Illustration showing the trapping of a single water molecule 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 Japan 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. 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.
