Exchange of Hydrogen Atoms Between BH₄ in LiBH₄

We report here boron-11 and hydrogen NMR measurements of the lifetime of BH 4 units in molten LiBH 4, with respect to hydrogen exchange. A well-resolved 11 B pattern of B−H J-couplings yields a lower limit of 16 ms on the BH 4 lifetime at 285 °C. Carr−Purcell−Meiboom−Gill multiple spin−echo measurements, both on 11 B and hydrogen, find decay time constants of ∼2 s. Most or all of these decays can be attributed to boron and hydrogen T 1 relaxation. Thus, any hydrogen exchange in molten LiBH 4 is slower than our ∼1 s time scale. In solid LiBH 4, H-exchange is expected to be even slower. This confirms that the diffusion of hydrogen in solid LiBH 4 observed recently results from motion of intact BH 4 units.