news of the week Prinzbach's group, working in collaboration with the groups of chemistry professor Lawrence T. Scott of Boston College and physicist Bernd von Issendorff at Freiburg, found that the photoelectron spectra of C20" ions derived from the cage and bowl precursors are markedly different from each other and also different from the spectrum of an20 other, previously observed, isomer that consists of a ring of 20 carbon atoms with alternating single and triple bonds. Furthermore, the researchers note, fter years of effort and having final- move. The chlorination route, however, ly resorted to "brute-force" meth- turned out to be impractical. And the the photoelectron spectrum of the C20" i ods, researchers at Albert Lud- bromination route was hampered by the ion derived from the dodecahedrane prewigs University in Freiburg, Germany, difficulty of replacing the petite hydro- cursor is "clearly more consistent with have claimed success at generating C20, gen atoms with the bulky, strain-induc- that expected for a closed 'polyolefinic' cage" than with that expected for open the smallest carbon-cage molecule that ing bromine atoms. can be considered a fullerene [Nature, Brute force was necessary to over- polycyclic isomers that have acetylenic 407,60(2000)]. come this obstacle: For three days, un- edges. Unfortunately, they add, the specEmeritus professor of chemistry der irradiation with visible light, a solu- tra do not allow them to unambiguously Horst Prinzbach and his coworkers re- tion of dodecahedrane in bromine was assign structures to these ions. port that they have produced C20 in refluxed in a pressurized flask that alEven so, "there's a very high probaminute amounts in the gas bility that Prinzbach is phase inside a mass specright," comments chemistrometer. In doing so, they try professor Philip E. Eaton C20 takes shape have provided evidence of the University of Chicago, that this elusive molecule, who has explored the chemwhose extremely strained istry of strained-cage moleframework makes it highly cules like cubane. Eaton reactive, can actually be tells C&EN that Prinzbach's created and studied specconclusions are "based on troscopically before it flicksound reasoning" and that Cage Bowl Ring ers out of existence. "he's done a terrific job," given the difficulty of the Significantly, C20 has problem. never been observed among the products of the usual synthetic route lowed the hydrogen bromide by-prodStill, Prinzbach's conclusions are not to C60 and other fullerenes, which in- uct to leak out. That did the trick. The definitive. In a Nature commentary, volves vaporizing graphite and condens- product consisted primarily of a multi- chemistry professor Martin F. Jarrold of ing the vapor. Rather, the Freiburg re- tude of isomeric trienes with the aver- Northwestern University points out the searchers generated the C20 cage from a age composition C20HBr13. possibility, "perhaps unlikely," that the dodecahedrane derivative—a molecule Electron-impact mass spectroscopy C20~ ions derived from the cage precurwhose C20 skeleton had been rationally was then used to debrominate the sor "could have distorted or partly synthesized in a stepwise manner. trienes in the gas phase and observe the opened up in some unexpected way." RulDodecahedrane (C20H20), a saturat- production of C20+ and C202+, as well as a ing out this possibility will require further spectroscopic measurements, he writes. ed cage hydrocarbon having 12 pentag- full range of brominated C20 ions. "Now that the bowl isomer has finally onal faces, wasfirstsynthesized in 1982 To exclude the possibility that the by Leo A Paquette's group at Ohio State C20 cage might have rearranged to a been prepared," Jarrold notes, "it should University. In the early 1990s, Prinz- bowl-shaped isomer that some calcula- be possible to resolve an important issue bach and coworkers developed an im- tions suggest is more stable than the about how fullerenes form." The C20 bowl proved synthesis that allowed them to cage form, the researchers generated is the starting point for one of two pathprepare multigram quantities of this and studied the equally elusive C20 bowl ways proposed to explain how fullerenes compound. That opened the possibility isomer in separate experiments. Coran- grow by the addition of small carbon fragthat a way might be found to strip off all nulene (C20H10), a bowl-shaped polycy- ments. The availability of the bowl isomer the hydrogens to yield the unsaturated clic aromatic hydrocarbon, was exten- should make it possible to test this mechC20 fullerene—a highly curved cage so sively brominated, and then the product anism, he believes. strained that its very existence was in mixture was debrominated in the gas "Thefinalresolution of this and other doubt. phase under the same conditions used issues will hinge on further experiPrinzbach realized that for C20H20 to with the brominated C20 cages. To dis- ments and on more extensive theoretibe transformed to C20, the strongly tinguish between these isomers, the re- cal studies, both of which will surely be bound hydrogen atoms wouldfirsthave searchers converted the C20 species to motivated by this latest development," to be replaced with atoms such as chlo- their anions and examined them by pho- Jarrold concludes. rine or bromine, which are easier to re- toelectron spectroscopy. Ron Dagani
SMALLEST FULLERENE SEEN
Elusive C cage and its bowl-shaped isomer are generated in the gas phase
A
SEPTEMBER 11,2000 C&EN
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