Transition-State Energy Measured

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.”