Bonding of Metals to Carbon Rings: Lac,,+ Isomers with La+ Inserted and Attached to the Ring

Doping carbon clusters with metal atoms provides an intriguing way to modify their properties and generate new materials. The metallofullerenes1.2 and metallocarbohedranes3.4 are good examples. Despite the intense interest in these systems, there is virtually no experimental information about how a metal atom binds to a carbon cluster, even in simple model systems. In this communication we describe studies of small (n = 12-40) Lac,+ clusters and report the observation of two families of Lac,,+ monocyclic rings. For clusters with an even number of carbon atoms, an isomer where the lanthanum atom appears to be inserted into the ring dominates, while for odd-numbered clusters, only isomers where the lanthanum appears to be attached to the outside (or inside) of the carbon ring are observed. The odd/even fluctuations in the relative abundances of the two isomers provide an important clue about the nature of the chemical bonding in these clusters. This information is necessary to unravel the mechanism of endohedral metallofullerene synthesis, since recent studies have shown that LaC60+ polycyclic rings can be annealed into the endohedral metallofullerene with near unit efficien~y.~ The apparatus and experimental techniques have been described in detail previously.6 Lac,,+ ions generated by pulsed laser vaporization of a La203/graphite composite rod1 were sizeselected with a quadrupole mass spectrometer. Fifty-microsecond pulses ofmass-selected ions were then injected intoa drift tubecontaining -5 Torr of helium. When the ions enter the drift tube they experience a rapid transient heating cycle as their injection energy is thermalized by collisions with the buffer gas. At high injection energies, the clusters may be heated to the point where they anneal or even fragment.' Further collisions with the buffer gas rapidly cool the clusters. They then travel across the drift tube under the influence of a weak electric field. The drift time depends on the structure of the cluster, such that clusters with compact geometries haveshorter drift times than clusters with lesscompact geometries.* After exiting the drift tube the ions are focused into a second quadrupole mass spectrometer that is used to monitor the fragmentation pattern and transmit only the ion of interest. The results presented here were recorded with an injection energy of 150 eV. At this energy a considerable amount of annealing and some fragmentation (usually less than 10%) occurs as the ions enter the drift tube. Thus, the drift time distributions of the surviving parent ions are dominated by relatively stable isomer^.^ Figure 1 shows drift time distributions recorded for LaC20+, Lacz1+, Lac2*+, and LaC23+.1° Clusters containing even numbers of carbon atoms (LaC2,,+) show two relatively large peaks, while odd-numbered clusters (LaCb+l+) show only one. As the number of carbon atoms increases, the drift times become