Science 333, 613–616 (2011)
Atoms and molecules can be trapped inside the hollow interiors of fullerenes using either physical or chemical methods. For example, high pressures and temperatures can be used to force noble gases into these cage-like compounds. Alternatively, chemical reactions can be used to open windows in the carbon framework that are large enough for small molecules to pass through under ambient conditions. In some cases, the original fullerene framework can be restored by another sequence of reactions, resulting in guest molecules being trapped inside.
Now, Kei Kurotobi and Yasujiro Murata from Kyoto University in Japan have used the latter strategy to make bulk quantities of H2O@C60 — a compound in which a single water molecule is trapped inside each C60 cage. Water molecules have been put inside fullerenes before, but the openings in the cages were not closed and the water was free to escape. Kurotobi and Murata made a 16-membered-ring opening — through which a water molecule can pass — in C60 in just a few steps. Sealing up the hole restored the original carbon cage and trapped a water molecule inside.
X-ray crystallography confirms that the structure of the C60 cage is unaffected by the encapsulated water molecule and UV–visible spectroscopy is unable to detect any electronic interactions between the two species. The inclusion of a water molecule in its cavity is, however, expected to make H2O@C60 a polar molecule — unlike C60 itself, which has a zero dipole moment.
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Cantrill, S. Water behind walls. Nature Chem 3, 753 (2011). https://doi.org/10.1038/nchem.1168