Analysis of Dihydrogen Bonding in Ammonium Borohydride

The structural and vibrational properties of ammonium borohydride, NH4BH4, have been examined by first-principles density functional theory (DFT) calculations and inelastic neutron scattering (INS). The H disordered crystal structure of NH4BH4 is composed of the tetrahedral complex ions NH4+ and BH4ˉ, which are arranged as in the fcc NaCl structure and linked by inter-molecular dihydrogen bonding. Upon cooling, the INS spectra revealed a structural transition between 45 and 40 K. The reversible transition occurs upon heating between 46 and 49 K. In the low temperature form reorientational dynamics is frozen. The libration modes for BH4ˉ and NH4+ are near 300 and 200 cm-1, respectively. Upon entering the fcc high temperature form, NH4+ ions attain fast reorientational dynamics, as indicated in the disappearance of the NH4+ libration band, whereas BH4ˉ ions become significantly mobile only at temperatures above 100 K. The vibrational behavior of BH4ˉ ions in NH4BH4 compares well to the heavier alkali metal ...