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Physical Chemistry

Transition-State Energy Measured

Chemical Dynamics: Researchers use vibrational spectroscopy to follow the reaction coordinate of simple reactions

by Stu Borman
December 21, 2015 | A version of this story appeared in Volume 93, Issue 49

GOOD VIBRATIONS
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Credit: Adapted from Science
Researchers used vibrational spectroscopy to trace the progress of reaction (red curve) and determine the transition-state energy (peak of red curve) of the conversion of HC≡N to HN≡C.
Graph shows the transition-state energy and reaction coordinate for conversion of hydrogen cyanide to hydrogen isocyanide.
Credit: Adapted from Science
Researchers used vibrational spectroscopy to trace the progress of reaction (red curve) and determine the transition-state energy (peak of red curve) of the conversion of HC≡N to HN≡C.

In an advance that enhances the depth of understanding of chemical reaction dynamics, researchers have used spectroscopic data to determine transition-state energies and reaction coordinates. The transition state, a point at which a reactant converts to a product, and the reaction coordinate, the energy pathway or trajectory a chemical system follows, have not generally been accessible experimentally. Robert W. Field and Joshua H. Baraban of Massachusetts Institute of Technology and coworkers now find that these physical parameters can be obtained by studying changes in the spacing of vibrational energy levels of reacting systems (Science 2015, DOI: 10.1126/science.aac9668). The researchers used the technique to examine the cis-trans isomerization of acetylene (HC≡CH) and the conversion of hydrogen cyanide (HC≡N) to hydrogen isocyanide (HN≡C). The values they measured agree with those determined previously using theoretical calculations. The technique “is not a realization of the dream of direct spectroscopy of the transition state,” says 1986 Chemistry Nobel Laureate John Polanyi of the University of Toronto. “But it brings that goal closer.”

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