Structures and Properties of Known and Postulated Interstellar Cations
Résumé
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.