NEWS OF THE WEEK
METALLO CARBOHEDRENES: Ti8C12 clusters found strikingly stable hydrocarbons, including methn a development reminisane, acetylene, ethylene, and bencent of the startling discovzene. ery of buckminsterfullerene (C60) in 1985, chemists at PennThe Penn State team ionized sylvania State University last and analyzed the products of week announced that a cluster these reactions in a mass speccontaining eight titanium atoms trometer. Each mass spectrum and 12 carbon atoms is a recontains a single, dominant peak markably stable species. They at 528 atomic mass units (amu). propose that Ti8C12 is, like C60, a No other prominent peaks exist highly symmetrical molecule. in the mass range below 1200 amu. The Penn State chemists call To identify the unusually stable the new cluster a "metallo-carbospecies that gives rise to this hedrene," or "met-car" for short. "magic" peak, the team carried Ti8C12 resembles a fullerene in out a series of studies with hydrothat its atoms are arranged in a carbons of varying isotopic comcagelike structure. However, b e position. Experiments in which cause it contains transition-metal the vaporized titanium reacted atoms on its surface, Ti8C12 is with hydrocarbons containing likely to possess properties quite deuterium demonstrated that the different from fullerenes. Computer-generated graphic of stable Ti8C12 cluster shows stable species does not contain The research, reported in last titanium atoms in green and carbon atoms in purple hydrogen. Carbon-13 labeling esweek's Science [255, 1411 (1992)], represents a significant expansion of the "It is a very interesting result/' says tablished that the cluster ion contains 12 molecular clusters field. 'The new met- Richard E. Smalley, chemistry profes- carbon atoms, which suggests that the + cars are not merely a variant of the sor at Rice University, who was one of ion giving rise to the peak is Ti8C12 . ,, fullerenes, says Penn State chemistry the codiscoverers of fullerenes. "It rais- High-resolution isotope distribution patprofessor A. Welford Castleman Jr., who es a number of intriguing possibilities. tern analyses of the mass spectra also directed the work. "We think the mole- I am interested to know whether larger supported the conclusion that the cluster cules represent an entirely new and titanium-carbon clusters form. And contains eight titanium atoms, Castlebroader class of molecular clusters that there are numerous theoretical calcula- man says. we expect to have unusual chemical and tions that need to be done on these new The chemists concluded from this rephysical properties.,, clusters to understand their stability." sult that Ti8C12 has a pentagonal dodecaThe research was carried out by CasAnother open question, Smalley hedron structure in which the titanium tleman, postdoctoral research associate points out, is whether the technique for atoms occupy eight unique positions Baochuan Guo, and graduate student producing large quantities of fuller- that are similarly coordinated. The strucKevin P. Kerns. It was supported by enes—developed by Donald R. Huff- ture's surface is made up of 12 pentathe Department of Energy. man at the University of Arizona and gons, each of which has at its vertices If the metallo-carbohedrene phenome- Wolfgang Kratschmer of Max-Planck-In- two titanium atoms and three carbon atnon is a general one—that is, if stable stitut fur Kemphysik, Heidelberg, Ger- oms. Each titanium atom bonds to three clusters other than Ti8C12 exist—then many—or some variation of it, can be carbon atoms, probably through titanimet-cars could have a number of poten- harnessed to produce metallo-carbohe- um-carbon sigma bonds. Each carbon tial applications. Because they contain drenes. Because numerous research atom bonds to two titanium atoms and transition-metal atoms, Castleman says, groups currently focusing on fullerenes one carbon atom, probably through a it might be possible to custom design are well-placed to turn their attention to carbon-carbon sigma bond. These Ti-C and C-C sigma bonds met-cars with desired electronic proper- these new clusters, rapid advances in "connect all atoms together and form ties. In addition, he notes, titanium and understanding them are likely. numerous other transition metals are Preparation of met-cars is based on the network of the dodecahedral titanicatalytically active, so met-cars contain- vaporizing titanium metal in a laser va- um-carbohedrene," the chemists reing these metals might form the basis of porization plasma reactor. The titanium port. The remaining valence electrons new types of catalysts. plasma reacts with one of a number of are probably tied up in titanium-carbon
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MARCH 16,1992 C&EN
double bonds, they suggest, rather than in carbon-carbon double bonds. The all-carbon analog of Ti 8 C 12 , dodecahedral C20, is not expected to be a stable molecule because of the severe strain inherent in its structure and the degeneracy of pi bonding that results from tight curvature of its cage, Castleman points out. C20, if it existed, would be the smallest possible fullerene, but the smallest known fullerene is C32. Why, then, is Ti8C12 so stable? Castleman stresses that a great deal of work is required to fully answer this question. Nevertheless, he and his coworkers tentatively suggest a number of possible explanations. For instance, because titanium is a transition metal, it can participate in sigma bonding through d-sp hybridized orbitals. As
such, incorporating two titaniums into the pentagons of Ti8C12 may reduce the strain of the rings to some extent. Another possibility is that the pi bonds in Ti8C12 double bonds may have better overlap because of the participation of d orbitals, and thus may have greater stability than would be possible for carbon-carbon double bonds in C20. Because the mass spectrometer used in their initial work could not analyze ions with masses higher than 1200 amu, the Perm State chemists could not report whether larger titanium-carbon clusters analogous to the fullerenes exist. However, they say, there are tantalizing hints in their data suggesting such species are present. Efforts are under way to detect and mass-analyze them. Rudy Baum
Shuttleflightto probe atmospheric chemistry If all goes as planned, space shuttle Atlantis will rocket into space next Monday, March 23, from Kennedy Space Center in Florida, launching the first in a series of up to 10 ATLAS (Atmospheric Laboratory for Applications & Science) missions to probe the chemistry and physics of the atmosphere. During eight days in a circular orbit 183 miles above Earth, ATLAS-1 will carry out one of the broadest and most comprehensive studies of the atmosphere's composition ever done by scientists, notes mission scientist Marsha Torr of the National Aeronautics & Space Administration's Marshall Space Flight Center in Alabama. And it will be the first shuttle mission to implement NASA's Mission to Planet Earth, a large-scale, unified study of Earth as a single dynamic system. ATLAS-1 will use 13 instruments to carry out 14 studies. The instruments are mounted on two Spacelab pallets in the shuttle cargo bay, which will be opened to expose them to space. The ATLAS missions will cover an entire 11-year solar cycle, during which solar flares, sunspots, and other magnetic phenomena move from intense activity to relative calm and back again. Thus, these studies will yield a more detailed picture of Earth's atmosphere and its response to changes in the Sun. Scientists from six nations in addition to the U.S. are participating in the studies, including Belgium, France, Germany, Japan, the Netherlands, and
Switzerland. Four of the seven astronauts hold doctorates in science, including one who is a Belgian physicist. The astronauts will alternate 12-hour shifts, following a minute-by-minute timeline. ATLAS-1 studies focus on four areas: atmospheric science, solar science, space plasma physics, and ultraviolet astronomy. Six atmospheric studies will probe the middle and upper atmosphere to help correlate atmospheric composition, temperature, and pressure with
altitude, latitude, longitude, and changes in solar radiation. Phenomena to be examined include global distribution of atmospheric components and temperatures, and atmospheric reactions to external influences such as solar input and geomagnetic storms. The mission will help scientists validate and refine models of the effects of chemical change in the stratosphere. The studies also may look at high-altitude effects of volcanic eruptions, Kuwait's massive oil fires, and other events. For example, the shuttle solar backscatter ultraviolet spectrometer will provide highly calibrated UV measurements of ozone to fine-tune observations from four currently orbiting satellites. The imaging spectrometric observatory will measure visible and UV spectra to determine atmospheric composition, down to parts per trillion trace amounts. It will study such chemical constituents as atomic oxygen, molecular oxygen, hydroxyl radicals, and nitric oxides, and their excited states. The atmospheric trace molecule spectroscopy and grille spectrometer studies will use infrared spectra to map trace molecules, including carbon dioxide and ozone, in the middle atmosphere. The atmospheric lyman-alpha emissions study will use far ultraviolet spectra to measure the ratio of atmospheric hydrogen and deuterium in order to examine the rate of water evolution. And the millimeter-wave atmospheric sounder study will measure trfetrength of milli-
Workers assemble ATLAS-1 payload components at Kennedy Space Center MARCH 16,1992 C&EN
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