups. Our study presents physical insight into the dynamic properties of Sr­(BH4)2(NH3)2 on a microscopic level of atomic groups, providing a link between the microscopic and bulk properties of this material.