ELECTRON-PARAMAGNETIC RESONANCE STUDIES OF LANTHANUM-CONTAINING C82

THE conjecture that atoms can be trapped inside closed carbon cages such as the fullerenes was first made by Kroto et al. 1. Mass spectroscopic evidence obtained soon after 2 suggested that lanthanum atoms were encapsulated in fullerenes prepared by laser vaporization of a lanthanum-impregnated graphite disk, and these results were later corroborated 3,4. Recently, helium atoms have been incorporated into fullerenes through collisions in the gas phase 5, and evidence has been obtained for the formation of metal-containing fullerenes during arc burning of composite graphite rods 6. All of these studies, however, have produced quantities too small for characterization using standard spectroscopic techniques. We report here the preparation of milligram quantities of lanthanum-containing C82, which can be solvent-extracted in yields of about 2% along with empty C60 and C70 cages. We have measured the electron paramagnetic resonance spectrum of this mixture, both in solution and in the solid state, which reveals that the lanthanum atom has a formal charge of 3+, and the C82 a charge of 3-. This runs contrary to some expectations that the doubly charged fulleride anions would be the most stable species 6,7; it also reveals that the fullerene cages have the same formal charge as in the superconducting alkali-metal-doped phases 8,9.