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January 31 , 2005
Bowman uncovers new state of chemical transition
BY beverly clark & Eric Rangus
For nearly 75 years, transition-state theory has guided chemists in how they view the way chemical reactions proceed. Recent research by Emory chemists is challenging the long-held theory, showing that in some cases chemical reactions can proceed via a path that completely bypasses the "transition state."
"Our understanding of chemical reactions rests on the notion of the transition state," said Joel Bowman, Samuel Candler Dobbs Professor of Physical Chemistry and chair of the chemistry department. "It is the key fundamental theory of chemical reaction rates across the board."
Transition-state theory assumes that reactants are separated from products by a crucial chemical configuration--the transition state--that governs the chemical reaction's rate or speed.
"If we think of reactions as occurring on an energy landscape, the transition state is the 'mountain pass' separating the reactants, and the resulting products from the reaction are valleys," Bowman continued.
According to transition-state theory, reactions proceed over this mountain pass, Bowman said. "But our results for a well-studied chemical reaction show that the reaction occurs during the transition state--and also through a surprising second path that is not near this transition state region."
Bowman's research, done in collaboration with physical chemist Arthur Suits of Wayne State University, was published in the Nov. 12 issue of the journal Science and was highlighted in the Nov. 15 issue of Chemical and Engineering News .
Using high-powered computational work and detailed experimental studies, the scientists demonstrated that formaldehyde exposed to light rays (or photoexcited) can decompose to hydrogen and carbon monoxide via a path that skirts that reaction's well-established transition state entirely.
Using detailed pictures and measurements developed by Suits in collaboration with Larry Harding of the Argonne National Laboratory, Bowman performed high-level calculations to create a "movie" of this second pathway. The visual model reveals that one of formaldehyde's hydrogen atoms breaks off and roams around before bumping into the second hydrogen atom and forming a hydrogen molecule. At no point in this second pathway does the reaction go through its transition state.
Formaldehyde decomposition long has been a model system for those studying transition-state theory because the reaction is simple enough to treat with high-level theoretical models, and the products are easily detectable. Bowman's research shows that such transition-state-skirting pathways may not be all that unusual in other chemical reactions.
"Although this discovery does not overturn traditional transition-state theory, our work is part of a growing body of evidence that is changing and expanding the way chemists and biochemists think about chemical reactions," Bowman said. "Transition-state theory remains important, but this work shows that other pathways of chemical reaction do exist."
Suits and Bowman had actually been working on transition-state research independently, but when Bowman found out about the duplication of effort, he and the members of his lab--which includes graduate student Jamie Rheinecker and postdoctoral fellow Xiubin Zhang--joined forces with Suits.
Currently Bowman and his team are performing follow-up research and submitting additional papers to other journals (future publications include a paper in an upcoming Journal of Chemical Physics ).
"I think this definitely opens the door for more work, not just for us but for other researchers," Bowman said, adding that he first learned his research might be at the forefront of something big when he presented preliminary findings at a meeting in Washington that included representatives from the Department of Energy.
"We ended up talking about it a fair amount," he said, "Especially with the chemical modelers."