Rotational Spectroscopy Pinpoints the Tetrahydrate as the Onset of Water Self-Aggregation in Sevoflurane Hydration

Characterizing the interactions between water and volatile anesthetics at a molecular level is crucial for understanding their mechanisms of action. We employed broadband molecular rotational spectroscopy (CP-FTMW) and extensive isotopic substitution experiments to generate and characterize the stepwise addition of up to four water molecules to the volatile anesthetic sevoflurane, a flexible molecule with multiple binding sites. The substantial amount of isotopic data enabled the conclusive derivation of accurate structural information. The observed structures contain the most stable conformer of the previously identified monomer, with water clusters favorably interacting with the molecule to form an open chain with up to three water molecules. Notably, two isomers were detected for the tetrahydrate, which exhibit a cyclic structure with either a clockwise or anticlockwise orientation, resembling that of the pure water tetramer. The four-water marks a transition where water–water interactions dominate over direct sevoflurane–water interactions driving the assembly of the water network.