M. Saunders et al., PROBING THE INTERIOR OF FULLERENES BY HE-3 NMR-SPECTROSCOPY OF ENDOHEDRAL HE-3-AT-C60 AND HE-3-AT-C70, Nature, 367(6460), 1994, pp. 256-258
FULLERENES have internal cavities large enough to encapsulate atoms1,2
. Recently, noble-gas atoms were introduced into about one in a millio
n fullerene molecules3. It is now possible to achieve far greater yiel
ds of these noble-gas endohedral compounds4. BecaUSe He-3 has a spin o
f 1/2 and is an excellent NMR nucleus5,6, it can be used as a probe fo
r the magnetic shielding environment inside the fullerene cavity. This
environment should reflect possible ring currents and hence the aroma
ticity in fullerenes7-15, an issue that measurements of magnetic susce
ptibility16,17 have not completely resolved. Here we present He-3 NMR
spectra of the endohedral compounds He-3@C60 and He-3@C70 (the @ symbo
l 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 millio
n respectively, relative to free He-3. These shieldings are unexpected
ly large, indicating significant diamagnetic ring currents in C60 and
very large ones in C70. Our results also show that, because of its sma
ll 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.