In an effort to broaden the search for high-capacity hydrogen storage materials, three triborane compounds, NaB3H8, NH 3B3H7, and NH4B3H 8, were studied. In addition to hydrogen, thermal decomposition also releases volatile boranes, and the relative amounts and species depend on the cations (Na+, NH4 +) and the Lewis base (NH 3). Static-sample hydrogen NMR is used to probe molecular motion in the three solids. In each case, the line width decreases from low temperatures to room temperature in accordance with a model of isotropic or nearly isotropic reorientations. Such motions also explain a deep minimum in the relaxation time T1. Translational diffusion never appears to be rapid on the 10 -5 s time scale of NMR. In an effort to broaden the search for high-capacity hydrogen storage materials, three triborane compounds, NaB 3 H 8 , NH 3 B 3 H 7 , and NH 4 B 3 H 8 , were studied. In addition to hydrogen, thermal decomposition also releases volatile boranes, and the relative amounts and species depend on the cations (Na + , NH 4 + ) and the Lewis base (NH 3 ). Static-sample hydrogen NMR is used to probe molecular motion 10 in the three solids. In each case, the line width decreases from low temperatures to room temperature in accordance with a model of isotropic or nearly isotropic reorientations. Such motions also explain a deep minimum in the relaxation time T 1 . Translational diffusion never appears to be rapid on the 10 -5 s time scale of NMR.

The nanoporous polymorph γ-Mg(BH 4 ) 2 with 33% void space has a much lower density than the α and β forms, suggesting that reorientation and diffusion dynamics of BH 4 groups may be enhanced. We report hydrogen NMR line shapes and T 1 and T 1D measurements from −125 to 225°C, along with some 11 B results. The main component of the hydrogen NMR line does not show motional narrowing from translational diffusion up to 175°C, although a small mobile fraction appears at 175°C. T 1D measurements show no evidence for "slow" motions (faster than 10 2 s −1 ) up to 175°C, ruling out enhanced overall diffusivity. The hydrogen T 1 is sensitive to reorientations of the BH 4 units. In the γ-phase, a T 1 minimum at 85 MHz is observed at 50°C, similar to α-phase; the high barrier to reorientation in γ (compared to β-phase where the minimum appears near −135°C) is confirmed by broadening of the γ-phase hydrogen spectrum at −50°C and lower temperatures. A sharp increase in T 1 at or above 175°C signals a transformation of γ-phase to another structure of higher density. T 1 of the transformation product is similar but not equal to that of the β-phase, suggesting the transformation product is primarily β-phase. This is confirmed by powder X-ray diffraction of the transformed material. In situ X-ray powder diffraction studies reveal that γ-Mg(BH 4 ) 2 transforms upon heating in vacuum into the β-phase, and undergoes an amorphization in the presence of gases that can be adsorbed into the pores.

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