Abstract
The interactions between water and aromatic rings are pervasive across various scientific and technological disciplines, including biochemistry, materials science, and environmental chemistry. In this study, we combine broadband rotational spectroscopy and quantum-chemical calculations to reveal detailed structural and binding motifs in the aggregation of benzene, as the prototypical aromatic molecule, in the presence of a few water molecules. The benzene dimer and trimer structures with up to two water molecules are conclusively identified through isotopic substitution. We observe that the π-stacking interactions are substituted by more favorable CH···π contacts, allowing the insertion of water molecules acting as bridges between aromatic rings. This induces a shortening of the O···O distances for the complexes with two water molecules compared to that of the isolated water dimer. A many-body decomposition analysis of the interaction energy reveals the interactions of water with the aromatic partners through three-body contributions. While in the prototypical hydrogen-bonded pure water clusters this contribution amounts to 20-25% of the total interaction energy, we observe a significant contribution on the order of 10% in the interactions with the benzene rings. These results experimentally rationalize the binding strength of π-systems with water.