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

摘要

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 H-1 and B-11 spin-lattice relaxation rates and the H-1 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 H-1 and B-11 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. (C) 2018 Elsevier B.V. All rights reserved.