Carbon Clusters Containing Two Metal Atoms: Structures, Growth Mechanism, and Fullerene Formation

Gas phase ion mobility measurements have been used to probe the structures and interconversion of La 2 C n + ( n = 1−100) isomers. The smallest La 2 C n + clusters ( n ≤ 10) appear to be planar rings. However, planar mono- and bicyclic rings (the dominant isomers for C n + and LaC n +, n ≤ 30, clusters) are not observed for the larger La 2 C n + species. Instead, isomers which appear to be three-dimensional ring complexes dominate for unannealed La 2 C n + ( n ≥ 17) clusters. The formation of these complexes is probably driven by electrostatic forces. For n ≥ 30 the three-dimensional ring complexes isomerize into metallofullerenes (and metal-containing graphite sheets for n = 30−37). The estimated activation energies for these isomerization processes are about 1 eV lower than those estimated for similar processes for planar C n + and LaC n + rings. Metallofullerenes with two non-endohedral metal atoms (for n = 28−29), one endohedral metal atom (for n = 31−100), and two endohedral metal atoms (for n ≥ 64, only even n ), are identified. Fullerene derivatives (presumably fullerene + ring complexes) are abundant in the unannealed isomer distributions for La 2 C n + ( n ≥ 50) clusters, but readily isomerize into regular fullerenes upon collisional heating.