Anharmonic Frequencies of (MO)2 & Related Hydrides for M = Mg, Al, Si, P, S, Ca, & Ti and Heuristics for Predicting Anharmonic Corrections of Inorganic Oxides.

The low-frequency vibrational fundamentals of D2h inorganic oxides are readily modeled by heuristic scaling factors at fractions of the computational cost compared to explicit anharmonic frequency computations. Oxygen and the other elements in the present study are abundant in geochemical environments and have the potential to aggregate into minerals in planet-forming regions or in the remnants of supernovae. Explicit quartic force field computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory generate scaling factors that accurately predict the anharmonic frequencies with an average error of less than 1.0 cm-1 for both the metal-oxygen stretching frequencies and the torsion and antisymmetric stretching frequencies. Inclusion of hydrogen motions are less absolutely accurate but are similarly relatively predictive. The fundamental vibrational frequencies for the seven tetraatomic inorganic oxides examined presently fall below 875 cm-1, and most for the hydrogenated species do, as well. Additionally, ν3 for the SiO dimer is shown to have an intensity of 562 km mol-1, with each of the other molecules having one or more frequencies with intensities greater than 80 km mol-1 again with most in the low-frequency infrared range. These inten- sities and the frequencies computed in the present study should assist in laboratory characterization and potential interstellar or circumstellar observation.