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December 24, 2001
Volume 79, Number 52
CENEAR 79 52 p. 10
ISSN 0009-2347
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Ultrafast electron diffraction finds new path for excited pyridine decay


In 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.

A group 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)].

GRAND OPENING Radiationless decay of pyridine leads to an unexpected opened ring.
Ordinary electron diffraction techniques give distances between nuclei, revealing the structure of a compound. But Zewail's 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 of wide classes of chemical and photochemical processes," he says.

"Any new method to determine detailed molecular structure on very fast timescales is important," says photochemistry 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-three- state threshold and watching it decay. To the researchers' surprise, all the intermediate structures previously proposed for the transition, such as the Dewar and Hückel 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 of us 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," Zewail says.

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