How many water molecules are needed to solvate one?

The comprehension at the molecular scale of the processes involved during solvation still remains a challenge in chemistry. Remarkably, the question concerning how many solvent molecules are necessary to solvate a solute one is still open. By exploring several water clusters of increasing size, we employ semiclassical spectroscopy [1-5] to determine on quantum dynamical grounds the minimal number of surrounding water molecules to make the central one display the same vibrational features of liquid water. We find out that double-acceptor double-donor tetrahedral coordination constituting the standard picture is necessary but not sufficient for solvation, and that particular care must be reserved to the quantum description of the combination band due to the coupling of the central monomer bending mode with network low-frequency modes. The minimal structure eventually responsible of proper solvation is made of a total of 21 water molecules and includes two complete solvation shells, of which the whole first one is tetrahedrally coordinated to the central molecule [6].