Spectroscopic characterization of isomerization transition states

• Published in: Science (American Association for the Advancement of Science) Vol. 350; no. 6266; pp. 1338 - 1342
• Main Authors: Baraban, Joshua H., Changala, P. Bryan, Mellau, Georg Ch, Stanton, John F., Merer, Anthony J., Field, Robert W.
• Format: Journal Article
• Language: English
• Published: United States American Association for the Advancement of Science 11.12.2015; The American Association for the Advancement of Science; AAAS
• Subjects: Barriers; Chemical reactions; Chemistry; Dynamical systems; Dynamics; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Isomerization; Mathematical analysis; Reagents; Spectra; Spectroscopy; Transitions
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.aac9668

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.