Minimum Hydration Shells of Small Polar Organic Hydrogen-Bond Acceptors in Explicit Water Clusters

Water plays an active role in organic chemistry, yet the number of explicit water molecules required to describe the local solvation environment of small organic substrates remains poorly established. Herein, we examine the solvation of five small polar hydrogen-bond (HB) acceptors using explicit molecular clusters containing two to twelve water molecules. Conformational searches, electronic structure calculations, and Interacting Quantum Atoms (IQA) energy decomposition were combined to assess energetic convergence and the nature of solute-water interactions. Binding and water deformation energies converge at approximately ten water molecules, defining a practical minimum hydration shell for these substrates. Solute-water interactions are largely confined to the first solvation shell, within ca. 3.5 Å of the heteroatom of the organic substrate, while more distant water molecules recover a bulk-like local environment. Except for trimethylamine, the strongest solute-water contact is weaker than the average HB among water molecules. IQA further shows that differences among interaction energies are governed mainly by the exchange-correlation component, revealing an important covalent contribution to hydrogen bonding. Multiacceptor hydrogen-bonded sites preferentially form water networks around only one HB acceptor rather than bridged independent hydration spheres around each HB acceptor. These results provide practical criteria for constructing compact, chemically meaningful explicit solvation models in aqueous organic chemistry.