SCIENCE/TECHNOLOGY

Research Suggests Alternative Route to Fullerenes

• Rearrangement of large carbon ring systems, rather than accretion of small carbon fragments, may lead to fullerene cages

Rudy M. Baum, C&EN West Coast News Bureau

R ecently published research re­ports from three independent lab­oratories suggest a hitherto unex­

pected mechanism for the formation of fullerenes from carbon vapor. The results indicate that instead of growing by accre­tion of smaller carbon fragments, as had been proposed previously, fullerene cages may form spontaneously from the col­lapse of large carbon ring systems con­taining about the same number of carbon atoms as the fullerene.

The mechanism suggested by the re­search is not intuitively obvious. But then, that the fullerenes exist at all isn't intuitively obvious either. As Robert F. Curl, a chemistry professor at Rice Uni­versity, Houston, who collaborated in the discovery of fullerenes, notes in a commentary in the issue of Nature that contains one of the new papers: 'The hardest part of the buckminsterfullerene story to accept has always been the for­mation of this highly symmetric mole­cule in condensing carbon vapor. If ex­tremely directed efforts by conventional organic chemistry cannot yet make this molecule, its spontaneous formation out of chaos stretches credulity/'

It is clear, however, that C60 and the other fullerenes do form. Previous ideas about how they form have focused on se­quential addition of small carbon frag­ments to a growing network that eventu­ally closes on itself to become a fullerene, or additions of C2 units to small fuller­enes, eventually to produce C60. Isotopic labeling experiments have pretty well es­tablished one fact about fullerene forma­

tion following vaporization of graphite: The reactant carbon source must undergo vaporization into atomic carbon or very small carbon fragments, C2 or C3.

Richard E. Smalley—the Rice Universi­ty chemistry professor who discovered fullerenes in collaboration with Curl, Rice graduate students James R. Heath and Sean C. O'Brien, and Harry W. Kroto, a chemistry professor at the University of Sussex, England—has formulated a mech­anism involving sequential addition of small carbon fragments onto the edges of growing graphitic sheets. Incorporation of pentagons into specific positions in the sheets causes them to curve and eventual­ly to close. Smalley refers to this mecha­nism as the "pentagon road."

Heath has argued for what he calls the "fullerene road," in which small fullerenes containing on the order of 30 to 40 carbon

atoms form in the plasma, and sequential addition of C2 units produces C60. The unusual stability of C60 makes it resistant to further attack by C2, Heath suggests, thereby accounting for its predominance. Other researchers have proposed schemes that involve the combination of different, specific precursors to produce C60.

The problem is that, although these mechanisms are logical, no experimen­tal evidence supports any of them. Al­though collapse of a 60-carbon mono­cyclic polyyne ring to the spherical C60

fullerene may seem improbable, the new research provides strong evidence that this rearrangement does, indeed, occur, whether or not it is the mecha­nism that produces buckminsterfuller­ene in the vapor from a carbon arc.

Two of the recent reports are related in that the two groups used very similar

C60 clusters form in a variety of geometries The amount of time required for Relative abundance mass-selected C60

+ clusters to travel through a drift tube is a reflection of cluster geometry, with the compact fullerene traveling faster than planar ring systems. Clusters injected into the drift tube at 100 eV do not under­go significant rearrangement. Thus, the distribution observed—a sharp spike at about 800 microseconds cor­responding to buckminsterfullerene, and a broad component at longer times corresponding to planar poly-cyclic polyyne ring isomers—accu­rately reflects the distribution of cluster geometries formed by laser vaporization of graphite. Higher in­jection energies excite the clusters to the point where they rearrange or dissociate. At higher injection ener­gies, the polycyclic rings rearrange to produce the monocyclic C60 polyyne ring; the buckminsterfullerene spike also increases, indicating that the ring systems also rearrange to pro­duce this species Source: M. F. Jarrold

l y A j w y w ^ 500 1000 1500 2000

Time, microseconds 2500

32 MAY 17,1993 C&EN

Polyyne rings coalesce to form fiiUerenes Relative abundance

100

c\ ,o

°vX o^f

\ \ Xv. _

0 if 0

C30

n = 1 , C 1 8

n = 3, C 3 0

r /

/ . _ ^i ' ~ V^0

0

(CO)10

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^

50

0.

x = 36

UW lU'ri-.-rJIJyt^^

70 Cis (CO)6

88

106

Positive- and negative-ion mass spectra show that laser de-sorption of precursors C18(CO)6 and C30(CO)10 produce the corresponding C18 and C30 monocyclic polyyne rings. The positive-ion mass spectra, shown at right, indicate that C18

+ and C30+ rings coalesce to form larger clusters.

The properties of these clusters suggest that they are fullerenes. The upper spectrum is from laser desorption of C18(CO)6. Almost no C60, the principal product of laser va­porization of graphite, is produced. By contrast, fullerenes that are integrals of 18—that is, C36/ C54, C72, C90, and so on—are especially prominent. C70, a particularly stable fullerene, likely forms by C2 loss from C72. By contrast, the lower spectrum shows the products of laser vaporiza­tion of C30(CO)10. The principal product, C60, results from coalescence of two C30 rings

200 400 600 800 1000 1200 1400 1600 1800 Mass-to-charge ratio (mlz)

Relative abundance

100

50 H

400 600 800 1000 1200 1400 1600 1800

Mass-to-charge ratio (mlz)

techniques to probe fullerene formation. At the University of California, Santa Barbara, chemistry professor Michael T. Bowers and coworkers showed that carbon clusters containing between 30 and 40 atoms, when they initially form, consist of a mixture of mono-, bi-, and tricyclic planar rings, three-dimensional ring structures, and fullerenes [Nature, 363,60 (1993)1. When heated, the bi- and tricyclic rings anneal to form both stable monocyclic rings and superheated ful­lerenes. These fullerenes subsequently cool themselves by evaporating small carbon fragments—-Q, C2, or C3.

At Northwestern University in Chica­go, chemistry professor Martin F. Jarrold and coworkers have shown that C60

+

clusters, initially consisting of the fullerene and a mixture of polycyclic ring systems, anneal to form both a C60

+ monocyclic ring and additional buckmrnsterfullerene [Science, 260,784 (1993)]. The buckmrnster­fullerene ions that form are so energetic that some lose C2 fragments to form the C58 and C56 fullerene fragments. Further heating of the monocyclic C60 polyyne ring causes it to fragment or isomerize to

produce buckminsterfullerene and small­er fullerene fragments.

The other report, published in March [Science, 259, 1594 (1993)1, describes an entirely different approach. Stephen W. McElvany of the Naval Research Labo­ratory, Washington, D.C., Francois N. Diederich of the Swiss Federal Institute of Technology, Zurich, and coworkers studied the coalescence reactions of pos­itively charged polyyne carbon ring sys­tems produced by laser vaporization of Cn(CO)n/3 (n = 18, 24, or 30) precursors. They found that coalescence of C30

+ with neutral C30 polyynes produced predom­inantly buckminsterfullerene ions, where­as coalescence of the smaller polyynes produced predominantly C70

+ through distinct intermediates.

Bowers believes that the research pro­vides strong evidence that fullerenes form by isomerization of species such as large monocyclic and polycyclic carbon rings. 'This research shows, first, that annealing these rings can produce fullerenes," Bowers points out. "Second, for both the positive and negative car­bon cluster ions, the only species we see

are linear chains; monocyclic, bicyclic, and tricyclic rings; and fullerenes. We do not see the sort of precursors that should exist if fullerenes formed from some curved graphitic network."

Smalley agrees that the work shows that large carbon rings can anneal to fullerenes. But he takes exception to the notion that the absence of "pentagon road" precursors argues against their role in the formation of fullerenes from carbon vapor. "Because they are highly reactive intermediates, you wouldn't expect to see them in the ion chroma­tography experiments," he says.

The research done at UCSB and at Northwestern employed a technique de­veloped by Bowers and coworkers at UCSB, which they call "ion chromatogra­phy." In this method, cluster ions are pro­duced by laser vaporization of a substrate, mass selected, and injected into a drift tube containing helium. Under the influ­ence of an electric field, the ions travel through the drift tube. Although the ions all have the same mass, clusters of differ­ent geometries travel at different rates, and the UCSB chemists have developed

MAY 17,1993 C&EN 33

SCIENCE/TECHNOLOGY SOFTWARE/DATABASE UPDATE

techniques to correlate the speed at which a cluster moves with its shape.

Jarrold and coworkers adapted the ion chromatography technique to study how clusters rearrange, or anneal, when they are energetically excited. When the mass-selected clusters are injected into the drift tube at increasing energies, they are transiently heated by collisions with the helium gas. At high enough ener­gies, this causes structural rearrange­ments or fragmentation, which can be analyzed by the subsequent passage of the clusters through the drift tube.

In research supported by the Air Force Office of Scientific Research and the National Science Foundation, Bow­ers, graduate student Gert von Helden, and postdoctoral fellow Nigel G. Gotts studied the annealing of carbon cluster cations containing 30 to 40 carbon atoms. Their data suggest that monocyclic rings and fullerenes are the most stable iso­mers for a given cluster size.

Jarrold, postdoctoral fellow Joanna Hunter, and graduate student James Fye have studied annealing of carbon clus­ters ranging in size from 34 to 64 carbon atoms. They, too, show that it is possible to anneal the nonfullerene isomers into just two dominant geometries, the ful-lerene and a large monocyclic ring.

With regard to C60 clusters, Jarrold says calculations indicate that the activa­tion barriers for annealing polycyclic rings or the monocyclic ring to buckmin-sterfullerene are low enough that these processes could play an important role in the carbon arc synthesis of fullerenes.

McElvany, Diederich, and coworkers Mark M. Ross of the Naval Research Laboratory and Nancy S. Goroff, a grad­uate student in the chemistry and bio­chemistry department at the University of California, Los Angeles, suggest in their paper that the coalescence reactions they observed are relevant to fullerene formation in carbon arcs. The research­ers note that "the formation of fullerene ions in these laser desorption experi­ments is rather remarkable, considering the extensive structural rearrangements necessary in the assembly of several 2-D large carbon rings into 3-D cages with five- and six-membered rings."

The observations, they continue, "do not necessarily contradict the isotopic la­beling studies. The small carbon species (Q, C2, and C3) formed in the vaporiza­tion of graphite may react to form larger cyclic species that then coalesce to yield fullerenes." •

34 MAY 17,1993 C&EN

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