Wide-Line Solid-State NMR Characterizations of Sodium Alanates

Pure NaAlH4, TiCl3-doped NaAlH4, and pure Na3AlH6 were characterized using 1H, 23Na, and 27Al solid-state NMR. The signal intensities and linewidths of 1H NMR spectra using several spin echo sequences and backprediction of a single pulse experiment were compared to find the optimal experiment to measure wide-line NMR spectra of the alanates. Second moment calculations using the Van Vleck equations compared with fits of the dipolar coupling line broadening confirm that NaAlH4 has a rigid crystal lattice. On the other hand, for Na3AlH6, a narrowing of the proton and aluminum lineshape was observed, indicating a fast rotational motion of AlH6 clusters at room temperature. A broadening of the 1H and 27Al linewidth was observed upon lowering the temperature. This process is successfully described using thermally activated rotational jumps of AlH6 clusters assuming a fast rotational motion around one single C4 axis and a slower rotation around the other two C4 axes with an activation barrier of Ea = 25 kJ/mol a...