Structures and Properties of Known and Postulated Interstellar Cations

TL;DR

This study provides detailed quantum chemical data on 262 interstellar cations, enhancing the understanding of their structures and energies to improve astrochemical reaction networks and assess reaction feasibility in cold interstellar environments.

Contribution

The paper offers the first comprehensive quantum chemical characterization of 262 interstellar cations, filling a significant gap in astrochemical data for reaction network validation.

Findings

Identified structures and energies for 262 interstellar cations.

Demonstrated that some reactions involving Si-bearing ions are endothermic and unlikely in cold ISM.

Provided datasets to evaluate reaction thermochemistry in astrochemical models.

Abstract

Positive ions play a fundamental role in the interstellar chemistry, especially in cold environments where chemistry is believed to be mainly ion-driven. However, in contrast with neutral species, most of the cations present in the astrochemical reaction networks are not fully characterized in the astrochemical literature. To fill up this gap, we have carried out new accurate quantum chemical calculations to identify the structures and energies of 262 cations with up to 14 atoms that are postulated to have a role in the interstellar chemistry. Optimised structures and rotational constants were obtained at M06-2X/cc-pVTZ level, while electric dipoles and total electronic energies were computed with CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ single point energy calculations. The present work complements the study by Woon & Herbst (2009), who characterised the structure and energies of 200 neutral species involved as well in the interstellar chemistry. Taken together, the two datasets can be used to estimate whether a reaction, postulated in present astrochemical reaction networks, is feasible from a thermochemistry point of view and, consequently, to improve the reliability of the present networks used to simulate the interstellar chemistry. We provide an actual example of the potential use of the cations plus neutral datasets. It shows that two reactions, involving Si-bearing ions and present in the widely used reaction networks KIDA and UMIST, cannot occur in cold ISM because endothermic.