Rotational Excitation and De-Excitation of Interstellar Thionylimide ( cis -HNSO) by Helium Collisions

Abstract The recent identification of cis-thionylimide (cis-HNSO) in the interstellar medium (ISM) has opened new avenues for exploring nitrogen-, sulfur-, and oxygen-bearing molecules in space. To support astrophysical modeling and improve abundance estimates, we present the first comprehensive theoretical investigation of the collisional excitation and de-excitation of cis-HNSO by helium atoms. An accurate three dimensional potential energy surface (3D-PES) for the weakly bound cis-HNSO−He complex is first computed at the CCSD(T)-F12/aug-cc-pVTZ level, where we also consider basis set superposition error corrections. Then, an analytical form of this PES is derived, which is incorporated later for quantum close-coupling scattering calculations for cis-HNSO colliding with He, yielding state-to-state cross sections and temperature-dependent rate coefficients across ISM-relevant conditions (5 − 100 K). The results exhibit a strong propensity for Δj = ±1 transitions at low collision energies, with Δj = ±2 transitions becoming dominant at higher energies, reflecting the interplay between long-range and short-range anisotropies in the interaction potential. Astrophysical implications are also discussed. The collisional data provided here are essential for non-LTE radiative transfer models and will enable more accurate interpretations of current and future astronomical observations dealing with such N −, S −, and O − containing molecules.