NEWS OF THE WEEK
BEYOND THE PERIODIC TABLE INORGANIC CHEMISTRY: Metal
clusters mimic chemical properties of atoms
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F SHIV N. KHANNA is right, he and his colleagues have found a portal to another periodic table of sorts, this one populated by metal cluster "superatoms" that mimic the atoms of the original table. Their latest superatom, an Al7~ cluster, behaves like a single multivalent germanium atom (Proc. Natl Acad. Sci USA, DOI: io.i073/pnas.o6o878ii03). The scientists say that if someone can find ways of making large supplies of such superatoms, instead of the minuscule amounts in their gas-phase experiments, it might become possible to develop a superatombased chemistry for creating new categories of catalysts, semiconductors, and other materials. In the past few years, Khanna, a physicist at Virginia Commonwealth University, and Pennsylvania State University's A. Welford Castleman Jr. have made aluminum clusters that consist of exactly 13 or 14 atoms and have electronic structures and chemical traits like those of halogen or alkaline earth atoms, respectively. They also found that Al13" clusters exhibit a chemical aloofness akin to that of argon and other noble gases (G&EN, April 5,2004, page 56). Such cluster-atom correspondences have fanned the notion that it might be possible to generate clusters that mimic all members of the periodic table. Now, with Al7~ clusters in hand, that possibility seems more likely than ever, Khanna says. "What is truly remarkable is that unlike previous members, this superatom exhibits multiple valence states, enabling it to form stable compound clusters
when combined with diverse ele ments," the researchers write in their PNAS paper. The compound clusters they make from the Al7" cluster include A17C" and A170~. "For us, superatoms are those clusters that are fairly stable but mimic the chemical features of atoms in the periodic table," Khanna explains. "This extends the periodic table into a third dimension." "These are tantalizing building blocks, but as yet, to my knowledge, no one has made macroscopic amounts," comments Mark Knickelbein, a cluster chemist at Argonne National Laboratory. To make sense of their findings and to make predictions of clusters that would behave like new superatoms, the researchers invoke the so-called Jellium model. This well-established model depicts the inside of a cluster as a homogeneous matrix of atomic nuclei and inner-shell electrons. This depiction results in a core of net positive charge that generates an electric potential by which valence, or bonding, electrons from the contributing atoms become organized. Using this theoretical treatment, the researchers describe the electronic structure of clusters with the same nomenclature chemists have long used to describe the electronic structures of the elements. For example, the electronic structure of Al7~, which has 22 valence electrons, is written as is 2 ip 6 id 10 2s 2 rP. Khanna says his group is now pushing to discover more superatoms using magnesium, gold, boron, and other elements.—IVAN AMATO
Optimized structures of Al7C~ (top) and Al70~ (bottom), two clusters madefrom an Al7~ metal cluster.
TAX BREAK Chemical makers call on Congress to restore federal R&D tax credit Chemical manufacturers and other U.S. businesses are urging Congress to pass legislation before the end of the year to restore and expand the federal research and development tax credit, which expired at the end of 2005. "ACC strongly supports bipartisan efforts in Congress to extend and strengthen the R&D tax credit," says American Chemistry Council President Jack N. Gerard. "An
expired [credit] will impose financial consequences for American chemistry and may jeopardize research projects that are essential to the American economy." The 2 0 % credit would again be available for certain R&D expenditures made only in the U.S., primarily spending on employees who perform qualified research activities. Originally enacted in 1981, the R&D tax credit has been
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extended 11 times by Congress. Nearly 16,000 companies have used the credit. The provision has wide support in Congress. But the tax break failed to advance before the elections when it was included in a controversial measure to reduce the estate tax (H.R. 5970). The legislation passed the House but was rejected by the Senate. "I'm going to work very hard to persuade the Repub-
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lican and Democratic leadership to pass a seamless extension of the R&D credit," says Senate Finance Committee Chairman Charles E. Grassley (R-lowa). "It's time to stop the annual exercise that threatens the R&D tax credit," adds the committee's ranking member Max Baucus (D-Mont.), who will chair the panel when Democrats take over the Senate in January.-GLENN HESS
