Structures and Properties of Known and Postulated Interstellar Cations
  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.
