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.