Nuclear magnetic resonance study of hydrogen dynamics in Al(BH4)4-based hypersalts M[Al(BH4)4] (MÂ = Na, K, Rb, Cs)

In order to study the dynamical properties of the novel series of complex hydrides M[Al(BH4)4] (M = Na, K, Rb, Cs), we have measured the 1H and 11B spin-lattice relaxation rates and the 1H nuclear magnetic resonance spectra in these compounds over broad temperature ranges (6-384 K) and resonance frequency ranges (14-90 MHz). For all the studied compounds, the behavior of the spin-lattice relaxation rates is governed by the reorientational motion of BH4 groups. For Na[Al(BH4)4], the temperature dependences of the measured 1H and 11B spin-lattice relaxation rates suggest a coexistence of two reorientational processes with different characteristic jump rates and the activation energies of 186(7) and 262(9) meV. For K[Al(BH4)4], Rb[Al(BH4)4], and Cs[Al(BH4)4], the relaxation data are satisfactorily described by the model with a Gaussian distribution of the activation energies and the average activation energies of 393(6), 360(5), and 353(5) meV, respectively. The barriers for reorientational motion in M[Al(BH4)4] are discussed on the basis of changes in the local environment of BH4 groups.