Structures and Properties of Known and Postulated Interstellar Cations

Abstract

Positive ions play a fundamental role in 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 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 interstellar chemistry. Optimized structures and rotational constants were obtained at the 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, who characterized the structure and energies of 200 neutral species also involved in interstellar chemistry. Taken together, the two data sets 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 data sets. It shows that two reactions, involving Si-bearing ions and present in the widely used reaction networks KIDA and UMIST, cannot occur in the cold interstellar medium because they are endothermic.

Tracking the Ice Mantle History in the Solar-type Protostars of NGC 1333 IRAS 4

“…Briefly, it is a time-dependent 3-phase grain-gas chemistry code that computes the layered grain mantles structure. The gas-phase reaction network is an updated version of the KIDA 2014 network (http://kida.obs.u-bordeaux1.fr; Wakelam et al 2015), with the reactions described in Tinacci et al (2022a). The surface reactions are assumed to occur only in the last two formed mantle layers, the latter being in contact with the gas-phase.…”

The GRETOBAPE Gas-phase Reaction Network: The Importance of Being Exothermic

“…All the species present in the astrochemical reaction network were characterized via QM calculations to obtain their optimized structure, absolute electronic energy, and zero-point energy (ZPE) correction. Most of the neutral species structures and their corresponding electronic spin multiplicity (m s ) in the ground state, a fundamental piece of information for the electronic QM calculations, are taken from Woon & Herbst (2009), while for the cations the information is taken from our previous work (Tinacci et al 2021). For the chemical species that have not been theoretically characterized yet, we inferred here the structure and m s using the same approach introduced in Tinacci et al (2021).…”

“…Most of the neutral species structures and their corresponding electronic spin multiplicity (m s ) in the ground state, a fundamental piece of information for the electronic QM calculations, are taken from Woon & Herbst (2009), while for the cations the information is taken from our previous work (Tinacci et al 2021). For the chemical species that have not been theoretically characterized yet, we inferred here the structure and m s using the same approach introduced in Tinacci et al (2021). The list of those species with their chemical data is reported in the supporting material.…”

“…The first one, by Woon & Herbst (2009), computed the QM data for a large number of neutral molecules involved in the KIDA reaction network. The second one, by Tinacci et al (2021), complemented the first one and provided QM data for all the ions and the remaining neutral species of the KIDA network. Please note that the two studies have been carried out at the same electronic level of theory, so that they can be reliably used for computing the exo/ endothermicity of all the reactions.…”

“…Please note that the two studies have been carried out at the same electronic level of theory, so that they can be reliably used for computing the exo/ endothermicity of all the reactions. In addition, Tinacci et al (2021) created a database 11 with the above QM data for all the cations.…”