title: Spectroscopic characterization of isomerization transition states.
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, ω(eff). 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.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Joshua H
authors.initials: JH
authors.lastname: Baraban
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: P Bryan
authors.initials: PB
authors.lastname: Changala
authors.affiliation: Physikalisch-Chemisches Institut, Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany.
authors.forename: Georg Ch
authors.initials: GCh
authors.lastname: Mellau
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas, Austin, TX 78712, USA.
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton
authors.affiliation: Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Merer
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. rwfield@mit.edu.
authors.forename: Robert W
authors.initials: RW
authors.lastname: Field