Recent Research Developments

Dec 25, 2002
 

Watching the solvent participate in chemistry in real time

It has long been appreciated that the solvent molecules that make up the local environment of a chemical reaction play an integral role in the transformation from reactant to product. During the fleeting moments as a reactant attempts to cross through the transition state (TS) between reactant and product the coherent motions of the molecular solvent can both promote and hinder its progress. Recent advances in ultrafast (tens of femtoseconds) spectroscopy has allowed direct monitoring of the rate at which a reactant passes through the TS. For example, when exciting 1-chloroacetylaminoanthraquinone (CAAQ) in dichloromethane with light at 400 nm it will undergo a proton transfer reaction with the passage from the normal form to the tautomer form taking place in ~200 fs (Neuwahl et. al., Phys. Chem. Chem. Phys., 2001, 3, 1277), see the figure for structures. In nonreactive systems it is common to infer the dynamics of the solvent molecules from the time dependent effect those motions have on the spectroscopic properties of a reporter solute molecule. However, in the CAAQ proton transfer system it is very difficult to infer the participation of the solvent based on changes in the resonant spectroscopic properties of the reactant and product due to the substantial changes in these properties that result from the reaction itself.

In order to investigate the participation of the molecular solvent motions in reactive systems in solution, at the University of Minnesota we have developed a new ultrafast nonlinear spectroscopic method that allows us to directly probe the solvent motions as a dynamics event takes place in solution. Our approach uses third-order nonresonant time domain Raman spectroscopy as a direct probe of the local molecular environment following a resonant laser pulse that initiates the dynamic event. We call this spectroscopy RaPTORS, Resonant Pump Third Order Raman probe Spectroscopy. The sequence of laser pulses are shown in the figure along with the results of this experiment for the CAAQ proton transfer system. The 2D plot of the data has the time after we initiate the proton transfer, t, along the vertical axis, and along the horizontal axis is the time variable that represents the nuclear motions of the solvent molecules, tau. Note that the data represents a difference spectrum with and without the resonant initiation pulse. This means that the nuclear solvent motions that appear in the spectrum are only those that coherently participate in the reaction. Negative values show an increase in the polarizability of a given nuclear motion and positive values show a decrease in the polarizability of a nuclear motion. The negative feature in blue around (t=100 fs, tau=280 fs) we interpret as the gain in rotational inertia of those solvent molecules that are reorienting their dipoles in concert with the transfer of the proton. The positive feature in red around (t=250 fs, tau = 100 fs) shows a decrease in polarizability for higher frequency, faster, solvent nuclear motions. We are not yet certain about the interpretation of these motions, however they may represent the friction that develops as some trajectories return along the back reaction to the normal form in an initial move toward equilibrium in the excited state. The nonresonant nature of our probe will allow us to extend this method to any dynamic event in solution that can be initiated with a pulse of light.

This work was done by postdoc Dr. David Underwood and graduate student Sarah Schmidtke in Professor David Blank's research group. We gratefully acknowledge financial support from NSF, The Petroleum Research Fund, The Camille and Henry Dreyfus Foundation, 3M, and the resources provided by the University of Minnesota Supercomputing Institute.