Reactions of Photoionization-Induced CO-H₂O Cluster: Direct Ab Initio Molecular Dynamics Study

The hydrocarboxyl radical (HOCO) is an important species in combustion and astrochemistry because it is easily converted to CO₂ after hydrogen reduction. In this study, the formation mechanism of the HOCO radical in a CO-H₂O system was investigated by direct ab initio molecular dynamics calculations. Two reactions were examined for HOCO formation. First, the reaction dynamics of the CO-H₂O cluster cation, following the ionization of the neutral parent cluster CO(H₂O) (n = 1-4), were investigated. Second, the bimolecular collision reaction between CO and (H₂O) ⁺ was studied. In the ionization of the CO(H₂O) clusters (n = 3 and 4), proton transfer, expressed as CO(H₂O) ⁺ → CO-(OH)H₃O⁺(H₂O) _-2, occurred within the (H₂O) ⁺ cluster cation, and the HOCO radical was yielded as a product upon addition of CO and OH. This reaction proceeds under zero-point energy. Also, this radical was effectively formed from the collision reaction of CO with water cluster cation (H₂O) ⁺, expressed as CO + OH(H₃O⁺)(H₂O) _-2 → HOCO-H₃O⁺ + (H₂O) _-2. If the intermolecular vibrational stretching mode is excited in the CO(H₂O) cluster (vibrational stretching between CO and the water cluster), the HOCO radical was detected after ionization when n = 2. The reaction mechanism was discussed based on the theoretical results.