Anharmonic Frequencies and Spectroscopic Constants of OAIOH and AIOH: Strong Bonding but Unhindered Motion

The astrophysical buildup of premineral nanocrystals from atoms to the smallest network-covalent solids will require observations of various small molecules containing the most common elements in minerals including aluminum and oxygen. The present work utilizes high-level quantum chemical quartic force field (QFF) approaches to produce anharmonic vibrational frequencies and spectroscopic constants for such species. The computed B-eff for the astrochemically known AIOH molecule at 15780.5 MHz is a mere 40 MHz above the experimental value implying that the B-eff for OAIOH at 5580.9 MHz is similarly accurate. The additional 7.31 D dipole moment in OAIOH implies that this molecule is a viable target for interstellar observation. Unlike the other anharmonic vibrational frequencies reported in this work, the AI-O-H bending frequencies in both AIOH and OAIOH are poorly described in the present QFF results. However, this failing actually highlights the fact that these bends are exceptionally floppy yet with counterintuitive exceedingly strong bonding. The AI-O bond energies are 128.2 and 107.2 kcal/mol, respective of AIOH and OAIOH, while the barriers to linearity are meager 16.6 and 380.7 cm(-1) (0.1 and 1.1 kcal/mol).