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

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 (Mg $$ O/Mg $$ O $$ ) may play a role in silicate formation, but little vibrational reference data exist. As such, anharmonic spectroscopic data are computed for $$ Mg $$ O, $$ Mg $$ O, and $$ Mg $$ O $$ 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 $$ Mg $$ O $$ QFFs circumvent previous symmetry breaking issues even in explicitly-correlated coupled-cluster results, motivating the need for EOMIP treatments at minimum for such systems. All three species are found to have high-intensity vibrational frequencies. Even so, the highly intense frequency ( $$ Mg $$ O: 894.7 cm^-1/11.18 μm; $$ Mg $$ O: 915.0 cm^-1/10.91 μm) for either neutral state may be astronomically obscured by the polycyclic aromatic hydrocarbon 11.2 μm band. Mg $$ O $$ may be less susceptible to such obfuscation, and its $$ intensity is computed to be a massive 4793 km mol^-1.