ISOLATION OF C76, A CHIRAL (D2) ALLOTROPE OF CARBON

A LONG search for molecular allotropic forms of carbon 1-4 culminated in the discovery 5-7 of a method for preparing large quantities of the C60 molecule and the subsequent confirmation 8,9 of its cage-like truncated-icosahedral structure 1. The C70 molecule prepared by the same method was later also isolated and found to have the predicted cylindrical (D5h) structure. Incomplete chromatographic separation of the large molecules C76, C78 and C84 was achieved at the same time 10,11, and small quantities of highly enriched samples were later isolated 12. Preliminary NMR and spectroscopic studies of these carbon clusters failed, however, to provide evidence for the symmetrical structures predicted earlier 13. Here we report the isolation and characterization of the C76 molecule, a third molecular form of carbon following C60 and C70. We isolated and purified substantial quantities of this species using the extraction technique of ref. 6 together with chromatography. The C-13 NMR spectrum consists of 19 lines of essentially equal intensity, confirming a compact, cage-like fullerene structure. Among the several thousand possible structures for C76, theoretical calculations by Manolopolous 14 predict a chiral structure with D2 symmetry, consisting of a spiralling, double-helical arrangement of edge-sharing pentagons and hexagons. Our NMR spectrum is uniquely consistent with this remarkable structure.