Wide-Line Solid-State NMR Characterizations of Sodium Alanates

Pure NaAlH4, TiCl3-doped NaAlH4, and pure Na3AlH6 were characterized using H-1, Na-23, and Al-27 solid-state NMR, The signal intensities and linewidths of H-1 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 H-1 and Al-27 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 C-4 axis and a slower rotation around the other two C-4 axes with an activation barrier of E-a = 25 kJ/mol and an attempt frequency of upsilon(0) = 4 x 10(10) Hz.