Structure of Higher Fullerenes and Growth Mechanisms

The actual number of isomers observed is much smaUer-there is, for instance, exactly one each for C o and C70. Other fuUerenes, hke 052-68. do not form at aU, even though they are theoretically possible. It is obviously impossible for such carbon cages to adopt a structure exclusively made from hexagons and pentagons with the latter being isolated. 

The above observations may be summarized to state some rules that quite reh-ably predict the formation of certain, stable fuUerenes . 

The number of possible isomers is drastically reduced by these rules, and it becomes clear why many C molecules do not adopt a cage-Hke structure. The magic numbers for stable fuUerenes obeying the rules above are n = 60, 70, 72, 76, 78,84. and reaUy all of them (except for C72) can be affirmed experimentally. Possibly any C72 initiaUy created is transformed in situ into the even more stable 

Higher fuUerenes are by far less abound than the archetypical compound 50. The formation of these cage-like compounds is kineticaUy controUed among the fuUerenes, is by no means the molecule with the least enthalpy per carbon atom. Normally the larger, thermodynamically more stable homologs (refer to Section 2.4) should be more abundant than 50 with its higher curvature-induced 

To the right the bold line illustrates a hypothetical polyyne chain that is closed in several (simultaneous) cyclization steps to become the fullerene cage ( Royal Soc. 1993). 

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