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) (n = 1-4), were investigated. Second, the bimolecular collision reaction between CO and (H2O)(n)(+) was studied. In the ionization of the CO(H2O)(n) clusters (n = 3 and 4), proton transfer, expressed as CO(H2O)(n)(+) -> CO(OH)H3O+(H2O)(n-2), occurred within the (H2O)(n)(+) cluster cation, and the HOCO radical was yielded as a product upon addition of CO and OH. This reaction proceeds under zeropoint energy. Also, this radical was effectively formed from the collision reaction of CO with water cluster cation (H2O)(n)(+), expressed as CO + OH(H3O+)(H2O)(n-2) -> HOCO-H3O+ + (H2O)(n-2). If the intermolecular vibrational stretching mode is excited in the CO(H2O)(n) 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.