Ab initio calculation of transition properties and infrared absorption spectrum of the NaS molecule.

High-level multi-reference configuration interaction (MRCI) calculations with Davidson (+Q) correction were employed to investigate the electronic structure and spectroscopic properties of sodium monosulfide (NaS). The rotational energy levels were found to exhibit nuclear-spin-dependent quantization, with half-integer rotational states consistent with experimental and astrophysical observations. Considering spin-orbit coupling effects, the total partition function and the fractional contributions of each electronic state were evaluated; below 100 K, the ground state (X2Π3/2) was found to contribute over 99.0% of the total population. The infrared rovibrational spectrum of the X2Π3/2 state was dominated by Δv = 0 transitions, with a maximum line intensity of 1.34 × 10-19 cm/molecule at 10 K. The rovibronic absorption spectrum for transitions between the X2Π3/2 state and the first excited state was simulated based on potential energy curves and transition dipole moments considering spin-orbit coupling effects. These results provide benchmark spectroscopic data for the detection of NaS in molecular clouds and contribute to the development of astronomical spectral databases.