Applications of 2DIR on Solution Dynamics and Reaction Chemistry.

[摘要] Two-dimensional infrared (2DIR) spectroscopy provides a powerful frameworkwith which to study equilibrium solvation dynamics and non-equilibrium reactionsolvation by directly providing the frequency-frequency correlation function. HereFourier Transform-2DIR is extended to non-equilibrium reactions byimplementing Pump-Probe-2DIR, which provides the frequency correlation of theresulting photoproduct. Home-built infrared and visible light sources enabled theimplementation of these novel experimental methods.The non-equilibrium photocleavage of tungsten hexacarbonyl was studied. Thephotoproduct was found to be vibrationally hot, resulting in transitions observed from thefirst excited manifold to the second manifold and ground state. The 2D spectrum allowedan unambiguous photoproduct peak assignment. The rotational reorientation time wasfound 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 energyredistribution (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 thebath phonon density of states evaluated at the vibrational energy difference of the twostates involved in IVR.The equilibrium solvation dynamics of triruthenium dodecacarbonyl [Ru3(CO)12]was studied. Significantly faster IVR and spectral diffusion were observed relative toother metal carbonyls studied in the laboratory. Spectral diffusion was also observed inrelatively weakly interacting solvents n-hexane and cyclohexane, which has not beenobserved for other metal carbonyls. This was attributed to detecting not the sampling ofmicroscopic solvent environments per se, but instead sampling a loose conformationalspace of the flexible Ru3(CO)12 molecule. For the case of polar, hydrogen bondingsolvents, calculations indicate an increase in solvent disorder causes an increase incarbonyl participation ratio, indicative of increased delocalization. This unusualobservation was attributed to the odd-membered ring symmetry of the metal center,which results in frozen, frustrated carbonyl motions. Together, unfreezing of carbonylmotions and faster IVR and spectral diffusion times were taken to indicate that Ru3(CO)12has a relatively loose solvated equilibrium structure. Calculations of the potentialminimum confirm that multiple conformations lie at nearly equivalent energies, with thehighly symmetric strained structure partially stabilized by ;;carbon bonds.”

[发布机构] University of Michigan

[效力级别] Chemistry

[关键词] 2DIR Spectroscopy;Chemistry;Science;Chemistry