Isomerism Effects in the Collisional Excitation of Cyanoacetylene by Molecular Hydrogen

Rotational excitation of the interstellar HC 2 NC and HNC 3 molecules, two isomers of HC 3 N, induced by collisions with H 2 is investigated at low collision energy using a quantum time-independent approach. The scattering calculations are based on new high-level ab initio four-dimensional (4D) potential energy surfaces (PESs) computed at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12b] level of theory. The method of interpolating moving least squares (IMLS) was used to construct 4D analytical PESs. Rotationally inelastic cross sections among the low-lying rotational levels of HC 2 NC and HNC 3 were obtained using a pure quantum close-coupling approach for total energies up to ∼100 cm –1 . The corresponding thermal rate coefficients were computed for temperatures ranging from 1 to 20 K. Propensity rules in favor of even Δ j 1 transitions were found for both HC 2 NC and HNC 3 in collisions with para -H 2 ( j 2 = 0), with j 1 being the rotational level of HC 2 NC and HNC 3 molecules. The new rate coefficients were compared to previously published HC 3 N– para -H 2 ( j 2 = 0) rate coefficients. As expected, differences were found, especially for the rate coefficients corresponding to Δ j 1 = 1 transitions. Such a comparison confirms the importance of having specific collisional data for the different isomers of a molecule. The new rate coefficients will be crucial to improve the estimation of the HC 3 N/HC 2 NC/HNC 3 abundance ratio in the interstellar medium.