Fullerene isomerism: Isolation of C_2v-C₇₈ and D₃-C₇₈

Early reports on the formation of the higher fullerenes C₇₆, C₇₈, C₈₄, C₉₀, and C₉₄ by resistive heating of graphite stimulated theoretical calculations of possible cage structures for these all-carbon molecules. Among the five fullerene structures with isolated pentagons found for C₇₈, a closed-shell D_3h-isomer was predicted to form preferentially. Two distinct C₇₈-isomers were formed in a ratio of ∼5:1 and could be separated by high-performance liquid chromatography. The carbon-13 nuclear magnetic resonance (NMR) spectrum of the major isomer is uniquely consistent with a C_2v-structure. The NMR data also support a chiral D₃-structure for the minor isomer. The isolation of specifically these two isomers of C₇₈ provides insight into the stability of higher fullerene structures and into the mechanism for fullerene formation in general.