Spectroscopy and characterization of AlNX (X = O and S): Triatomic circumstellar molecules

Three isomers of the triatomic [Al, N, O] molecular system have been observed in a solid argon matrix by infrared absorption spectroscopy using ¹⁵N and ¹⁸O isotopic substitution. The present work provides high-level quantum chemical predictions of their spectroscopic parameters to observe this system in the interstellar medium. The spectroscopic parameters, stability, and geometries of the lowest stable isomers of its isoelectronic system [Al, N, S] were characterized using coupled-cluster CCSD(T), explicitly correlated coupled-cluster CCSD(T)-F12, and multireference configuration interaction. The three-dimensional potential energy surfaces of all isomers were computed at the CCSD(T)-F12/aug-cc-pV5Z level, and a set of spectroscopic parameters were calculated. In both systems, the most stable isomer is linear with an X³Σ^- electronic ground state, and all linear isomers are characterized by small bending modes of less than 200 cm^-1. Due to their large dipole moments, the high intensities of such modes, and the nonexistence of anharmonic resonance complicating their spectra, our results facilitate the detection of AlNO and AlNS in the laboratory or in the interstellar medium.