Elusive chemical reaction transition state captured

New technique reveals details of mysterious crossover point in molecule’s transformation

A new technique offers a sharp picture of an elusive and ephemeral stage of a chemical’s life. By precisely measuring the energy of a molecule at the instant it morphs from one form to another, scientists have revealed previously unseen details about the mysterious intermediates in chemical reactions.

“This is a major breakthrough in our ability to understand how chemical transformations happen,” says chemist Richard Zare of Stanford University.

During chemical reactions, molecules pass through a transition state — an unstable, high-energy form that immediately changes into the final product. “Transition states have always been thought of as these things that don’t really exist,” says Josh Baraban of the University of Colorado Boulder, a coauthor of the report in the Dec. 11 Science. But the state most definitely does exist, and now Baraban and colleagues have the specs to prove it.

The researchers worked with the simple molecule acetylene. Made of two carbon atoms each flanked by a hydrogen atom, the molecule can morph from a U-shaped structure, with both hydrogens above the carbon-carbon bond, to a lightning bolt, with one hydrogen above the carbons and one below. This type of shape-shifting reaction, called isomerization, is found in many places, including a light-detecting eye protein and gasoline manufacturing.

For all of these reactions, the transition states rest at the top of an energetically steep mountain. And the details of that landscape control the rate of the reaction. “It’s like you have a mountain range between reactants and products, and the transition state is the path,” says Baraban, who conducted the study at MIT. “It’s the easiest way to get from one to the other.”

But studying these elusive states is anything but easy, says study coauthor Robert Field of MIT, who calls them “molecules behaving badly.” As molecules march up the mountain toward their transition state, their energy profile grows so complicated that most scientists don’t bother trying to study them, he says.

To get a glimpse, Field, Baraban and colleagues used lasers to carefully pump energy into a jet of acetylene molecules. All the while, the team used laser spectroscopy to monitor changes in the molecules’ vibrations and rotations. At a certain point, the predictable pattern of vibrational changes broke down. This breakdown, marked by unexpectedly low vibrational frequencies, is the key feature marking the transition state, Field says. “When you’re going over a barrier, at the top, you basically stop.”

In the transition state, these broken patterns were related to the structural contortions of the molecule as it shape-shifts, the team found. The complete description of the transition state, including information about the molecule’s energy, structure and movement, agrees with theoretical predictions. But “there’s never been any independent way of looking at this problem,” Baraban says.

Because the transition state is “the ingredient that controls everything,” this new way of studying it could provide more information about how chemical reactions progress, says physical chemist Patrick Vaccaro of Yale University. Any new method that reveals details about transition states can “affect our basic understanding of chemistry,” he says. 

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