AVS 52nd International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThA

Paper SS2-ThA2
Experimental and Computational Probes of Transition States on Surfaces

Thursday, November 3, 2005, 2:20 pm, Room 203

Session: Gas-Surface Reaction Dynamics
Presenter: A.J. Gellman, Carnegie Mellon University
Authors: A.J. Gellman, Carnegie Mellon University
D.S. Sholl, Carnegie Mellon University
P.P. Ye, Carnegie Mellon University
X. Li, Carnegie Mellon University
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Transition states determine the rates of surface catalyzed reactions, however, direct characterization of surface transition states is challenging. Experimental measurements of substituent effects on reaction barriers have been combined with Density Functional Theory to probe the transition states to the @beta@-hydride elimination in ethoxy groups on Cu(111) and the hydrogenation of alkyl groups on Pt(111). Our results allow us to test the recently advanced proposition that surface reactions with reactant-like (product-like) transition states are relatively sensitive (insensitive) to the nature of the catalyst surface. The barrier to @beta@-hydride elimination in fluorine substituted ethoxy groups increases significantly as a result of fluorine substitution. This is ascribed to a transition state in which the @beta@-carbon atom is cationic with respect to the initial state ethoxy group. Comparison of the experimental values of @Delta@E@sub act@ and the predictions of DFT shows impressive agreement and verifies the predictions based on previous experimental measurements. Substituent effects have been used to probe the characteristics of the transition states to hydrogenation of alkyl groups on the Pt(111) surface and the transition state to @beta@-hydride elimination in alkyl groups on the Pt(111) surface. Co-adsorption of hydrogen with alkyl groups and subsequent heating of the surface results in hydrogenation of the alkyl groups to alkanes which desorb into the gas phase. The influence of the substituents on the activation barriers to hydrogenation has been correlated with the field and polarizibility substituent constants of the alkyl groups in the form of a linear free energy relationship. This indicates that the @alpha@-carbon in the transition state is cationic with respect to the initial state alkyl group and that the reactant has greater charge density on the @alpha@-carbon than the transition state.