Mass Spectrometry of Metallofullerenes
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MASS SPECTROMETRY OF METALLOFULLERENES MARK M. ROSS', H.H. NELSON, J.H. CALLAHAN and S.W. MCELVANY
Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000.
ABSTRACT Fullerenes with a metal atom inside of the cage, metallofulierenes, were produced by either laser vaporization or a graphite arc and characterized using a variety of mass spectrometric methods. First, yttrium-fullerene adducts were formed by direct laser vaporization of samples consisting of graphite, yttrium oxide and fullerenes. Fragmentation and oxidation ion/molecule reactions showed that the laser-generated adducts are endohedral complexes (Y,@Cj), in contrast to externally-bound Y(C,)+ species formed by gas-phase reactions. In addition, evidence was obtained for laserinduced bulk coalescence reactions yielding the metallofullerenes. Second, negative ion/desorption chemical ionization mass spectrometry was used to characterize metallofullerenes in arc-generated soot, pyridine extracts and the extract residue. The pyridine extracts of La20 3/graphite soot contain mostly La@Cs2 and La 2 @Cs0 , in addition to (empty) fullerenes. However, the raw soot and the extract residue contain a broader range of metallofullerenes with relative abundances different than those observed from the extract (e.g. abundant La@C60, La@C 70) and La@C7 4). The thermal desorption behavior of the doped and undoped fullerenes indicate an interaction between the C. and LaQ@C, species. Analysis of aqueous solutions of dried pyridine extracts of LaO 3/graphite soot show C. and La•@C,, which is consistent with the possible presence of metallofullerene/ fullerene ionic complexes, (La,@C)+C,-. INTRODUCTION The production and characterization of metallofullerenes is a rapidly increasing area of research. The idea of encapsulating a metal atom inside of a fullerene cage was an important part of early experiments of Smalley, Kroto and coworkers [1]. The availability of large quantities of fullerenes [2] and improved fullerene production methods [3] have advanced this field significantly. Several groups have used different methods to generate soot from metal compound-impregnated graphite rods and have shown that the soot contains metallofullerenes. For example, Smalley and coworkers [4] have shown that La- and Y-encapsulated fullerenes (La,@C,, YX@C,), in addition to fullerenes (C.), can be produced by laser vaporization of graphite rods impregnated with the appropriate metal oxide. Laser desorption/mass spectrometric (LD/MS) analysis of a film of sublimed material from the soot generated by laser vaporization of a La 2 0 3/graphite rod showed mostly La@C, , La@C7 0, La@C7 4 and La@Cg2 while extraction with toluene yielded mostly La@C, 2 . Similar results were obtained from studies of yttrium-fullerene complexes, with the additional observation of an anomalous abundance of Y 2@Cs2 [5]. A dilanthanumfullerene, La2@Cs0 , was observed first by Whetten and coworkers [6] to be very abundant in the LD/MS analysis of the toluene soot extract, yet, this dimetallofullerene h
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