Ion-ice astrochemistry: Barrierless low-energy deposition pathways to HCOOH, CH₃OH, and CO₂ on icy grain mantles from precursor cations

A new family of very favorable reaction pathways is explored involving the deposition of ions on icy grain mantles with very low energies. Quantum chemical cluster calculations at the MP2/6-31+G^** level in 4H₂O clusters and at the B3LYP/6-31+G^** level in 17H₂O clusters indicate that HCO⁺ and CH ₃⁺ are able to react spontaneously with one of the water molecules in the cluster to form protonated formic acid (HCOOH₂ ⁺) and protonated methanol (CH₃OH₂ ⁺), respectively. It is furthermore found that these initial adducts spontaneously transfer their excess protons to the cluster to form neutral formic acid and methanol, plus solvated hydronium, H₃O⁺. In the final case, if a CO molecule is bound to the surface of the cluster, OH⁺ may react with it to form protonated carbon dioxide (HCO ₂⁺), which then loses its proton to yield CO₂ and H₃O⁺. In the present model, all of these processes were found to occur with no barriers. Discussion includes the analogous gas-phase processes, which have been considered in previous studies, as well as the competitive abstraction pathway for HCO⁺ + H₂O.