Cagey about water: NMR in a fullerene

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  • Published: Jul 1, 2014
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Cagey about water: NMR in a fullerene

Water cage

The all-carbon, soccerball-shaped fullerene, C60, molecule provides the most compact of cryoscopic NMR tubes for studying the spin behaviour of water, according to researchers at the Universities of Nottingham and Southampton, in UK and Columbia University in New York, USA.

The all-carbon, soccerball-shaped fullerene, C60, molecule provides the most compact of cryoscopic NMR tubes for studying the spin behaviour of water, according to researchers at the Universities of Nottingham and Southampton, in UK and Columbia University in New York, USA.

The discovery of [60]fullerene, buckminsterfullerene, and the fullerenes in general in the mid-1980s earned Harry Kroto, Robert Curl and the late Richard Smalley the Nobel Prize in Chemistry in 1996. Since the fullerenes were first posited, chemists have wondered about what might occur if smaller molecules were trapped inside. Now, the UK-US researchers are continuing their pioneering work on "caging" water molecules and cooling them, work on which SpectroscopyNOW reported previously. The hope now is to study the change in orientation of the magnetic nuclei at the centre of each hydrogen atom that gives rise to molecular spin isomers. Water molecules can exist as one of two isomers, depending on how the spins of their two hydrogen atoms are orientated: ortho, where the nuclear spins are parallel to one another, and para, where the spins are essentially in the opposite direction, antiparallel. It is thought that any given molecule can transform from ortho- into para- spin states and vice versa, a process known as nuclear spin conversion.

Chilled, but not frozen

"Currently, mechanisms for this conversion are not completely understood, nor how long it takes the molecules to transform from one spin isomer to the other," explains Southampton physicist Salvatore Mamone. "To study this, we had to figure out how to reduce the strong intermolecular interactions that are responsible for grouping of molecules and lowering the rotational mobility of the water molecules." The answer was to isolate a single water molecule from the external environment or any interference from other molecules by caging it within the interior of [60]fullerene.

The team used an ingenious chemical reaction to cut an opening in the fullerene, implant a water molecule and then stitch the fullerene closed to form H2O@C60. "The reaction has several steps and requires excellent synthetic skills to be completed successfully," Mamone told SpectroscopyNOW. "The chemists, Komatsu and Murata and the late Nick Turro and recently Whitby and Krachmalnicoff, deserve high praise for inventing and improving the molecular surgery procedure." At the end of the process which involves a multitude of fullerenes rather than single molecules they find that between 70 and 90 percent of the cages contain water, which gives them sufficient numbers to study. Because the water molecules are so well isolated in each cage, there is a large rotational freedom that makes observation of the ortho and para isomers possible.

Crucially, because the water molecules are separated in this way, they cannot freeze and so experiments at 50 Kelvin and down to 5 K are possible with effectively freely moving water molecules. During the cooling process, the team monitored the NMR spectra every few minutes over the course of several days.

Days of NMR

"As the observed NMR signal is proportional to the amount of ortho-water in the sample (para-water is "NMR silent"), we can track the percentages of ortho and para isomers at any time and any temperature," Mamone explains. "At 50 K, we find that 75 percent of the water molecules are ortho, while at 5 K, they become almost 100 percent para. Therefore, we know that after the quick temperature jump, equilibrium is restored by conversion from ortho to para; we see that conversion in real time." The team adds that their cross-polarization experiments did not reveal any evidence that it was simply the presence of carbon-13 nuclei in the fullerene cages that was catalysing the spin conversion of the endohedral water molecules.

The next step is to study what role, if any, isomer concentration and temperature have on the back conversion process of para-water to ortho-water. They will also investigate whether it is possible to detect single ortho- and para-water molecules on surfaces and to observe spin isomers of other small molecules implanted into fullerenes.

"The next step consists of studying the low concentration (water filled C60/C60) limit and pinpointing the physical mechanism behind the spin conversion in water," Mamone told SpectroscopyNOW.

Related Links

J Chem Phys, 2014, 140, 194306: "Nuclear spin conversion of water inside fullerene cages detected by low-temperature nuclear magnetic resonance"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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