Quantum dynamics shake fullerene cages

An international collaborative research project has established a method whereby individual hydrogen and water molecules can be inserted into spherical buckminsterfullerene molecules. Also known as ‘bucky balls’, 60 carbon atoms are arranged in 20 hexagons and 12 pentagons to form these special molecules. In 1996, Professors Harry Kroto, Robert Curl and Richard Smalley were awarded the Nobel Prize in Chemistry for their discovery of buckminsterfullerene and the other related fullerenes.

In a paper which was recently published in Proceedings of the National Academy of Sciences (PNAS), researchers from the United States, Japan, France, Estonia and the United Kingdom describe how the technique they developed enabled observations of the wavelike behaviour of molecules caged within buckminsterfullerene ‘prisons’.

I spoke to Professor Tony Horsewill from the School of Physics and Astronomy at the University of Nottingham in order to find out more about the research, its theoretical and practical applications, and why this technique is so exciting. Professor Horsewill began by explaining his own position in relation to the subject.

"My research interests lie in the field of quantum dynamics, which is concerned with finding out how the wavelike nature of matter influences the motion of atoms, molecules, and so on," he said. "If you open a quantum mechanics textbook one of the first systems you will see is a particle inside a box. This is one of the systems that undergraduates learn about from the earliest days of their training. A system with a molecule inside a buckminsterfullerene is exactly that; it’s a beautiful model on which we can test out the theories of quantum mechanics and use to illustrate this fascinating area."

It was scientists from Kyoto in Japan who made the breakthrough which allowed them to surgically open up a C₆₀ ball. They then inserted a hydrogen (H₂) or water (H₂O) molecule into the ‘cage’ under high temperature and pressure. Cooling the system then stabilises the molecule so that the C₆₀ shell can be repaired with it captive inside.

"The interaction of the imprisoned molecule with the bucky ball is sufficient to confine the particle to the cage, but is not such that the molecule binds permanently to the surface of the cage," Professor Horsewill explained. "It’s a rather passive interaction because the surface is relatively isotropic and simply holds the molecule in place."

Entrapping the molecule in this way has afforded the physicists and chemists involved the rather unique opportunity to study the properties of an individual water molecule in the absence of interactions. Professor Horsewill told me that this was one of the factors that particularly attracted him to the investigation in the first place.

"This technique allows us to study a single isolated water molecule, which is not necessarily easy. In bulk water the molecules interact very strongly with each other, so you’re not looking at one molecule but an interacting system.

"At the same time, of course, there are the quantum effects due to the wavelike nature of matter. When the particles are locked up inside a box, basically what you can observe is a standing particle wave in that box. What we have here is an example of a standing wave which is a particle wave."

Professor Horsewill and his colleagues at Nottingham utilised inelastic neutron scattering (INS), an experimental technique which uses a beam of neutrons, to enable detailed investigation of the 'cage rattling' effects of the entrapped H₂ or H₂O particles. They were thus able to observe the two different isomers of each molecule, which vary according to the spin of protons in their nuclei.

"H₂ exists in two subtly different forms – ortho-hydrogen and para-hydrogen – which are quite distinct species that don’t interact or interchange very strongly at all, illustrating something called the Pauli Exclusion Principle," described Professor Horsewill. "This is a simple idea but one which has profound effects due to the quantum nature of matter."

Aside from the intrinsically fascinating process of learning about such fundamental properties and the implications for quantum mechanical theory, the scientists are hopeful that many practical applications may stem from their findings in future, as Professor Horsewill outlined.

"This is where we have to open our imaginations a little. One of the fields in which I work is nuclear magnetic resonance (NMR) and one of the big growth areas in NMR is in hyperpolarisation. There is, conceivably, a way that one could use ortho- and para-hydrogen or ortho- and para-water to create hyperpolarised spin states.

"If we can attain a system which has hyperpolarisation with a large proportion of the spin going in the same direction – like having many magnetic bar magnets in a magnetic field, where they all align parallel with that field – then we could significantly enhance the sensitivity of the NMR technique and possibly enhance the sensitivity and brightness of magnetic resonance imaging (MRI) as well.

"We could perhaps target particular cells to give us much more detailed information about the biochemistry of living organisms. This is just one of the possibilities one could imagine from making use of the ortho- and para- states."

According to Professor Horsewill, the researchers have given thought to attaching bucky balls with ortho- or para- molecules inside to other functionalised molecules. These in turn might preferentially bind to a particular site in a protein, so that it would be possible to effectively ‘shine a light’ on that protein molecule, finding out its structure and dynamics.

Although he is largely interested in H₂O and that possibilities that are being opened up by the imprisonment of water and hydrogen, the physicist conceded that it is more than likely possible to insert other types of molecule into buckminsterfullerene spheres.

"It has to be a molecule small enough to fit inside, but of course there are other molecules one could envisage the technique working with," he said. "We are looking at putting in other small molecules of a similar size to H₂ or H₂O; these could be things like nitrogen, but equally could be something like ammonia, NH₃."

The field of molecular electronics is another that could stand to benefit from advances in this experimental field.

"Another interesting point about water is that it has a dipole moment. This opens up interesting possibilities for molecular electronics. One could imagine making a field effect transistor, and making materials which would be useful dielectrics for electronics components.

"It is very early days and we’re crystal ball-gazing here, but there are many interesting possibilities. It will be fascinating to see whether, having these relatively isolated water molecules with their individual dipole moments, one will be able to control the extent to which those dipole moments interact with one another within the bucky balls to produce some interesting dielectric materials."