Could you explain the significance of your article to the non-specialist?

A fascinating range of compounds has been synthesized in which hydrogen molecules are trapped inside nanometre-scale carbon cages. The hydrogen molecules are very light and behave in a quantum-mechanical fashion. The trapped molecules are free to rotate and behave as quantum rotors. We study these encapsulated rotors by using solid-state nuclear magnetic resonance, a technique that combines magnetic fields, radiowaves, and mechanical rotation of the sample. We show that the motion of the quantum rotors depends on the symmetry of the cage, and that the presence of the hydrogen molecules influences the interactions between neighbouring cages.

What has motivated you to conduct this work?

The work is part of a wider program to develop high-resolution NMR techniques at cryogenic temperatures (below 77 K). We are hoping to perform high-resolution solid-state NMR experiments under very cold conditions. There are two main reasons for this. The first reason is that NMR signals are much stronger under cryogenic conditions, so it should be possible to study much smaller amounts of sample than is currently the case. The second reason is that there is a lot of interesting physics in the cryogenic regime. Cryogenic high-resolution NMR should allow quantum phenomena such as superconductivity, magnetic ordering, quantum rotation and quantum tunneling to be investigated with chemical site selectivity. As part of this program we are investigating the use of hydrogen-fullerene complexes as "cryorelaxors" - agents that can rapidly cool down the nuclear spins of neighbouring molecules.

3. Where do you see this work developing in the future?

If high-resolution cryogenic NMR is implemented successfully, there may be many applications to structural biology and materials science, since the possibility of using smaller amounts of material will be a big advantage. The use of very low temperatures to trap highly mobile or unstable species and reaction intermediates will also be very attractive. It could open up the study of a lot of important systems that have been off-limits to high-resolution NMR. In addition, the physics of these encapsulated rotors is a very interesting topic by itself. They are true molecular gyroscopes.

Are there any particular challenges facing future research in this area?

There are considerable technical obstacles against performing stable and reliable magic-angle-spinning NMR experiments under cryogenic conditions. There are also considerable theoretical challenges in understanding the NMR behaviour of the quantum rotors, especially the relaxation properties, which are quite counter-intuitive. The systems are particularly fascinating because they are basically rather simple yet still puzzling. Fortunately, it is possible to do chemistry on these systems in order to change the dimensions of the cages, and test the theoretical models. It's a dream combination of interesting organic chemistry and very basic quantum physics.