Probing the interior of fullerenes by 3He NMR spectroscopy of endohedral 3He@C60 and 3He@C70

Abstract

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 3He 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 3He NMR spectra of the endohedral compounds 3He@C60 and 3He@C70 (the @ symbol denotes a compound that is endohedral). We find that the 3He nuclei encapsulated in C6o and C70 are shielded by 6 and 29 parts per million respectively, relative to free 3He. 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 3He NMR spectra of stable helium compounds.