NMR characterization of isomers of C₇₈, C₈₂ and C₈₄ fullerenes

Abstract

FOLLOWING the development of a method for bulk synthesis of C⁶⁰ and other fullerenes¹, the isolation of higher fullerenes ranging from C₇₆ to C₉₆ has been achieved using chromatographic techniques^2–5. Whereas C₆₀ and C₇₀ have unique, high-symmetry structures⁶, theoretical calculations for fullerenes larger than C₇₆ have suggested that each may exist in at least two isomeric forms⁷. For C₈₄, 24 isomers have been postulated⁷, and for C₉₆calculations have yielded 196 distinct isomers⁸. Diederich et al.⁹ have used liquid chromatography and ¹³C NMR to identify two isomers of C₇₈, but previous experimental studies of other higher fullerenes² have produced ambiguous results. Here we use ¹³C NMR to determine the structures of some principal isomers of C₇₈, C₈₂ and C₈₄. We find a third isomer of C₇₈, which was not reported in ref. 9. Characterization of the structures of these larger fullerenes should provide new understanding of the factors determining the stability of hollow carbon clusters.