Seventeen water molecules form the smallest cage that traps a single water molecule

How few water molecules does it take to build a cage around a single water molecule? Researchers led by Prof. JIANG Ling and Assoc. Prof. LI Gang of the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, working with Prof. LI Jun’s team at Tsinghua University, have now answered that question experimentally: 17. The study was published in the Journal of the American Chemical Society.

Determining the minimum size of an endohedral water cage matters because it bears on how scientists understand liquid water, hydrophobic solvation, and ice nucleation. Yet the minimum number had remained an open question, and the new result addresses a long-standing debate about the molecular limit of a solvation shell.

To reach the answer, 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. The Tsinghua University group then carried out high-precision quantum chemical calculations to interpret the measured spectrum.

Together, the experiment and calculations identified three lowest-energy structural models for the 17-molecule cluster: an endohedral water cage, a fused pentagonal prism, and a fused cube. Among these candidate structures, the endohedral cage was found to be dominant, providing experimental evidence that (H2O)17 forms the smallest endohedral water cage.

The cage’s oxygen-atom framework turned out to be an irregular polyhedron with four quadrilateral faces, four pentagonal faces, and two hexagonal faces. This geometry differs from the 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.

Beyond settling the size question, the work offers insights into the hydrogen-bond network in the first coordination shell of liquid water, the immediate molecular neighborhood that surrounds individual molecules in the liquid. The researchers suggest the findings could inform how the molecular-scale structure of water is understood in contexts ranging from solvation to the earliest stages of ice formation.

Subject of Research: The smallest endohedral water cage, formed by 17 water molecules

Article Title: Researchers identify smallest endohedral water cage: 17 water molecules

Article References: Researchers identify smallest endohedral water cage: 17 water molecules. Original publication (Bibliographic details unavailable.)

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Keywords: water clusters, hydrogen bonding, endohedral cage, infrared spectroscopy, solvation shell, ice nucleation, hydrophobic solvation, quantum chemistry, Dalian Coherent Light Source, Journal of the American Chemical Society, Researchers, identify