Determining Transition State Geometries in Liquids Using 2D-IR Page: 1 of 35
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Determining transition state geometries in liquids using 2D-IR
James F. Cahoon, Karma R. Sawyer,* Jacob P. Schlegel,* and Charles B. Harrist
Many properties of chemical reactions are determined by the transition state connecting reactant
and product, yet it is difficult to directly measure any information on these short-lived structures in
liquids. We show that two dimensional infrared (2D-IR) spectroscopy provides direct information on
transition states by tracking the transformation of vibrational modes as a molecule crosses a
transition state. We successfully monitored a simple chemical reaction, the fluxional rearrangement
of Fe(CO)5; the exchange of axial and equatorial CO ligands causes an exchange of vibrational
energy between the normal modes of the molecule. This energy transfer provides direct evidence for
the time-scale, transition state, and mechanism of the reaction.
One-sentence summary: Ultrafast two-dimensional infrared spectroscopy is used to determine the time-
scale and transition state of a simple chemical reaction, the exchange of carbonyls in Fe(CO)5.
Transition states determine many properties of chemical reactions, yet there are limited experimental
methods to directly probe the details of transition state geometries and symmetries in liquids. Molecular
structures primarily reflect local minima on a potential energy surface and chemical reactions occur only
during brief excursions up and over transition states on these surfaces. The excursions are short-lived,
greatly complicating any measure of their properties. Most methods with sufficient time resolution rely on
photoinitiation of a chemical reaction and provide information on the intermediates formed during the
course of the reaction rather than direct information on transition state structures (1).
Here, we demonstrate that ultrafast two dimensional infrared (2D-IR) spectroscopy provides evidence
for the transition state involved in a simple thermal chemical reaction, the fluxionality of Fe(CO)5.
Fluxionality refers to the rearrangement of a molecule between chemically indistinguishable structures.
These reactions produce no net change in molecular structure, yet they are important for understanding the
basic chemical behavior and reactivity of molecules in solution.
Department of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences
Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720
* These authors contributed equally and are listed alphabetically
t corresponding author email address: firstname.lastname@example.org
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Harris, Charles; Cahoon, James F.; Sawyer, Karma R.; Schlegel, Jacob P. & Harris, Charles B. Determining Transition State Geometries in Liquids Using 2D-IR, article, December 11, 2007; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc893986/m1/1/: accessed October 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.