Investigating Formation Pathways and the Spectroscopic Constants of Aluminum Nitrides from AlH and NH3

Reactions of AlH and NH₃ are shown to produce Al–N containing species that could potentially contribute to the composition of dust grains, like the Murchison CM2 chondritic meteorite. The present computational study couples explicitly correlated coupled cluster theory and density functional theory to produce a gas-phase chemical formation pathway from AlH and NH₃ to Al₄N₄H₈, a stable, cubic structure with the proper Al/N ratio expected for larger aluminum nitride species. Rovibrational spectroscopic analyses of HAlNH₃, AlNH₂, Al₂N₂H₄, and Al₄N₄H₈ reveal consistently intense vibrational transitions and large absorption cross sections of modes associated with spectral features typically associated with dust grain formation in circumstellar media. The ν₅ wagging frequency for Al₄N₄H₈ at 748.8 cm^–1 (13.35 μm) exhibits a vibrational transition intensity of 428 km mol^–1, more than 5× larger than the antisymmetric stretch in water. Hence, the present formation pathway and rovibrational analysis should assist in the potential detection of a class of inorganic molecules that may contribute to the nucleation and formation of dust in protoplanetary disks and stellar outflows of AGB stars.