Electron paramagnetic resonance studies of lanthanum-containing C₈₂

Abstract

THE conjecture that atoms can be trapped inside closed carbon cages such as the fullerenes was first made by Kroto et al¹. Mass spectroscopic evidence obtained soon after² 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 phase⁵, and evidence has been obtained for the formation of metal-containing fullerenes during arc burning of composite graphite rods⁶. 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 C₈₂a charge of 3-. This runs contrary to some expectations that the doubly charged f ulleride 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.