A single molecule of water has been isolated for the first time by trapping it in a fullerene cage.
Water molecules are never found alone — they are always hydrogen-bonded to other molecules of water or polar compounds. It is the character of this pervasive hydrogen bonding that is responsible for the familiar bulk properties of water, such as its high boiling point and ability to flow. To study in more detail the role of these hydrogen bonds, researchers have tried with little success to isolate water as a single molecule. Kei Kurotobi and Yasujiro Murata from Kyoto University in Japan1 have now achieved this for the first time by trapping a water molecule in a hydrophobic fullerene cage.
Kurotobi and Murata carried out a series of organic reactions to ‘poke’ a 16-carbon-wide ‘hole’ into soccer ball-like C60 fullerene cages — a hole just large enough for a single water molecule (see image). The reactions also placed oxygen atoms around the cage opening to allow the water molecule to slip into the cage more easily. The researchers then refluxed the modified fullerene in an aqueous solvent and confirmed by nuclear magnetic resonance spectroscopy that they had trapped the water molecules as expected. Another series of chemical transformations was conducted to close up the opening, leaving a single water molecule trapped within each intact fullerene cage. “We completely isolated a single molecule of H2O, without any hydrogen bonds, within the confined subnanometer space inside fullerene C60,” says Murata.
Other techniques for encapsulating atoms and ions inside fullerenes are known, but none are suitable for molecules such as water due to the difficulty in closing up the relatively large opening needed to trap the larger compounds. Solving this issue was the cleverest part of Kurotobi and Murata’s work. The opening in the fullerene only expands from 13 atoms wide to the required 16 atoms wide momentarily during the reflux process when water enters the cage. It then promptly shrinks back to 13 atoms. Restoration of the small opening by organic synthesis is easier than for the larger one, Murata explains.
“I now plan to study the intrinsic properties of a single molecule of water,” says Murata, adding that the technique could also be easily adapted to the encapsulation of other molecules with potential applications in solar cells and medicinal products.
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