Spectroscopic characterization of isomerization transition states

Chemists liken reaction energetics to a landscape with hills and valleys. In this context, the transition state represents the highest barrier that reagents must pass over en route to forming products. Baraban et al. introduce a framework for extracting details about the transition state of rearrangement reactions directly from vibrational spectral data. They identified a characteristic pattern in the spacing between vibrational energy levels near the transition state, which revealed its energy as well as the specific motions involved in surmounting the barrier.Science, this issue p. 1338Transition state theory is central to our understanding of chemical reaction dynamics. We demonstrate a method for extracting transition state energies and properties from a characteristic pattern found in frequency-domain spectra of isomerizing systems. This pattern—a dip in the spacings of certain barrier-proximal vibrational levels—can be understood using the concept of effective frequency, ωeff. The method is applied to the cis-trans conformational change in the S1 state of C2H2 and the bond-breaking HCN-HNC isomerization. In both cases, the barrier heights derived from spectroscopic data agree extremely well with previous ab initio calculations. We also show that it is possible to distinguish between vibrational modes that are actively involved in the isomerization process and those that are passive bystanders.