Fullerenes are a new class of carbon molecules, the first truly molecular form of pure carbon yet isolated. Fullerenes consist of hollow cages composed of three-connected networks of carbon atoms. The name fullerene was chosen to honor R. Buckminster Fuller, the creator of the geodesic dome, and fullerenes are also known as "bucky-balls." The most famous fullerene molecule, C60 or "buckminsterfullerene," has sixty carbon atoms arranged in the same geometry as the vertices of the seams on a soccer ball. Fullerenes and their discoverers, Richard E. Smalley, J. R. Heath, S. C. O'Brien, and Robert F. Curl of Rice University and Howard W. Kroto of the University of Sussex, England, were honored this fall by the 1996 Nobel Prize in Chemistry, eleven years after their discovery.
The cage-like structure of fullerenes can be used to encapsulate metal atoms on a molecular scale creating an endohedral metallofullerene (a metal atom within a closed-cage fullerene). In fact, within a few days of the original realization that C60 may be shaped like a soccer ball, the first experimental indication was obtained that a metal atom can be put inside. Smalley writes, "For most observers on first learning of the proposed soccer ball structure of C60, there often is an almost irrepressible urge to ask 'Can you put something inside?'" We asked, "Can you put an actinide inside?" Here was an opportunity to trap actinide atoms within one of the most stable molecules or clusters ever encountered and isolate it chemically from its environment.
Figure 1. An idealized structure of U@C60.* The uranium atom in the center of the molecule is too large to fit through any of the holes in the five-membered and six-membered carbon rings that make up the fullerene cage. It is expected that the true structure of U@C60 will have the uranium atom off-center as the hollow within the cage is much larger than the uranium atom. It is also expected that the uranium atom will donate electrons to the cage and exist within the cage as an ion although the molecule as a whole will be uncharged.
*Note on nomenclature: A molecule identified as a fullerene always has a closed-cage structure. Endohedral metallofullerenes are written as M@CX where M is the metal, @ indicates that the metal is inside the fullerene cage as opposed to being bound to the outside of the cage, and X indicates that the fullerene cage is composed of X carbon atoms.]
The first application to occur to us was to use endohedral metallofullerenes as the basis for a superior waste form for actinides. An encapsulated actinide atom cannot escape from its fullerene cage-the holes in the cage are too small even for a helium atom to fit through. The problem of a stable waste form is reduced to the simpler problem of the immobilization of the much larger and hydrophobic fullerene with the actinide trapped inside. In all conventional waste forms, metal ions have mobility and can be leached out of the matrix. The migration of a metal atom trapped within a fullerene is expected to be much lower because the whole fullerene must migrate for the trapped metal atom to move.
Our interest in this approach was stimulated in 1991 by a string of discoveries in fullerene science. The first and most significant discovery was made at the University of Arizona by Huffman and Krätschmer, who developed a method to produce macroscopic quantities of fullerenes in a carbon arc. Before this discovery was made, fullerenes were studied by sweeping the vapor created by an intense laser pulse, which had been focused onto a carbon target, into a sensitive mass spectrometer. The second discovery was the detection of fullerenes in Precambrian rock believed to be more than 600 million years old. Fullerenes thus appear to have been chemically stable in the environment for much longer times than those required for a plutonium waste form. The third discovery was of a uranium endohedral metallofullerene, U@C28, whose efficiency of production was much greater than other endohedral metallofullerenes. The relative absence of empty fullerenes, the authors conclude, suggests that fullerene cages are to some extent actually nucleated in the gas phase around uranium atoms or ions. One could not only make actinide endohedral metallofullerenes, but the production efficiency was much higher when the metal was an actinide than it was for other metals.
Investigations of endohedral metallofullerenes have been plagued by low production efficiencies, sensitivity of the endohedral fullerenes to air, and a lack of separation methods to isolate a pure product. The last difficulty arises from the fact that endohedral fullerenes, with the single exception of M@C82, are insoluble in all solvents. We have built a carbon arc apparatus for the production of actinide metallofullerenes under anaerobic conditions. We have developed a sublimation method for the separation of metallofullerenes from the "soot" produced in the carbon arc and, by careful control of the temperature, have accomplished separation of metallofullerenes from empty fullerenes. A thin film can be made by subliming fullerenes onto a target. Certain carbon atoms can be removed preferentially by sublimation at relatively low temperatures, leaving the metallofullerenes in the soot. The U@C60 is sublimed after the empty fullerenes have been removed from the soot by raising the temperature by just a few degrees. The ability to produce relatively pure films of metallofullerenes will allow the investigation of their chemical and physical properties.
Safe production and manipulation of actinide-containing fullerenes has been demonstrated using anaerobic handling techniques. Sublimation of empty and endohedral metallofullerenes has proved its utility as a technique for separation of fullerenes into cage-size groups. This allows the preparation of films containing only C60, C70, and M@C60 as well as films greatly enriched in higher fullerenes relative to C60 and C70. The temperatures required for sublimation of M@C60 indicate that its enthalpy of vaporization is a few kcal/mol higher than C60, the likely result of stronger interactions between M@C60 and its environment in the soot.
Collaborators on this project include Michael D. Diener, Rice University; and Coleman A. Smith, James T. McFarlan and D. Kirk Veirs, NMT-6.
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