2021MNRAS.503.2902M

Protonated cyanoacetylene, HC₃NH⁺, is detected in astrophysical media, where it plays a key role as an intermediate in the chemistries of HCN/HNC and of cyanopolyynes. We first generated a potential energy surface (PES) describing the intermonomer interaction between HC₃NH⁺ and He in Jacobi coordinates using the highly correlated CCSD(T)-F12/aug-cc-pVTZ ab initio methodology. Then, scattering calculations based on an exact close-coupling quantum-scattering technique were done to obtain pure rotational cross-sections for the rotational (de-)excitation of HC₃NH⁺ after collision with He for total energies up to 2500 cm^–1. These cross-sections are used to deduce the collision rates in the 5-350 K temperature range for the low-lying rotational levels of HC₃NH⁺ (up to j = 15). In addition, we generated an average PES for the HC₃NH⁺-H₂ system. The preliminary results show that the H₂(j_H2_ = 0) and He state-to-state de-excitation cross-sections have similar magnitudes, even though the H₂ cross-sections are larger by a factor of 2-2.5. This work should help with the accurate derivation of protonated cyanoacetylene abundances in non-local thermodynamical equilibrium astrophysical media. These will put more constraints on the chemical pathways involving the formation and destruction of HC₃NH⁺ while going back to the cyanopolyyne family and more generally those parts of nitrogen-containing molecular chemistry.