Intracluster Reaction Dynamics of Ionized Micro-Hydrated Hydrogen Peroxide (H₂O₂): A Direct Ab Initio Molecular Dynamics Study

Abstract

Hydrogen peroxide (H 2 O 2 ) is a unique molecule that is applied in various fields, including energy chemistry, astrophysics, and medicine. H 2 O 2 readily forms clusters with water molecules. In the present study, the reactions of ionized H 2 O 2 –water clusters, H 2 O 2 + (H 2 O) n , after vertical ionization of the parent neutral cluster were investigated using the direct ab initio molecular dynamics (AIMD) method to elucidate the reaction mechanism. Clusters with one to five water molecules, H 2 O 2 –(H 2 O) n ( n = 1–5), were examined, and the reaction of [H 2 O 2 + (H 2 O) n ] ver was tracked from the vertical ionization point to the product state, where [H 2 O 2 + (H 2 O) n ] ver is the vertical ionization state (hole is localized on H 2 O 2 ). After ionization, fast proton transfer (PT) from H 2 O 2 + to the water cluster (H 2 O) n was observed in all clusters. The HOO radical and H 3 O + (H 2 O) n −1 were formed as products. The PT reaction proceeds directly without an activation barrier. The PT times for n = 1–5 were calculated to be 36.0, 9.8, 8.3, 7.7, and 7.1 fs, respectively, at the MP2/6-311++G(d,p) level, indicating that PT in these clusters is a very fast process, and the PT time is not dependent on the cluster size ( n ), except in the case of n = 1, where the PT time was slightly longer because the bond distance and angle of the hydrogen bond in n = 1 were deformed from the standard structure. The reaction mechanism was discussed based on these results.