Tuesday, January 24, 2012
4:00 pm
147 Noyes, J. Holmes Sturdivant Lecture Hall
Chemical Physics Seminar
Series:Chemical Physics Seminar
Dynamics of Liquids and Interfaces Studied with Ultrafast 2D IR Vibrational Echo Spectroscopy
Michael D. Fayer, David Mulvane Ehrsam and Edward Curtis Franklin Professor of Chemistry, Department of Chemistry, Stanford University
Catalysts are important throughout chemistry. Heterogeneous catalysts, in which a
surface is functionalized with the catalyst, are particularly useful. A heterogeneous catalyst
makes it a different phase than the reactants in solution, simplifying separation and preventing
leaching of the catalyst into the reaction mixture. Until now, it has not been possible to study the
structural and environmental dynamics of heterogeneous catalysts, and in general the dynamics
of molecules at surfaces and interfaces. Here ultrafast 2D IR vibrational echo experiments on a
submonolayer of a heterogeneous catalyst are presented. To introduce the 2D IR vibrational
echo technique, first two other problems are discussed. Vibrational echo chemical exchange
spectroscopy is used to directly measure the dissociation and formation of organic solute-solvent
complexes in solution. Then experiments that determine the hydrogen bond dynamics of water
are discussed. The water experiments use spectral diffusion as the observable. Then the 2D IR
vibrational echo experiments that measure spectral diffusion of phenanthroline Rhenium
tricarbonyl chloride ((phen)Re(CO)3Cl) bound with a linker to an SiO2 surface are presented.
The very small number of molecules in a submonolayer presents a severe experimental
challenge. The heterogeneous photocatalyst (phen)Re(CO)3Cl reduces CO2 to CO.
(phen)Re(CO)3Cl bound to the SiO2 solvent in air (dry, no solvent) shows structural dynamics on
the 150 ps time scale. Thus, even in the absence of solvent, there are significant rapid molecular
motions of the catalytic monolayer. When the identical surface sample is immerse in chloroform
(wet), the dynamics speed up to 50 ps. These values are compared to (phen)Re(CO)3Cl as a
homogeneous catalyst dissolved in chloroform where the dynamics occur on the 5 ps time scale.
To extract the structural spectral diffusion dynamics, they must be separated from vibrational
excitation transport induced spectral diffusion. The nature of the excitation transport problem is
described. These studies are the first 2D IR vibrational echo experiments of any surface or
interfacial molecular system. They open a new approach for understanding the nature of and
processes involving surface bound and interfacial molecular systems.
Contact Priscilla Boon pjboon@caltech.edu at 6524
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