How many water molecules are needed to form the smallest cage that encapsulates a single water molecule? Researchers have experimentally identified the answer: 17 water molecules.

The study, led by Prof. JIANG Ling and Assoc. Prof. LI Gang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. LI Jun's team from Tsinghua University, was published in the Journal of the American Chemical Society.

Determining the minimum number of water molecules needed to form an endohedral cage is important for understanding liquid water, hydrophobic solvation, and ice nucleation. However, this minimum size has remained an open question.

In this study, the DICP team used a home-built neutral-cluster infrared spectroscopy station based on the Dalian Coherent Light Source (DCLS) to measure the infrared spectrum of neutral (H₂O)₁₇ clusters.

Combined with high-precision quantum chemical calculations by the Tsinghua University team, the results identified three lowest-energy structural models: an endohedral water cage, a fused pentagonal prism, and a fused cube. Among these structures, the endohedral cage was found to be dominant.

The findings provide experimental evidence that (H₂O)₁₇ forms the smallest endohedral water cage, addressing a long-standing question about the molecular limit of a solvation shell. The work also offers insights into the hydrogen-bond network in the first coordination shell of liquid water.

Interestingly, the cage's oxygen-atom framework forms an irregular polyhedron with four quadrilaterals, four pentagonal faces, and two hexagonal faces. Its structure differs from those of classical Platonic, Archimedean, and Johnson solids, raising new questions about whether classical geometric constraints are sufficient to describe the structures that hydrogen-bonded systems can form.