Thermodynamical stability of [CNN and NCN] sequences as indication of most abundant structures in the ISM

¹ Univ Rouen Normandie, INSA Rouen Normandie, CNRS, Normandie Univ, COBRA UMR 6014, INC3M FR 3038, F76000 Rouen, France and CNRS, Sorbonne Université, LCT UMR 7616, 75005 Paris, France
² Institut Parisien de Chimie Physique et Théorique IP2CT (FR2622), Sorbonne Université, Paris, France
³ Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR UMR 6226, 35000 Rennes, France
e-mail: olivier.tasseau.1@ensc-rennes.fr; yves.ellinger@ensc-rennes.fr; jean-claude.guillemin@ensc-rennes.fr

Received: 22 February 2024
Accepted: 13 June 2024

Abstract

Context. Most of the molecules identified in the interstellar medium (ISM) are organic compounds and more than 50 have one isomer or more. Statistically, the most stable isomer of a given chemical formula is the most abundant. This occurrence is verified up to ~90% of the detected species leading to the so-called minimum energy principle (MEP).

Aims. Our main objective is to increase the list of the 14 bis-nitrogen species already detected. We focus on ten C_xH_yN_z isomer families with x = (1, 2, 3), y = (0, 2, 4, 6, 8), z = 2. To this end, we look for a reliable and economic way to provide energy scales.

Methods. We employed standard quantum chemistry methods to determine the relative position of each isomer on the energy scales of each family. We systematically applied density functional theory (DFT) treatments using basis sets of increasing size and quality (6-311++G** and cc-pVQZ). When reasonably feasible, we then performed high-level coupled cluster calculations (CCSD) using the same basis sets to refine relative energies.

Results. All 14 bis-nitrogen species already identified in the ISM indeed satisfy the MEP. We determine the relative thermodynamic stability of the isomers with a C_xH_yN₂ formula of each of the ten sets (94 compounds altogether), and hightlight those that are potentially detectable. By increasing the number of carbon atoms, we find 15 compounds that are by far the most stable candidates.

Conclusions. We confirm that, within the limits of thermodynamics, MEP is an efficient and easily applicable tool for identifying the isomers in a given series that have a greater probability of being detected. Computationally, the combination “B3LYP/cc-pVQZ” provides a suitable compromise for determining energy differences and dipole moments. Clearly, the isomers containing the [NCN] sequence should be prioritized over those with [CNN] in future observation campaigns.