Science 11 December 2015:
DOI: 10.1126/science.aac9668
RESEARCH ARTICLE
Spectroscopic characterization of isomerization transition states
Joshua H. Baraban 1,*, P. Bryan Changala 1,†, Georg Ch. Mellau 2, John F. Stanton 3, Anthony J. Merer 4,5, Robert W. Field 1,‡
1Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2Physikalisch-Chemisches Institut, Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany.
3Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712, USA.
4Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.
5Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
↵‡Corresponding author. E-mail: rwfield{at}mit.edu
↵* Present address: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
↵† Present address: JILA, National Institute of Standards and Technology, and Department of Physics, University of Colorado, Boulder, CO 80309, USA.
Shaking out details of 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.
Abstract
Transition 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.
Received for publication 1 September 2015.
Accepted for publication 29 October 2015.
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