Rotationally inelastic scattering of cyanocyclopentadiene by helium atoms

TL;DR

This study calculates the interaction potential and rotational excitation rates of cyanocyclopentadiene with helium, providing essential data for understanding its abundance and role in interstellar chemistry, especially in dark molecular clouds.

Contribution

The paper presents the first potential energy surface and scattering calculations for cyanocyclopentadiene-He interactions, crucial for modeling interstellar molecule excitation.

Findings

High anisotropy of the PES with a -101.8 cm$^{-1}$ well depth.

Rotational excitation rates computed for temperatures up to 20 K.

Propensity for transitions with Δk_a=0 observed.

Abstract

In the interstellar medium (ISM), polycylic aromatic hydrocarbons (PAHs) are believed to be an important carbon reservoir, accounting for up to a quarter of all interstellar carbon in our galaxy. This makes the investigation of their potential formation precursors highly relevant in the context of ISM chemistry. This, in turn, includes knowing the abundance of the precursor species. One of the possible precursor molecules for PAHs is the recently detected cyanocyclopentadiene, c-C$_5$H$_5$CN. Given the physical conditions of the dense dark molecular cloud TMC-1 where the cyclic species was detected, it is crucial to consider that local thermodynamic equilibrium conditions may not be satisfied. In such case an accurate estimation of the molecular abundance involves taking into account the competition between the radiative and collisional processes, which requires the knowledge of rotational excitation data for collisions with the most abundant interstellar species - He or H$_2$. In this paper the first potential energy surface (PES) for the interaction of the most stable isomer of cyanocyclopentadiene (1-cyano-1,3-cyclopentadiene) with He atoms is computed using the explicitly correlated coupled-cluster theory [CCSD(T)-F12]. The obtained PES demonstrates a high anisotropy and is characterized by a global potential well of -101.8 cm$^{-1}$. Scattering calculations of the rotational (de-)excitation of 1-cyano-cyclopentadiene induced by He atoms are performed with the quantum mechanical close-coupling method for total energies up to 125 cm$^{-1}$. The resulting rotational state-to-state cross sections are used to compute the corresponding rate coefficients for temperatures up to 20 K. A propensity favoring the transitions with $\Delta k_a=0$ is observed.