Unlocking the Water Trimer Loop: Isotopic Study of Benzophenone‐(H2O)1‐3 Clusters with Rotational Spectroscopy

The synergistic interplay between different kinds of noncovalent interactions is crucial to explain the structures of molecular clusters. In this study, we examined the structures of complexes of benzophenone microsolvated with up to three water molecules by using broadband rotational spectroscopy and the cold conditions of a molecular jet. The analysis of the experimental structures shows that the water molecules dock sideways on benzophenone for the water monomer and dimer moieties, and they move to the top of one of the aromatic rings when the water cluster grows to the trimer. The analysis of the rotational spectra reveals that the water trimer moiety in the complex adopts an open-looped arrangement. In contrast, ab initio calculations face a dilemma of identifying the global minimum between the open loop and the closed loop, which is only solved when zero-point vibrational energy correction is applied. A noncovalent interaction analysis indicates that an OH···π bond and a Burgi-Dunitz interaction between benzophenone and the water trimer are present in the cluster. This work throws light on the subtle balance between water-water and water-solute interactions when the solute molecule offers several different anchor sites for water molecules.