Exploring Condensed Phase Equilibrium Dynamics via Ultrafast Two-Dimensional Infrared Spectroscopy.

Abstract

Understanding condensed phase equilibrium dynamics is essential to describing and predicting chemical reactions and their outcomes. To gain insight into equilibrium dynamics, ultrafast two-dimensional infrared (2DIR) spectroscopy is applied to metal carbonyl systems in order to observe two dynamic equilibrium processes: picosecond isomerization and intramolecular vibrational energy transfer. The picosecond isomerization between two different isomers of a fluxional metal carbonyl complex, dicobalt octacarbonyl, is characterized using 2DIR spectroscopy. The isomerization times are extracted from the congested spectra using the well characterized coherent modulation –a signature of non-exchanging signals – to isolate the exchange contribution to the signal. From temperature dependent 2DIR spectra, the temperature dependent rate constants are extracted. Analysis of these rate constants through the Arrhenius and Eyring equations enables the extraction of both the activation energy and the entropic contribution associated with the barrier crossing process. Using this well characterized reaction as a probe, the solvent’s influence on a barrier crossing process is systematically investigated. It was found that for a series of linear alkanes, the potential energy surface does not change as a function of the solvent, indicating that any changes in solvent-dependent rate constants are solely due to the dynamic solvent effect. Through a combination of linear FT-IR measurements and quantum and classical computations, the static and dynamic contributions to the rate constant are separated, enabling the first direct test of Kramers theory in the time domain on a picosecond reaction occurring on the ground electronic state. The experimental data show agreement with a simple Markovian Kramers theory for the isomerization rate constant’s dependence on solvent viscosity. Intramolecular vibrational energy redistribution (IVR) in two metal complexes, (Cp)2Fe2(CO)4 and its ruthenium analog, (Cp)2Ru2(CO)4, is also studied via 2DIR spectroscopy. The equilibrium energy transfer dynamics between different vibrational modes of the cis-B (Cp)2Fe2(CO)4 and the gauche-NB (Cp)2Ru2(CO)4 isomers is observed. Treating the energy transfer as an equilibrium process, rate constants associated with both the uphill and downhill transfer of vibrational energy are obtained. It was found that the difference in the rate constants maps to the difference in the energy gap between the two modes involved in IVR.