Magic Alkali-Fullerene Compound Clusters
The appearance of clusters of pronounced stability, so-called magic clusters, is usually associated with the formation of filled geometrical or electronic shells. For example, the stability of C60Ba32 was explained by the decoration of all 32 faces of a fullerene with Ba atoms [1], whereas the enhanced stability of (C60K6)nK+ was associated with the complete filling of the lowest unoccupied t1u molecular orbital of C60 with the 4s1 valence electrons of K [2]. Recently, experiments employing a different heating/cooling technique have been performed and a different set of magic peaks which cannot be explained by either geometrical or electronic shell filling arguments, has been identified [3].

We have used DFT to address the bonding and stability of the experimentally observed magic clusters [3]. For the fullerene-potassium clusters the bonding was found to be primarily ionic, whearas ionic and covalent bonding were both determined to be important for those clusters containing barium. The latter can be seen in Fig. 2 below, which shows that the HOMO of Ba(C60)2 displays ionic bonding between the Ba 5d and C60-Π* orbitals. Entropy was found to be vital in determining the stability of the clusters, especially for those containing K.


Fig. 2. The highest occupied molecular orbital (HOMO) of Ba(C60)2.

[1] Zimmermann, U.; Malinowski, N.; Naher, U.; Frank, S.; Martin, T.P. Phys. Rev. Lett, 1994, 72, 3542.
[2] Martin, T.P.; Malinowski, N.; Zimmermann, U.; Naher, U.; Schaber, H. J. Chem. Phys, 1993, 99, 4210.
[3] Enders, A.; Malinowski, N.; Ievlev, D.; Zurek, E.; Autschbach, J.; Kern, K. J. Chem. Phys, accepted.