Water Inside the Carbon Cage-H₂O@C₆₀ and Related Fullerene Structures: Synthesis, Experimental Characterization,...

The encapsulation of a single water molecule inside C₆₀ has established H₂O@C₆₀ as a benchmark system for investigating molecular behavior under precisely defined subnanometer confinement. This review integrates advances in the synthesis, experimental characterization, theoretical description, and emerging applications of H₂O@C₆₀ and selected related water-containing fullerenes. Particular attention is given to molecular-surgery strategies, in which the fullerene cage is opened, loaded with water, and subsequently reconstructed, as well as to improvements that have increased encapsulation and cage-closure efficiencies and enabled isotopically labeled and heterofullerene derivatives. Structural, nuclear magnetic resonance, infrared, terahertz, neutron-scattering, dielectric, photoelectron, and single-molecule transport studies collectively show that the C₆₀ framework remains highly rigid, while the confined water molecule retains pronounced translational, rotational, vibrational, and nuclear-spin dynamics. At the same time, weak but measurable host-guest interactions produce symmetry breaking, rotational-level splitting, electrostatic screening, modified vibrational responses, and environment-dependent ortho-para conversion. Computational studies, ranging from density functional and correlated wave-function methods to multidimensional quantum-dynamical and molecular-dynamics approaches, have been essential for interpreting these observations and defining the underlying potential-energy landscape. Related systems, including H₂O@C₇₀, H₂O@C₅₉N, charged azafullerenes, and isotopically substituted endofullerenes, provide routes to tune cage geometry and electrostatic interactions. Although technological applications remain largely at the proof-of-principle stage, these systems offer promising platforms for quantum spectroscopy, molecular electronics, and the rational design of functional host-guest nanomaterials.