Fayer Lab: Ultrafast Chemical Exchange Spectroscopy

Ultrafast Chemical Exchange Spectroscopy
Junrong Zheng and Kyungwon Kwak

Introduction: Chemistry involves the structural evolution of molecular systems. To obtain an understanding of chemical dynamics, direct measurements on the appropriate time scales are necessary. Many methods for examining chemical reaction and structural changes involve perturbing a system away from its equilibrium conditions and then observing its approach to a new equilibrium.

However, multidimensional NMR can be used to examine chemical dynamics under thermal equilibrium conditions, but only on slow time scales (ms or longer). These NMR measurements directly observe chemical exchange.
We have experimentally and theoretically demonstrated 2D IR vibrational echo spectroscopy can be used to observe chemical exchange on ultrafast time scales, down to 10-13 s under thermal equilibrium conditions. The first experiments examine the association and dissociation of phenol-benzene complexes in liquid solution. The phenol-benzene complex is in thermal equilibrium with free phenol. The association and dissociation are very rapid. The results are illustrated in Figs. 1 and 2.

Methodology: The vibrational echo method uses 3 ultrafast IR pulses to cause the generation of a fourth pulses, the vibrational echo, which emerges from the sample in a unique direction. The first 2 pulses label the original structures of the molecules. After a period of waiting for a time, Tw, the 3rd pulse is applied, which is followed by the vibrational echo emission. Chemical exchange, the changing of molecular structure that influences a vibrational frequency during the period Tw can be monitored by the growth of the cross peaks in the 2D spectrum (see Figs. 1 and 2). The merit of this method is that the ultrafast kinetic measurements can be conducted under thermal equilibrium conditions. The experimental setup is shown in figure 3.

Research: In addition to solute-solvent interactions and dynamics, the vibrational echo chemical exchange approach can be applied to a wide variety of problems. We have been begun to study isomerization, proton transfer, electron transfer. How solvents interact with solutes, how energy transfers from one molecule to another, and what governs the interactions, are main focuses. Experiments are aimed at these issues in both liquids and supercritical fluids. We are also interested in reaction dynamics in which a system passes through a transition state but under thermal equilibrium conditions for the reaction. As one of the most basic chemical reactions, fast proton transfer (from a hydrogen bonded species to an ion pair) can be directly studied using chemical exchange 2D vibrational echo spectroscopy. Ground state electron transfer driven by thermal equilibrium fluctuations can also be investigated. In addition, ultrafast 2D vibrational echo spectroscopy has many other applications that involve the observation of the changing shapes or positions of the 2D vibrational echo spectral peaks rather than the appearance of cross peaks in the spectrum.

“Ultrafast Solute-Solvent Complex Chemical Exchange Observed in Real Time: Multidimensional Vibrational Echo Correlation Spectroscopy,” Junrong Zheng, Kyungwon Kwak, John Asbury, Xin Chen, I. Piletic, and M. D. Fayer, Science 309, 1338-1343 (2005). pdf

“Dynamics of Water Probed with Vibrational Echo Correlation Spectroscopy,” John B. Asbury, Tobias Steinel, Kyungwon Kwak, S. A. Corcelli, C. P. Lawrence, J. L. Skinner, and M. D. Fayer, J. Chem. Phys. 121, 12431-12446 (2004). pdf

“Vibrational Echo Correlation Spectroscopy Probes of Hydrogen Bond Dynamics in Water and Methanol,” John B. Asbury, Tobias Steinel, and M. D. Fayer, J. Lumin. 107, 271-286 (2004). pdf

“Using Ultrafast Infrared Multidimensional Correlation Spectroscopy to Aid in Vibrational Spectral Peak Assignments,” John B. Asbury, Tobias Steinel, M. D. Fayer, Chem. Phys. Lett. 381, 139-146 (2003). pdf