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 structure¹. The C₇₀ molecule prepared by the same method was later also isolated and found to have the predicted cylindrical (D _5h) 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 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 D ₂ symmetry, consisting of a spiralling, double-helical arrangement of edge-sharing pentagons and hexagons. Our NMR spectrum is uniquely consistent with this remarkable structure.