Reorientational Motion of BH₄ Ions in Alkali Borohydrides MBH₄ (M = Li, Na, K) as Studied by Solid-State NMR

Ion dynamics in alkali borohydrides, LiBH 4, NaBH 4, and KBH 4, was studied by solid-state NMR. NMR spectra and spin–lattice relaxation times, T 1, of 1 H and 11 B were measured and analyzed. The T 1 results of 1 H and 11 B indicated that two types of motions take place in the low-temperature (LT) phase of LiBH 4 . The second moments of 1 H and 11 B NMR spectra suggested that the sum of the two types of motions resembles isotropic reorientation of the BH 4 unit. The T 1 minimum values of 1 H and 11 B supported that the C 3 rotation takes place at first. The second type of motion might be either C 3 rotation or C 2 rotation. The second moments of NMR spectra and the T 1 minimum values of 1 H and 11 B supported that the BH 4 unit reorients isotropically in NaBH 4 and KBH 4 .