c-AlO2, c-HAlO2, and c-(HN)OAlH spectroscopic constants and anharmonic frequencies

Aluminum forms strong bonds to oxygen, nitrogen, and hydrogen, but these motifs are relatively rare in the interstellar medium (ISM). However, no chemical rationale accounts for this observation. The cyclic molecules AlO2, HAlO2, and (HN)OAlH are quintessential examples of the aforementioned phenomenon, and the latter two are isoelectronic with each other. Matrix-isolation spectroscopic data and prior theoretical computations exist for cyclic AlO2, although it has yet to be observed astronomically. However, high-level theoretical data provided herein generate useful predictions for and insights into spectral data for these three molecules. Anharmonic vibrational frequencies and rotational constants are determined in this work for these molecules via quartic force fields at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory and with canonical CCSD(T) for consideration of basis set convergence, core electron correlation, and relativity (CcCR). The present work finds that these aluminum oxides are weak-to-moderate infrared emitters but are much stronger microwave emitters with dipole moments of 4.95 D, 4.55 D, and 3.76 D respective of cyclic AlO2 (c-AlO2), cyclic HAlO2 (c-HAlO2), and cyclic (HN)OAlH (c-(HN)OAlH). Correlation to argon-matrix experiments for c-AlO2 is within the expected matrix shift for the ν3 bend computed here to be 527.8 cm−1. Astrophysical detection of these molecules could imply their role in the creation or degradation of aluminum-containing nanocrystals and interstellar dust of importance for the formation of rocky bodies, and the spectral data computed in this work should be able to assist in such classification.