The Spectroscopy and Quantum Mechanics of C70-Based Endofullerenes

Fullerene molecules are hollow 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.

"Molecular surgery" is a remarkable synthetic method. First, a series of chemical reactions is used to open a hole in the fullerene cages. A small molecule such as water (H2O) is inserted into each fullerene cage by using high temperature and pressure. Finally, a further series of chemical reactions is used to "sew" the holes back up again. The result is the remarkable chemical compound called water-endofullerene, denoted H2O@C60. Our team has succeeded in developing new synthetic routes which have allowed the synthesis of endofullerenes containing a broader range of molecules, such as HF@C60 and CH4@C60.

Larger fullerenes than C60 exist. The fullerene C70 consists of ellipsoidal carbon cages, surrounding a cavity which is larger than that of C60. The cavity of C70 may accommodate two atoms or small molecules. We propose to create such systems, and study the properties of the encapsulated molecules and atoms. One example is C70 containing two 3He atoms. The two 3He atoms are squeezed together by confinement inside the same cavity, and comprise an "artificial molecule" which cannot exist without confinement.

We will synthesise such C70 endofullerenes, and study their quantised rotational, vibrational, and translational motions, using a variety of electromagnetic spectroscopic techniques as well as inelastic neutron scattering. The study of such systems will provide a wealth of experimental data on non-covalent interactions. Such information is very valuable since (1) non-covalent interactions are critically important for a wide range of materials and biomolecular properties, and (2) non-covalent interactions are hard to estimate for systems of reasonable size by current computational chemistry techniques.

Some C70 endofullerenes will display spin isomerism, meaning that there are different varieties of the same compound, differing only by the way the nuclear magnetic moments are aligned with respect to each other. Such compounds will be particularly interesting if they also contain unpaired electron spins. For such systems, the energy splitting associated with the spin isomerism may be brought into coincidence with the energy splitting between the electron spin states, induced by an applied magnetic field. We expect to observe unique spectroscopic phenomena in such systems including highly selective magnetic-field-induced spin-isomer conversion. This conversion may be accompanied by enhancement of nuclear magnetic resonance signals. This phenomenon can eventually lead to new ways to enhance magnetic resonance imaging signals, with applications to the imaging of materials and in the clinical sciences.