Determining Transition State Geometries in Liquids Using 2D-IR Page: 3 of 35
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dissociation of solute-solvent complexes (11-13). These experiments use 2D-IR spectroscopy to
vibrationally tag a specific, localized vibrational mode and monitor its shift in frequency as the system
undergoes chemical dynamics. Here we demonstrate a conceptually different experiment on Fe(CO)5. We
are imparting vibrational energy into a specific but delocalized vibrational mode and monitoring how that
energy is distributed between the delocalized modes as the molecule crosses a transition state.
Quantification of this energy exchange provides direct information on the time-scale, transition state, and
consequently, mechanism of the reaction.
Fe(CO)5 has two IR active vibrational modes involving the stretch of the CO ligands. For the D3h
symmetry of Fe(CO)5, these absorptions correspond to a doubly degenerate e' band and an a2" band,
respectively at 1999 and 2022 cm-1 in n-dodecane solution (Fig. IA). Density functional theory (DFT)
vibrational frequency calculations (14) yield CO displacements for the a2" and e' modes as illustrated in
Fig. 1B. The e' mode involves nearly exclusive vibration of the three equatorial CO groups whereas the a2"
mode involves vibration of the axial CO groups.
In principal, fluxionality can cause the coalescence and collapse of the two IR absorptions into a
single peak, analogous to the coalescence of line shapes observed in NMR spectra. The frequency of
exchange must be comparable to the frequency separation of the absorption bands, which is ca. 1 ps-1 for
these absorptions. The Fourier transform (FT) IR spectrum at 100 C shows some evidence of coalescence
relative to the room temperature spectrum (Fig. IA), but it is unclear if the changes in line shape are the
result of exchange or homogenous broadening (see Fig. Si). The boiling point (103 C) and thermal
instability of Fe(CO)5 prevent acquisition of spectra at sufficiently high temperatures to conclusively
observe IR coalescence. Nevertheless, several examples of this phenomenon have been reported and
attributed to fast exchange (15-17), yet the true physical origin of the effect has been debated in some
instances (18). The 2D-IR spectra reported below, however, show strong evidence for the time-scale of
fluxionality and exchange in room-temperature solution and highlight the advantage of 2D-IR as a more
general method of observing IR exchange, analogous to the advantages of 2D-NMR over conventional
NMR spectroscopy (19).
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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. (https://digital.library.unt.edu/ark:/67531/metadc893986/m1/3/: accessed March 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.