A Water Bridge Rewires CO2 Binding to Maleic Anhydride

The earliest stages of atmospheric particle formation involve the cooperative assembly of oxygenated organic molecules with abundant atmospheric species, yet how intermolecular binding preferences depend on the presence of an additional molecular component remains poorly understood. Here, high-resolution Fourier transform microwave spectroscopy, combined with quantum-chemical calculations, is used to determine the gas-phase structure of the maleic anhydride–CO2–H2O cluster. The observed cluster adopts a nearly coplanar geometry in which H2O bridges maleic anhydride and CO2 through an O–H···O hydrogen bond and a C···O tetrel bond. Compared with the binary complexes, the ternary complex exhibits a different CO2-centered interaction motif: CO2 interacts with the oxygen atom of H2O rather than with a carbonyl oxygen of maleic anhydride. Orbital-interaction analysis further reveals a corresponding difference in the dominant donor–acceptor interaction channel, while energy decomposition identifies electrostatics as the largest attractive contribution in the ternary complex. These findings show that binding preferences established in isolated binary complexes are not necessarily retained in a chemically heterogeneous three-component environment.