Research Topics

• Chemical Exchange Dynamics
• Dynamics of Water
• Proton Transfer Dynamics
• Protein Dynamics and Biological Water
• Complex Liquids

Experimental Methods

• 2D-IR Echo Spectroscopy
• Polarization Selective Pump-Probe
• Optical Kerr Effect Method
• UV-Vis Transient Absorption and Fluorescence

Chemical Exchange Dynamics – Solute Solvent Complexes and Other Systems

Organic solvents play a substantial role in practical chemistry by influencing the reactivity of dissolved solutes. Specific intermolecular interactions, such as hydrogen bonding, can lead to structurally characterizable solute-solvent complexes that are constantly forming and dissociating on very short time scales under thermal equilibrium conditions. Dynamics of these transient species can play an important role in the physical and chemical properties of a solute-solvent system by affecting reaction rates, reaction mechanisms, and product ratios. For organic solutions, we have shown that solute-solvent complexes form and dissociate on picosecond timescales that cannot be measured by NMR and other methods.

Figure 1 shows a cartoon of an anisotropic solute molecule and a solute-solvent complex. The anisotropic intermolecular interactions can lead to the formation of a well defined complex. The interactions are weak, comparable to the thermal energy in the system. Therefore, the complexes are not long lived. There is equilibrium between the free solute, that is a solute that is not complexed, and the solute-solvent complex. Under thermal equilibrium conditions, complexes are continually dissociating to form free solvent molecules, and free solvent molecular a constantly associating with solvent molecules to form complexes. Because the system is in thermal equilibrium, the numbers of complexes and free solvent molecules are time independent.

The upper left portion of figure 2 shows the spectrum of the hydroxyl stretch of phenol-OD (OH hydroxyl replaced with OD) in the mixed solvent of benzene and CCl₄ (29 mol % benzene, 71 mol % CCl₄). Phenol forms a p hydrogen bonding complex with benzene. Phenol-OD is used to shift the hydroxyl stretch absorption below the C-H stretch frequencies. The mixed solvent of benzene/CCl4 is used to shift the equilibrium so that the two peaks are about the same amplitude. The structure of free phenol and the phenol-benzene complex is shown in the figure to the upper right. The spectrum shows that the two species coexist, but the spectrum cannot yield information on the time dependence of the dissociation and formation of the complexes.

The lower left portion of figure 2 shows a 2D IR vibrational echo chemical exchange spectrum taken at a time short compared to the chemical exchange. The 2D IR vibrational echo method is akin to 2D NMR but operates on times scales 9 orders of magnitude faster and directly probes the structural degrees of freedom of molecular systems. The method is discussed below in the Experimental Methods section. The two bands on the diagonal (dashed line) arise from the two peaks in the absorption spectrum. As discussed below, the red bands are positive going and correspond to vibrational echo emission at the 0-1 vibrational transition frequency. The two blue bands (negative going) below the diagonal arise from vibrational echo emission at the 1-2 transition frequency. These peaks are shifted along the wm axis by the vibrational anharmonicity of the OD hydroxyl stretching potential. The bottom right side of figure 2 shows the 2D IR spectrum at a time that is sufficiently long for a substantial amount of chemical exchange to have occurred, that is, phenol-benzene complexes have dissociated to form free phenol and free phenol has formed complexes with benzene. Off-diagonal peaks have grown in. The off-diagonal peaks arise from the chemical exchange. The peaks on the diagonal correspond to species that have not changed their character. The important point is that the time dependent growth of the off-diagonal peaks yields direct information on the rate of chemical exchange. The experiment effectively labels the molecules at zero time and then watches them interconvert as time progresses. The experimental method does not perturb the thermal equilibrium of the system.

By measuring the growth of the off-diagonal peaks and the decay of the diagonal peaks and using the proper data analysis, the chemical exchange rate can be readily determined. Because the rate of complex dissociation is equal to the rate of complex formation, the dynamics can be characterized by the single parameter, the dissociation rate td. All of the necessary parameters to fit the data are know except for td. Figure 3 shows the data and the fits for the phenol-benzene system. The single adjustable parameter, t_d = 10 ps.

Using ultrafast 2D IR vibrational echo chemical exchange spectroscopy we have measured a variety of solute-solvent complex dynamics for alcohols, silanols, and C-H O complexes of, for example, chloroform with acetone and DMSO. Using the same method have also made the first measurements of rotational isomerization around a carbon-carbon single bond in a room temperature liquid and the structural switch between well defined substates of a protein. The following is a list of publications describing our work on chemical exchange dynamics of molecular systems.

Ultrafast 2D IR Vibrational Echo Chemical Exchange Spectroscopy Publications

>> Next: Dynamics of Water

[ Top | Previous: Research Overview ]

[ Home | Research Overview | Group Members | Quantum Mechanics Book | Publications ]