Capturing the most-stable C56 fullerene cage by in situ chlorination.

The most-stable (#916)C(56) carbon cage has been captured by in situ chlorination during the radio frequency furnace process. The resulting exohedral (#916)C(56)Cl(12) was separated and unambiguously characterized by single crystal X-ray structure determination. The discovery of (#916)C(56) provides evidence for a thermodynamically controlled mechanism of fullerene formation, and on the other hand shows that the in situ chlorination does not remarkably influence the fullerene formation itself but just results in the capture of preformed cages. A detailed analysis of the chlorination pattern of (#916)C(56)Cl(12) reveals the main factors controlling the reactivity of non-IPR fullerenes. A high degree of aromatization was observed in the remaining π-system by considering geometric criteria and nucleus-independent chemical-shift analysis (NICS). Along with the well-known stabilization of pentagon-pentagon junctions during chlorination, the formation of aromatic islands plays an important role in the stabilization of the fullerene cage and also in the determination of the chlorination pattern. Based on these empirical rules, the preferable addition patterns for non-IPR fullerene cages can be easily predicted.