Rotational (de)-excitation of HNSi and HSiN in collisions with He: <i>ab initio</i> potential energy surfaces, cross-sections, and rate coefficients

ABSTRACT Accurate modelling of molecular emission from interstellar environments requires reliable rate coefficients for rotational (de)-excitation by collisions with abundant species, such as He. Despite their current non-detection in the interstellar medium, the silicon-bearing isomers HNSi and HSiN are important targets for astrochemistry due to their structural analogy with HNC and HCN. Collisional data for these species are needed to expand molecular data bases and probe the behaviour of this molecular family. This work presents quantum scattering investigation of rotationally inelastic collisions of HNSi and HSiN with He. Two-dimensional potential energy surfaces were constructed using the CCSD(T)-F12 method in conjunction with the aug-cc-pVTZ basis set. These PESs reproduce interaction energies of near-complete basis set quality. Using the close-coupling formalism, state-to-state integral cross-sections were computed for rotational excitation and de-excitation between $\Delta j = \pm 1$ and $\pm 2$ transitions, for rotational quantum number $j \le 8$. These cross-sections were then thermally averaged to obtain rate coefficients for the temperature ranging from 3 to 300 K. These results reveal a clear propensity rule: odd $\Delta j$ transitions are favoured in HNSi–He collisions, whereas even $\Delta j$ transitions dominate for HSiN–He. The collisional rate coefficients presented here provide fundamental input for future astrochemical models that may include these silicon-bearing species, enabling a more accurate treatment of their excitation and radiative transfer in astrophysical environments.