On the Mechanism and Quantum Tunneling of the CO₂ + OH Anion Reaction in Ice: A Computational Study

The mechanism of the reaction between CO₂ and OH^– (anion) in ice cluster models was determined using density functional theory (DFT), employing the ωB97X-D functional and def2-TZVP basis sets for all atoms. A range of reaction barriers, 0.08–0.43 eV, were found, and the lowest energy path has a barrier of 0.08 eV, giving rise to the bicarbonate ion (HCO₃^–). Computed rate constants, accounting for quantum tunneling by employing the Eckart potential, suggest that the CO₂ + OH^– → HCO₃^– reaction can operate in ice at low temperatures (e.g., 10 K). In contrast, relatively high reaction barriers (0.52–0.74 eV) were found for the CO₂ + OH^• (radical) → HCO₃^• (radical) reaction, and the computed rate constants at low temperatures (e.g., 10 K) are extremely small. Based on the computed data, we argue that OH^– can react with CO₂ trapped in interstellar ice at 10 K, and the product of the reaction, HCO₃^–, is stable in ice. On the other hand, the OH radical does not react with CO₂ in ice. Therefore, we propose that OH anions in interstellar ice play a role in the formation of precursors of complex organic molecules (COMs) in the interstellar medium. The present findings will open a new dimension to explore the chemical evolution in the interstellar medium through the chemistry of anions in interstellar ices.