Isomerism Effects in the Collisional Excitation of Cyanoacetylene by Molecular Hydrogen

Rotational excitation of the interstellar HC2NC and HNC3 molecules, two isomers of HC3N, induced by collisions with H2 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 HC2NC and HNC3 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 Δj 1 transitions were found for both HC2NC and HNC3 in collisions with para-H2(j 2 = 0), with j 1 being the rotational level of HC2NC and HNC3 molecules. The new rate coefficients were compared to previously published HC3N–para-H2(j 2 = 0) rate coefficients. As expected, differences were found, especially for the rate coefficients corresponding to Δj 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 HC3N/HC2NC/HNC3 abundance ratio in the interstellar medium.