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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.