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 La2Cn+ (n = 1-100) isomers. The smallest La2Cn+ clusters (n less than or equal to 10) appear to be planar rings. However, planar mono- and bicyclic rings (the dominant isomers for, C-n(+) and LaCn+, n less than or equal to 30, clusters) are not observed for the larger La2Cn+ species. Instead, isomers which appear to be three-dimensional ring complexes dominate for unannealed La2Cn+ (n greater than or equal to 17) clusters. The formation of these complexes is probably driven by electrostatic forces. For n greater than or equal to 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 LaCn+ 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 greater than or equal to 64, only even n), are identified. Fullerene derivatives (presumably fullerene + ring complexes) are abundant in the unannealed isomer distributions for La2Cn+ (n greater than or equal to 50) clusters, but readily isomerize into regular fullerenes upon collisional heating.