Raman spectroscopy study of ammonia borane at high pressure

Ammonia borane, NH3BH3, has attracted significant interest as a promising candidate material for hydrogen storage. The effect of pressure on the bonding in NH3BH3 was investigated using Raman spectroscopy to over 20 GPa in a diamond anvil cell, and two new transitions were observed at approximately 5 and 12 GPa. Vibrational frequencies for the modes of the NH3 proton donor group exhibited negative pressure dependence, which is consistent with the behavior of conventional hydrogen bonds, while the vibrational frequencies of the BH3 proton acceptor group showed positive pressure dependence. The observed behavior of these stretching modes supports the presence of dihydrogen bonding at high pressure. In addition, the BH3 and NH3 bending modes showed an increase in spectral complexity with increasing pressure together with a discontinuity in d nu/dP which suggests rotational disorder in this molecule. These results may provide guidance for understanding and developing improved hydrogen storage materials.