Metal-Containing Carbon Clusters: Structures, Isomerization, and Formation of NbCn+ Clusters

Abstract

Injected ion drift tube techniques, including ion mobility measurements and annealing and fragmentation studies, have been used to examine the isomers present for NbC,+ (n = 15-50) clusters. Isomers attributed to niobium-containing monocyclic and bicyclic rings, graphitic sheets, and metallofullerenes have been identified. Monocyclic rings, where the niobium atom appears to be either inserted into or bound to the outside of the ring, dominate for NbC,+ with n 22. Isomers assigned to bicyclic rings are first observed and become dominant around NbC22+. Unlike the bicyclic rings for C,' and Lac,+, the NbC,+ bicyclic rings do not anneal into monocyclic rings. They probably consist of two rings joined together by a niobium atom. An isomer attributed to NbC,+ graphitic sheets is present for n > 22 and becomes important for clusters with around 30 carbon atoms. Metallofullerenes are first observed for NbC28+ and become a major isomer for clusters with n 31. Both endohedral metallofullerenes and networked metallofullerenes (where the metal atom is part of the cage) have been identified. For clusters with more than around 30 carbon atoms the NbC,+ bicyclic rings can be annealed into metallofullerenes and, for the smaller ones, metal-containing graphitic sheets. The isomers observed for NbC,+ are similar to those found for pure C,+ and Lac,+, but the niobium atom has a substantial effect on the properties and the abundances of the different isomers.