Reexamining Al₂H₂: Evidence That the Unidentified Butterfly Ground-State Isomer Was Previously Observed

Although popular throughout the 1990s and early 2000s, interest in Al₂H₂ suddenly diminished, leaving an unresolved chasm between theory and experiment for over 20 years. Our computations reconcile the two, unveiling evidence for the unidentified but earlier observed Al₂H₂ butterfly global minimum in matrix IR. We confirm the experimental IR assignments of the cyclic and monobridged structures. The critical ingredient in our case is the b₂ symmetry Al-H stretching frequency predicted here to be 1096 cm^-1. This feature was observed at 1090 cm^-1 by Andrews et al., but never identified. Andrews et al. also assigned a feature at 1647 cm^-1 to the linear structure of Al₂H₂. However, this is unlikely, as we do not find the linear structure to be an equilibrium geometry. CCSD(T) geometries and energies for four neutral Al₂H₂ isomers in this work are computed at the highest levels of theory among theoretical investigations in the literature. For final energetic predictions, basis sets as large as augmented quadruple zeta were used, as were theoretical methods as advanced as CCSDT(Q). We also report Al₂H₂ VPT2 fundamental frequencies for the first time. In addition, two anionic structures are investigated at the same levels of theory, the butterfly ground state and monobridged local minimum, which have never been characterized. We conclude that these anion structures could be identified in future mass-selected gas-phase spectroscopy experiments.