NEWS OF THE WEEK PHYSICAL CHEMISTRY
NEW TOOL TO TAKE MOLECULAR MOVIES Ultrafast electron diffraction finds new path for excited pyridine decay
I GRAND OPENING Radiationless decay of pyridine leads to an unexpected opened ring.
PULLING
N A STRIKING EXAMPLE OF THE
power of a new ultrafast diffraction technique, scientists have probed the evolving structure of an excited molecule as it decays and have discovered a previously unknown structural intermediate in the process. Agroup at California Institute of Technology headed by Nobel Laureate and ultrafast chemistry pioneer Ahmed H. Zewail has used electron diffraction to show that a pyridine molecule excited into a special state sheds its excess energy by opening up its ring—something never before predicted \J. Phys. Chem.A, 105, 11159(2001)}. Ordinary electron diffraction techniques give distances between nuclei, revealing the structure of a compound. But ZewaiTs group has developed a methodology with an instrument—whose
construction was funded by NSF—that repeatedly takes electron diffraction "snapshots" of a reacting system every fraction of a picosecond. Its success in producing such a detailed chemical movie has exceeded Zewail's expectations. "It's so clean and neat—usually experiments don't cooperate like this," he says. A. Welford Castleman Jr., chemistry professor at Pennsylvania State University, says the study is impressive. "The ability to directly observe the breakage of bonds and the formation of new ones opens up a new approach to the study ofwide classes of chemical and photochemical processes," he says. "Any new method to determine detailed molecular structure on very fast timescales is important," says photochem-
BACK
Dow Reacts To Tough Plastics Market
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oor economic circumstances are forcing Dow Chemical's plastics business to back off aggressive expansion plans and brace itself for an uncertain year. In a surprise move in polyurethanes, Dow will temporarily idle a 130 million-lbper-year toluene diisocyanate (TDI) production train in Freeport, Texas, in coming weeks. It also won't convert an idle methylene diisocyanate (MDI) unit in Camacari, Brazil, to TDI. The company had decided in October to shutter an
old MDI plant as it starts up a new unit in Freeport in 2004. Robert L. Wood, president of Dow's thermosets business, said at Dow Plastics' annual New York City press conference last week that the polyurethanes market, which normally grows at 5% annually, had contracted this year. "Unfortunately, we didn't deliver," he said. At the conference, executives predicted that Dow will end the year with plastics sales of $14 billion, versus $U.8 billion in 2000, and will post lower profits. Even Dow'ss
polyethylene business has had to regroup; because of high feedstock prices and capacity expansions, Dow idled lines in Seadrift and Freeport, Texas, this year. "This is the most difficult year I've been through in the polyolefin industry," said Romeo Kreinberg, president of Dow's polyolefins and elastomers business, noting his 25 years of industry experience. Dow executives don't expect a recovery in polymers until the second half of 2002.-ALEX TULLO
istry expert Nicholas J, Turro, chemistry professor at Columbia University In particular, radiationless transitions are extraordinarily difficult to study because of their short timescales—on the order of a picosecond. The ultrafast electron diffraction technique is well suited to study this type of decay For 30 years, scientists have known that when some aromatic molecules in their excited singlet state are given enough vibrational energy, their usual form of decay— fluorescence—decreases significantly while radiationless decay takes over. The phenomenon, known as the channel-three process, has been something of a mystery. Chemists have proposed a number of possible ways that the molecule could return to a lower energy state—by twisting itself into different isomers, for example. Zewail's group decided to examine the process by exciting pyridine above its channel-threestate threshold and watching it decay To the researchers' surprise, all the intermediate structures previously proposed for the transition, such as the Dewar and Huckel isomers, weren't forming. Instead, the researchers saw an extremely large internuclear distance between the pyridine's nitrogen and an adjacent carbon, indicating that the C-N bond had broken and the ring had spread open. "None ofus anticipated opening of the ring," Zewail says. This previously unknown decay channel could be important in other channel-three-type processes. The group plans to study many more reactions and is also building another generation of ultrafast electron diffraction instruments to do experiments in the condensed phase and on biological systems. "This is going to keep us excited for many years to come," Z e w a i l says.-ELIZABETH WILSON
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C&EN / DECEMBER 24, 2001
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