An uncharacterized phase of ammonia borane (AB) was previously identified while investigating the thermal aging of this hydrogen rich solid state material. This phase was identified as a key intermediate for the release of hydrogen from AB prompting the current characterization study. The new phase (AB*) was investigated extensively using in situ solid state MAS NMR spectroscopy, including 11B, 15N and 1H NMR. Single-pulse excitation, cross polarization and T1 relaxation experiments for all nuclei collectively demonstrated a significant increase in mobility of AB in the new phase compared to the room temperature phase of AB, consistent with a highly mobile solid. This implies a disruption of the extensive dihydrogen bonding network, allowing increased motional freedom. Complementary studies by in situ X-ray diffraction showed AB* to have the same crystal lattice as AB, but suggested an expanded lattice, providing support for increased mobility. NMR measurements of AB embedded into a mesoporous scaffold were also consistent with high mobility in AB*. These data suggest that a breakdown of the extensive hydrogen bonding network occurs before hydrogen can be released and will be an important factor in the practical use of AB as a hydrogen storage material.
