Smallest Endohedral Water Cage Featuring a Nonclassic Polyhedron: Experimental Observation of (H2O)17

The structural transition from clustered water molecules to the hydrogen-bonding networks of bulk liquid defines one of the most fundamental problems in physical chemistry, yet the precise size at which the endohedral cages emerge remains experimentally elusive. While theory has long predicted that a fully encapsulated water molecule requires a minimum of 16–23 surrounding molecules, direct experimental identification has been hindered by the difficulty of size selection of neutral clusters. Using infrared spectroscopy coupled to vacuum-ultraviolet threshold photoionization, we provide the first experimental evidence that (H2O)17 forms the smallest water endohedral cage, achieving a decades-standing goal of establishing the molecular limit of a solvation shell. Surprisingly, the oxygen framework of this (H2O)17 cage defines an unprecedented, nonclassic polyhedron with four tetragonal, four pentagonal, and two hexagonal faces that breaks the symmetry constraints of all classical Platonic, Archimedean, and Johnson solids. This work establishes a critical structural precursor to the hydrogen-bonding network topology observed in condensed-phase water and offers new perspectives at the multidisciplinary sciences of mathematics, materials, physics, and chemistry.