Atomic and Molecular Endofullerenes: Spins in a box

Fullerenes are football-shaped cages of carbon atoms, for the discovery of which the British scientist Harry Kroto won the Nobel prize in 1996. Inside the cage is an empty space. Chemists and physicists have found many ingenious ways of trapping atoms or molecules inside the tiny fullerene cages. These encapsulated compounds are called endofullerenes and denoted A@C60.

A remarkable method is called "molecular surgery" in which a series of chemical reactions is used to open a hole in the fullerene, a small molecule or atom is inserted into each fullerene cage, and a further series of chemical reactions is used to "sew" the holes back up again to reform the pristine cage with the atom or molecule inside. Initial examples were hydrogen (H2@C60) and water (H2O@C60). Our team greatly improved the reported method and extended it to HF@C60.

Our team recently achieved a breakthrough in encapsulating methane to give CH4@C60 - the first time an organic molecule has been put inside C60. The route developed, using a larger hole than before, opens the way to encapsulating other interesting molecules such as ammonia (NH3), oxygen (O2) and formaldehyde (CH2O).

In the gas phase, ammonia (NH3) displays an unusual resonance in the microwave region of the electromagnetic spectrum. This resonance is associated with the "inversion" of the pyramid-shaped ammonia molecule, similar to an umbrella being inverted in a strong wind. This ammonia resonance is of great historical significance, since it was used for the very first MASER experiment (microwave amplification by stimulated emission of radiation), which was the precursor of the laser. This MASER resonance is quenched for ammonia in ordinary experimental conditions, by the interaction of the ammonia with neighbouring molecules. However it may exist for ammonia trapped inside the closed cavity of a C60 molecule. We intend to find out.

Many small symmetrical molecules display a phenomenon called spin-isomerism. This means that they exist in several forms distinguished by the configurations of their magnetic atomic nuclei, and which convert only slowly into each other. We will study the spin-isomerism of confined molecules such as methane, ammonia, and formaldehyde by using techniques such as nuclear magnetic resonance (NMR), which detects radio frequency emissions from the atomic nuclei in a strong magnetic field. In some circumstances, spin-isomerism may be exploited to give strongly enhanced NMR signals. This is potentially important since NMR is widely used throughout science for examining the structure and motion of matter - the most famous example being MRI (magnetic resonance imaging). Any technique that increases the strength of NMR signals is potentially of great importance.

Oxygen (O2) is an unusual molecule since it has two unpaired electron spins in the ground state. For this reason, oxygen is slightly magnetic. We will study the behaviour of the unpaired electron spins in fullerene-encapsulated oxygen by using a technique called electron paramagnetic resonance (EPR) in which the unpaired electrons are monitored for microwave emission in a strong magnetic field. We have reason to believe that oxygen molecules in which one of the oxygen atoms has atomic mass number 16, and the other one has atomic mass number 18, will have very unusual and useful EPR properties at low temperature.

The element Helium (He) has two stable isotopes, called helium-3 and helium-4. Helium-3 (3He) is a very favourable nucleus for NMR, giving a strong, narrow signal. However it is a very rare and expensive gas. We will encapsulate 3He inside fullerene cages and greatly enhance the 3He NMR signals of the helium-endofullerene by exposing the solid material to 3He gas which has been brought into a strongly polarized state by using lasers. The polarized 3He-endofullerene solid may have applications as a tracer substance, for example in magnetic resonance imaging.

1. Academic impact

1.1 New knowledge and scientific advancement.
The research in this proposal is basic in nature since it is directed to very basic questions at the heart of quantum physics: how do nuclear angular momenta interact with molecular angular momenta? Can molecular angular momentum be converted into nuclear polarization, giving rise to enormously enhanced NMR signals in the solid state? How does the interaction between a small molecule and atom and a carbon surface depend on distance?

In addition the proposal is highly interdisciplinary involving also highly novel compounds in organic chemistry. Simple and familiar molecules such as water, ammonia, and methane are married to the simplest and most symmetrical cage (C60-fullerene) and in order to generate unusual and fundamentally interesting quantum properties.

1.2 Worldwide scientific advancement
The proposal is part of an ongoing global collaboration with project partners from USA, France, Poland and Estonia.

1.3 Development of new methodologies, equipment, techniques, cross-disciplinary approaches.
The project fits perfectly into all of these categories. It uses new methodologies, equipment and techniques, in particular the low-temperature magnetic resonance equipment newly installed in Southampton and which is in many aspects world-unique. The project is highly cross-disciplinary, involving organic chemistry, quantum physics, quantum chemistry, surface science, neutron scattering, magnetic resonance, and electromagnetic spectroscopies.

1.4 Health of academic disciplines
It is essential for the health of academic disciplines that they are not locked into "silos". This project will establish close contact between many different academic disciplines such as synthetic chemistry, quantum molecular physics, quantum chemistry, and nuclear and electron magnetic resonance, to great benefit of all. As one example, the current proposal will provide quantitative data on the non-bonded interactions of atoms and molecules with carbon surfaces. This is expected to serve as benchmarking data for the next generation of quantum chemistry algorithms directed at the quantitative treatment of intermolecular, interatomic, and dispersion interactions.

1.5 Delivering and training researchers.
All researchers involved in this project will be exposed to multiple disciplines and will acquire an excellent oversight of spectroscopic and physical techniques applied to molecular quantum systems. These are skills of great general applicability to a very wide range of scientific problems. In particular the postgraduate student funded by ILL-Grenoble will be explicitly trained in these interdisciplinary techniques.

2. Economic and Societal Impact
2.1 Cultural. Science is a cultural activity - especially basic science. Basic research of this nature is therefore culturally enriching.

2.2 Societal benefits. In the long term the research described here is directed towards the development of more readily available methods for enhanced NMR spectroscopy, which would have considerable benefits in the medical, chemical and engineering areas. NMR is an extraordinarily broad field of science so fundamental advances in NMR have the potential for broad societal impact, as evidenced by the adoption of MRI technology worldwide. In addition, quantum chemistry is of enormous predictive power for the development of new materials and pharmaceuticals.

2.3 Economic benefits. Magnet technology is a strength of UK engineering so enhancements in NMR and MRI are of long-term economic benefit to the UK. This has been recognized by the recent strong investment in "quantum technology".

2.4 National security and social welfare. Improvements to medical treatment improve social welfare. Improvements to information processing and storage may improve social welfare if handled wisely. Cross-continental scientific cooperation is beneficial for national security.