Water Arrangements upon Interaction with a Rigid Solute: Multiconfigurational Fenchone-(H₂O)_4-7 Hydrates

Insight into the arrangements of water molecules around solutes is important to understand how solvation proceeds and to build reliable models to describe water-solute interactions. We report the stepwise solvation of fenchone, a biogenic ketone, with 4-7 water molecules. Multiple hydrates were observed using broadband rotational spectroscopy, and the configurations of four fenchone-(H₂O)₄, three fenchone-(H₂O)₅, two fenchone-(H₂O)₆, and one fenchone-(H₂O)₇ complexes were characterized from the analysis of their rotational spectra in combination with quantum-chemical calculations. Interactions with fenchone deeply perturb water configurations compared with the pure water tetramer and pentamer. In two fenchone-(H₂O)₄ complexes, the water tetramer adopts completely new arrangements, and in fenchone-(H₂O)₅, the water pentamer is no longer close to being planar. The water hexamer interacts with fenchone as the least abundant book isomer, while the water heptamer adopts a distorted prism structure, which forms a water cube when including the fenchone oxygen in the hydrogen bonding network. Differences in hydrogen bonding networks compared with those of pure water clusters show the influence of fenchone's topology. Specifically, all observed hydrates except one show two water molecules binding to fenchone through each oxygen lone pair. The observation of several water arrangements for fenchone-(H₂O)_4-7 complexes highlights water adaptability and provides insight into the solvation process.