Applications of 2DIR on Solution Dynamics and Reaction Chemistry.

Abstract

Two-dimensional infrared (2DIR) spectroscopy provides a powerful framework with which to study equilibrium solvation dynamics and non-equilibrium reaction solvation by directly providing the frequency-frequency correlation function. Here Fourier Transform-2DIR is extended to non-equilibrium reactions by implementing Pump-Probe-2DIR, which provides the frequency correlation of the resulting photoproduct. Home-built infrared and visible light sources enabled the implementation of these novel experimental methods. The non-equilibrium photocleavage of tungsten hexacarbonyl was studied. The photoproduct was found to be vibrationally hot, resulting in transitions observed from the first excited manifold to the second manifold and ground state. The 2D spectrum allowed an unambiguous photoproduct peak assignment. The rotational reorientation time was found to depend on visible-pump/2DIR-probe delay, relaxing in at most 60 ps. Moreover, an additional peak was found that explicitly requires intramolecular vibrational energy redistribution (IVR) during the 2DIR waiting time. The IVR was found to slow from ~200 fs to ~2 ps as the pump-probe delay increased, consistent with calculations of the bath phonon density of states evaluated at the vibrational energy difference of the two states involved in IVR. The equilibrium solvation dynamics of triruthenium dodecacarbonyl [Ru3(CO)12] was studied. Significantly faster IVR and spectral diffusion were observed relative to other metal carbonyls studied in the laboratory. Spectral diffusion was also observed in relatively weakly interacting solvents n-hexane and cyclohexane, which has not been observed for other metal carbonyls. This was attributed to detecting not the sampling of microscopic solvent environments per se, but instead sampling a loose conformational space of the flexible Ru3(CO)12 molecule. For the case of polar, hydrogen bonding solvents, calculations indicate an increase in solvent disorder causes an increase in carbonyl participation ratio, indicative of increased delocalization. This unusual observation was attributed to the odd-membered ring symmetry of the metal center, which results in frozen, frustrated carbonyl motions. Together, unfreezing of carbonyl motions and faster IVR and spectral diffusion times were taken to indicate that Ru3(CO)12 has a relatively loose solvated equilibrium structure. Calculations of the potential minimum confirm that multiple conformations lie at nearly equivalent energies, with the highly symmetric strained structure partially stabilized by “carbon bonds.”