Isomerism Effects in the Collisional Excitation of Cyanoacetylene by Molecular Hydrogen

Abstract

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 4-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 ∆j 1 transitions were found for both HC 2 NC and HNC 3 in collisions with para-H 2 (j = 0), j 1 being the rotational level of HC 2 NC and HNC 3 molecules. The new rate coefficients were compared with previously published HC 3 N-para-H 2 (j = 0) rate coefficients. As expected, differences were found, especially for the rate coefficients corresponding to ∆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.

Non-LTE modelling of cyanoacetylene: evidence for isomer-specific excitation

“…To gain further insight into propensity rules, we plot in Fig. 2 the de-excitation rate coefficients of the HC 3 N isomers out of the 1 = 10 level at 50 K. The HC 3 N rate coefficients were taken from Faure et al (2016). For collisions with p-H 2 , for both isomers, one can see that the propensity in favor of even Δ 1 transitions observed in our previous work (Bop et al 2019) persists at higher temperatures. In contrast, for collisions with o-H 2 , the propensity rules are different for the two isomers.…”

“…These vdW complexes present each two minima; with well-depths of 174.1 cm −1 and 178.0 cm −1 for HC 2 NC-H 2 , and 270.8 cm −1 and 467.9 cm −1 for HNC 3 -H 2 . More details concerning the computational details, as well as the analytical representation, well-depths and its precise locations for each PESs, can be obtained in Bop et al (2019).…”

“…The final expansion included 380 angular basis functions and each expansion coefficient was interpolated radially in the range = 5-30 a 0 with a smooth connection to standard short-and long-range extrapolations. Full details can be found in Bop et al (2019).…”

“…It was recently shown (Hernández Vera et al 2017) that isomerism effects can be important in collisional studies, and that rate coefficients can differ significantly for different isomers. Hence, we recently reported the first four dimensional (4D) PESs for the HC 2 NC-H 2 and HNC 3 -H 2 systems, as well as excitation rate coefficients due to para-H 2 ( 2 = 0) [hereafter p-H 2 ( 2 = 0)] collisions for temperature below 20 K (Bop et al 2019). These data highlighted different excitation efficiencies for the HC 3 N isomers.…”

Inelastic scattering in isotopologues of O2–Ar: the effects of mass, symmetry, and density of states

“…The two dimensional (2D) PES was constructed using an automated interpolating moving least squares (IMLS) methodology, which has been used in several previous studies 24,[83][84][85][86][87] and has been recently released as a software package under the name AUTOSURF. 88 As usual, 89 a local fit was expanded about each data point and the final potential is obtained as the normalized weighted sum of the local fits.…”