PROBING THE INTERIOR OF FULLERENES BY HE-3 NMR-SPECTROSCOPY OF ENDOHEDRAL HE-3-AT-C60 AND HE-3-AT-C70

FULLERENES have internal cavities large enough to encapsulate atoms1,2. Recently, noble-gas atoms were introduced into about one in a million fullerene molecules3. It is now possible to achieve far greater yields of these noble-gas endohedral compounds4. BecaUSe He-3 has a spin of 1/2 and is an excellent NMR nucleus5,6, it can be used as a probe for the magnetic shielding environment inside the fullerene cavity. This environment should reflect possible ring currents and hence the aromaticity in fullerenes7-15, an issue that measurements of magnetic susceptibility16,17 have not completely resolved. Here we present He-3 NMR spectra of the endohedral compounds He-3@C60 and He-3@C70 (the @ symbol denotes a compound that is endohedral). We find that the He-3 nuclei encapsulated in C60 and C70 are shielded by 6 and 29 parts per million respectively, relative to free He-3. These shieldings are unexpectedly large, indicating significant diamagnetic ring currents in C60 and very large ones in C70. Our results also show that, because of its small size and inertness, helium can serve as a useful probe of magnetic molecular properties. In addition, our work represents the first He-3 NMR spectra of stable helium compounds.