Isolation of C76, a chiral (D2) allotrope of carbon

A LONG search for molecular allotropic forms of carbon1–4 culminated in the discovery5–7 of a method for preparing large quantities of the C60 molecule and the subsequent confirmation8,9 of its cage-like truncated-icosahedral structure1. 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 time10,11, and small quantities of highly enriched samples were later isolated12. Preliminary NMR and spectroscopic studies of these carbon clusters failed, however, to provide evidence for the symmetrical structures predicted earlier13. 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 13C 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 Manolopolous14 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.