Structures and Isomerization of Lac,+ Clusters

Injected ion drift tube techniques have been used to examine the structures and to study the isomerization of Lac,+ (n = 2-90) clusters. Three families of metal-containing carbon rings ("ring Ia", "ring Ib", and "ring II"), metal-containing graphite sheets, and metallofullerenes have been resolved. Several plausible geometries with similar mobilities can be suggested for all of the ring isomers. The most likely geometries are a La inserted into a carbon ring (ring Ia), a La inside a carbon ring (ring Ib), and various Lac, and C, rings fused together (ring 11). The relative abundances of the graphite sheet and the metallofullerene isomers are substantially larger than for the pure carbon clusters. Both endohedral and non-endohedral Lac,+ metallofullerenes have been resolved. La&+ and all Lac,+ fullerenes with n = 38-90 are found to be endohedral, while LaC29+-LaC35+ fullerenes are non-endohedral. Annealing studies show that Lac,+ ring I1 isomers with an even number of carbon atoms convert into ring Ia isomers, while the odd-numbered ring I1 isomers convert into ring Ib isomers. For larger clusters, fullerenes and graphite sheets are formed when the rings are annealed. The efficiency of this annealing process in the Lac,+ system is much larger than in the C,+ system, although the activation energies appear to be approximately the same. Studies of the dissociation of Lac,+ show that loss of LaC4' is generally the dominant dissociation process for the Lac,+ rings with an even number of carbon atoms, while loss of C3 is also important for the odd-numbered ones.