Quantum Chemical Cluster Studies of Cation-Ice Reactions for Astrochemical Applications: Seeking Experimental...

ConspectusInterstellar clouds and the outer reaches of protostellar and protoplanetary systems are very cold environments where chemistry is limited to processes that have little or no reaction barrier (in the absence of external energy input). This account reviews what is known about cation-ice reactions, which are not currently incorporated in astrochemical network models. Quantum chemical cluster calculations using density functional theory have shown that barrierless reactions can occur when gas phase cations such as HCO⁺, OH⁺, CH₃⁺, and C⁺ are deposited on an icy grain mantle with energies commensurate with other gas phase species. When cations react with molecules on ice surfaces, the pathways and products often differ significantly from gas phase chemistry due to the involvement of water and other molecules in the ice. The reactions studied to date have found pathways to abundant and important astromolecules such as methanol, formic acid, and carbon dioxide that are very favorable and may be more efficient pathways than gas phase processes. Other products that can be produced include glycolonitrile, its precursors, and related isocyanide compounds. This account describes for the first time ice surface reactions between the carbon cation, C⁺, and two common astromolecules, methanol (CH₃OH) and formic acid (HCOOH), which can yield precursors to glyoxal, hydroxyketene, vinyl alcohol, and acetaldehyde. The quantum chemical methodology used to explore reaction surfaces is also used to predict both vibrational and electronic spectra of reactant and product ices, which offers guidance for possible experimental studies of these reactions. While theoretical calculations indicate that cation-ice reactions are efficient and offer novel pathways to important astrochemical compounds, experimental confirmation would be very welcome. Cations and ice-covered grain mantles are certainly present in cold astrophysical environments. The account concludes with a discussion of how cation-ice reactions could be incorporated into reaction network models of the formation and destruction of molecules in interstellar clouds and protoplanetary systems. Further studies will involve characterizing additional rcactions and more extensive treatment of the most important cation-ice reactions to better ascertain reaction branching outcomes.