methods. "At the level we're dealing with here, we're getting the most detail you can get," says coauthor David W. Schwenke of the National Aeronautics & Space Administration's Ames Research Center, Moffett Field, Calif. Piergiorgio Casavecchia and colleagues at the University of Perugia, Italy, performed exacting tests, firing beams of CI and H2 at each other over a range of energies and observing the products' angular distributions and time-of-flight spectra. From these data, the group was able to compare their results with the theory developed by chemistry professor Donald G. Truhlar and colleagues at the University of Minnesota, Minneapolis. Meanwhile, Schwenke, Truhlar, and colleagues solved the Schrôdinger equation for the system. This gave them a dethe research with graduate student Mattailed picture of the reaction at the quanthew S. Bratcher and postdoctoral fellow tum mechanical level, which describes Stefan Hagen. phenomena such as tunneling that would "It is quite a spectacular result bebe missed by a classical treatment. cause of its simplicity," says K. Peter C. Such calculations are more difficult Vollhardt, a chemistry professor at the than classical methods, but over the next University of California, Berkeley. "It few years, chemists will increasingly add pushes the frontier of carbon bowls to quantum mechanical aspects to their thebeyond 50% of the C60 framework." ories, Truhlar says. However, methods In 1992, Scott—then a chemistry prousing classical mechanics are still very fessor at the University of Nevada, Reno— popular for simulating chemical reacand coworkers reported the FVP synthetions, and so F. Javier Aoiz and colsis of corannulene (C20H10), another leagues at University Complutense, bowl-shaped PAH. Independent research Madrid, also described the system with by Scott and by University of California, quasi-classical trajectory methods. "By San Diego, chemistry professor Jay S. Sie- Successful pairing of theory and experi- comparing them to quantum mechanical gel with derivatized corannulenes ment has pushed understanding of the results, you can get more confidence as showed that the molecule is surprisingly fundamental reaction of chlorine atoms to when classical simulations work and flexible. with hydrogen molecules to new heights. when they don't," Schwenke explains. Not so for C36H12. Although X-ray The scientists found excellent agreeWith state-of-the-art crossed-molecuquality crystals have not yet been ob- lar-beam techniques and quantum me- ment between both theories and experitained, theoretical calculations "all point chanical scattering methods, chemists in ment, Truhlar says. to a structure with steep curvature," es- the U.S., Italy, and Spain have produced With quantum mechanical treatment sentially equivalent to that of C6o, Scott an intimate picture of the behavior of CI validated, the scientists were able to exsays. Calculations also suggest the mole- and H2 as they interact to form hydrogen plore details of the reaction currently uncule has a high barrier to inversion, chloride (HC1) and hydrogen atoms [Sci- available to the experimentalist, Schwenke meaning it is quite stiff. ence, 273, 1519(1996)]. notes. For example, the theory shows that The ultraviolet absorption spectrum Other reactions have been studied in very short lived, metastable complexes of C36H12 is quite similar to C6o's, sug- this manner, but CI reactions are of par- can form during the reaction. Although gesting that "as the network of rings in- ticular importance in many chemical sys- they exist for only femtoseconds, their excreases from corannulene to C36H12, tems, such as the atmosphere. The CI + istence can cause an oscillation in the refullerene fragments appear to take on H2 reaction, which scientists have stud- action rate, he says. more and more of the electronic charac- ied for more than 100 years, is also a Truhlar likens the process to rolling a ter of C60," Scott says. model for understanding fundamental ball up and over a hill: As the system "One question we have is whether chemical kinetics. moves from a position of low potential C36H12 will undergo fullerene chemisDuring the past decade, physical energy up a potential energy barrier, the try," he adds. "PAH chemistry is edge chemists increasingly have been able to reaction gradually slows down. But once chemistry," involving the hydrogens that harness tedmology to study individual at- the metastable species are perched pretie up the dangling bonds around the pe- oms and molecules. Combining these ca- cariously at the top of the barrier, the rimeter of the PAH. "We are looking at pabilities with advances in theory, they slightest push over the edge sends the whether fullerene-like chemical reactivi- can tease out exotic characteristics that reaction speeding up again, like a ball ty will occur at the highly strained interi- would be masked by studying average re- rolling down the other side of the hill. or carbon atoms of C^H, 7. " Elizabeth Wilson action probabilities by traditional kinetics Scott notes that, shortly after the successful synthesis of corannulene, his group and others attempted to use FVP at 1,100 °C to effect cyclodehyrogenation of decacyclene to C36H12. But the only unpolymerized product at that temperature was unchanged decacyclene. "Quartz is used for pyrolysis tubes, and it softens around 1,150 °C," he points out. "But a glass blower told us that quartz won't melt until it reaches 1,500 to 1,600 °C. This work was done at 1,250 to 1,300 °C." Scott hopes to use FVP on a suitable PAH precursor to produce C6o, as well as a variety of "baskets, bowls, and tubes. We are actually more interested in those species than the fullerenes." Rudy Baum
Theory, experiment jibe for chlorine reaction
SEPTEMBER 16, 1996 C&EN
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