Infrared spectra of evolved stars have a rise in signal between the 7.0 μm and 8.0 μm wavelengths; the antisymmetric Si−O stretches of small silicon oxide clusters fall in this range and have large intensities. Hence, this quantum chemical analysis provides data for such molecules that may be of significance for astrochemical classification and could play a role in the formation or degradation of mineral nanocrystals from or into their constituent atoms. Both explicitly computed anharmonic fundamental vibrational frequencies and those determined from scaled harmonic frequencies agree well with known experimental data, and spectroscopic constants are provided herein such that astronomical rotational spectral characterization may also be possible for the C2v SiO3 and Si2O3 molecules.