Determining Transition-State Geometries in Liquids Using 2D-IR

Abstract

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 † corresponding author email address: cbharris@berkeley.edu 2 Fe(CO) 5 , an organometallic complex with five CO ligands arranged in a trigonal bipyramidal geometry, is a textbook example for fluxionality. In the late 1950s, nuclear magnetic resonance (NMR) spectroscopy revealed that this molecule rapidly exchanges its CO ligands between axial and equatorial sites. The 13 C NMR spectrum of 13 C labeled Fe(CO) 5 exhibits only a single peak at all accessibly measured solution temperatures, indicating that 13 C nuclei shift between axial and equatorial positions faster than NMR spectroscopy is able to distinguish these two chemical environments (2)(3)(4). Careful analysis of IR, Raman, and NMR spectra of Fe(CO) 5 and various derivatives suggests that the exchange process possesses a low barrier and occurs on a time-scale of picoseconds (4)(5)(6). Nevertheless, the dynamics in solution have not been quantified.From a general perspective, fluxional processes are simple chemical reactions in which a molecule briefly rearranges to a new symmetry and geometry as it crosses a transition state and then returns to its original geometry as it completes the reaction. We show for Fe(CO) 5 that during this process energy is exchanged between the vibrational modes of the reactive ligands. Quantification of this energy exchange provides direct information on the time-scale, transition state, and consequently, mechanism of the reaction.2D-IR spectroscopy has recently received much attention for its ability to monitor thermal reactions and chemical exchange on the femtosecond and picosecond time-scales. Conventionally, ultrafast timing of chemical reactions is achieved by photoinitiating the reactions with a short, intense laser pulse which electronically excites the molecules and typically leaves them with significant excess energy (1). In comparison, 2D-IR spectroscopy only requires vibrational excitation with an ultrafast IR laser pulse and allows the investigation of an entirely different class of thermally activated reactions in liquids at or close to equilibrium. Although similar to one-dimensional IR-pump, IR-probe experiments (1D-IR), 2D-IR spectroscopy separates the contributions to the 1D-IR spectrum into two frequency dimensions, which provides information on the correlations, anharmonicities, and exchange dynamics...

The Study of Reactive Intermediates in Condensed Phases

“…[80][81][82][83][84] 2DIR has also been used to observe conformational exchange in fluxional molecules such as Fe(CO)5, 85 or chemical exchange in solution, 86 processes which are too fast to be followed by NMR. Fig.…”

Waveguide-enhanced 2D-IR spectroscopy in the gas phase

“…Iron (0) pentacarbonyl, Fe(CO)5, provides an ideal demonstration of the present technique as it has significant vapor pressure (21 Torr at 293 K), intense absorptions in the 2000 cm -1 region of the mid-IR spectrum and because there are previous solution-phase studies with which to compare [17]. Experimental 2D-IR spectra of Fe(CO)5 vapor were taken using a double-resonance approach [18], with a narrowband (~ 10 cm -1 , 2 ps duration) pump pulse and a broadband (~ 300 cm -1 , 50 fs duration) probe pulse.…”

Communication: Probing non-equilibrium vibrational relaxation pathways of highly excited C≡N stretching modes following ultrafast back-electron transfer

“…[7][8][9][10][11][12] Recently, transient two-dimensional infrared spectroscopy has been used to probe vibrational dynamics following electronic excitation in chemical, biological, and material systems. [13][14][15][16][17][18][19][20] Nonlinear IR probes are ideally suited to study energy transfer among coupled vibrational modes in ultrafast charge transfer processes. Cyano-bridged transition metal mixed valence complexes serve as very interesting model systems for probing relaxation dynamics among highly excited vibrational modes populated in <200 fs by ultrafast backelectron transfer (BET).…”