AlOSO: Spectroscopy and Structure of a New Group of Astrochemical Molecules

Abstract

With the ever-increasing detection of sulfur-bearing molecules and the high abundance and refractory nature of aluminum, the [Al, S, O2] isomers may play an important role in the gas-phase chemistry of circumstellar envelopes and the chemistry on the surface of dust grains. High-level theoretical exploration of the [Al, S, O2] molecular system yielded five isomers, and predictions of their rotational, vibrational, and electronic spectroscopic properties are provided to inform experimental and observational searches. Cis-AlOSO and diamond isomers are isoenergetic and connected via a very small (∼1 kcal mol−1) transition-state barrier. These isomers may act as intermediates along the chemical pathway between Al + SO2 and AlO + SO. Other isomers OAlOS and SAlO2 are stable relative to their corresponding dissociation asymptotes. Large permanent dipole moments of 2.521 D (cis-AlOSO), 1.239 D (diamond), and 5.401 D (OAlOS) predict strong rotational transitions and indicate these molecules as prime candidates for experimental study. Due to the low transition-state barrier, mixing of the vibrational levels is anticipated, complicating the vibrational spectrum. Electronic spectroscopy may be used as a means to differentiate between the two isomers. Strong electronic transitions are predicted to occur in the 200–300 nm range for cis-AlOSO and diamond. Simulated electronic absorption spectra provide a starting point for experimental characterization and spectral deconvolution of these isomers.