Ab initio Potential Energy Surfaces and Low–Temperature Collisional Dynamics for Rotational De-excitation of HNC and HCN by Ar

Accurate modeling of nitrogen-rich atmospheres, such as that of Titan, requires precise collisional data with key secondary partners such as Ar. This work presents a quantum scattering investigation of rotational (de-)excitation of HCN and HNC molecules in their ground vibrational state in collisions with Ar. Two-dimensional potential energy surfaces were constructed using the CCSD(T)-F12A/aug-cc-pVTZ method, achieving accuracy near the complete basis set limit. State-to-state integral cross-sections for rotational transitions were computed by solving the quantum close-coupling equations for all rotational quantum number j ≤ 8, over collision energies up to 1000 cm⁻¹, and were then thermally averaged to obtain de-excitation rate coefficients for temperatures of 5−200 K. A propensity rule for clear rate coefficients is observed: in contrast to the HCN−He system, which favors even Δj transitions, the HCN−Ar system exhibits a strong propensity for odd Δj transitions at j < 4 and for even transitions at j ≥ 4. This propensity gradually weakens at higher temperatures but remains observable. The provided rate coefficients supply essential data for astrochemical models involving Ar, enabling more accurate treatment of HCN/HNC (de-)excitation and radiative transfer in interstellar and planetary environments, thereby improving the reliability of derived molecular abundance ratios.