Ionization-Driven Oxygen-Oxygen Bond Formation in Mixed CO2-H2O Clusters: The Role of Solvation and Cluster Composition.

Ionization of hydrogen-bonded molecular clusters can initiate reaction pathways that are inaccessible in isolated molecules, yet the role of local solvation in directing this chemistry remains poorly understood. Here, we employ weakly bound CO2-H2O clusters as experimentally tractable model systems to investigate how microsolvation influences ionization-driven molecular growth and bond formation. Using synchrotron-based vacuum ultraviolet photoionization mass spectrometry over the 10-14 eV photon-energy range, we investigate mixed CO2-H2O clusters and observe new oxygen-bearing products formed through molecular growth and bond reorganization. We detect m/z 78, assigned to [H2CO4]+, consistent with peroxide bond formation within mixed CO2-H2O clusters. We further identify an indirect route to H2O2 through cluster rearrangement and evaporation, evidenced by m/z 112 and assigned to [(H2O2)2(CO2)]+. Product branching depends strongly on cluster composition, with water-rich environments favoring peroxide-forming chemistry. Additional products, including formic acid and carbonic acid, reveal extensive ionization-induced molecular growth within hydrogen-bonded clusters. Density functional theory at the double-hybrid level reveals roaming-like intermediates, charge localization, and low-barrier rearrangements unique to the mixed cluster environment. Together, the experimental and theoretical results identify nanoscale CO-H2O aggregates as chemically active reactors.