The inside story of fullerene anions: a 3He NMR aromaticity probe.

A fundamental chemical question concerning the fullerenes involves their aromatic nature. For many reasons, it is impossible to experimentally obtain anything that is similar to a resonance energy for a fullerene. We must therefore rely on information obtained from magnetic properties. Since fullerenes are borderless polycyclic conjugated systems with internal cavities, it is possible to study the diamagnetism from the inside. Incorporating He inside a fullerene cage (endohedral) and measuring its NMR chemical shift compared with the shift of helium outside gives a direct measurement of the shielding of the magnetic field by the fullerene. 3] The magnetic field inside the fullerene is a consequence of diamagnetism and is related to the ring currents in the fullerene molecular orbitals. The aromaticity of C60 and C70 has been thoroughly investigated theoretically and experimentally. An interesting aspect of these investigations was the observation that the addition of electrons drastically changes the magnetic properties of fullerenes. Reducing these fullerenes to their hexaanions inverted their aromatic character. He NMR chemical shifts have clearly shown the extraordinary changes that occur in the aromaticity of C60 and C70 upon reduction. While the He chemical shift of He@C60 appears at d= 6.4 ppm, that of He@C60 is shifted to an extremely high field (d= 49.5 ppm), which exemplifies the much higher aromatic character of the C60 6 ion. On the other hand, the He NMR of He@C70 6 is shifted to low field, when compared with the neutral state (d(He@C70)= 28.8; d(He@