Theoretical Rotational and Vibrational Spectral Data for the Hypermagnesium Oxide Species Mg₂O and Mg₂O⁺

Abstract

While magnesium is astronomically observed in small molecules, it largely serves as a contributor to silicate grains, though how these grains form is not well‐understood. The smallest hypermagnesium oxide compounds (Mg2O/Mg2O+) may play a role in silicate formation, but little vibrational reference data exist. As such, anharmonic spectroscopic data are computed for X 1Σ+g Mg2O, ã 3Σ+u Mg2O, and X2Σ+g Mg2O+ using quartic force fields (QFFs). Explicitly‐correlated coupled‐cluster QFFs for the neutral species perform well, implying that full multireference treatment may not be necessary for such systems if enough electron correlation is included. Equation‐of‐motion ionization potential (EOMIP) methods for X2Σ+g Mg2O+ QFFs circumvent previous symmetry breaking issues even in explicitly‐correlated coupled‐cluster results, motivating the need for EOMIP treatments at minimum for such systems. All three species are found to have high‐intensity vibrational frequencies. Even so, the highly intense fundamental (X 1Σ+g Mg2O: 894.7 cm‐1/11.18 μm; ã 3Σ+u Mg2O: 915.0 cm‐1/10.91 μm) for either neutral state may be astronomically obscured by the polycyclic aromatic hydrocarbon 11.2 μm band. Mg2O+ may be less susceptible to such obfuscation, and its ν1 intensity is computed to be a massive 4793 km mol‐1.