FLUORINE SUBSTITUTION EFFECTS ON THE ALKYL COUPLING REACTION ON A AG(111) SURFACE

We have investigated fluorine substitution effects on the rate of coupling of adsorbed alkyl groups on a Ag(111) surface. Alkyl groups are formed by thermal dissociation of the C-I bond in adsorbed alkyl iodides. Variable heating rate temperature programmed reaction (TPR) studies were used to determine the kinetic parameters for the coupling of ethyl groups and propyl groups. They are E(a) = 15.1 +/- 0.6 kcal/mol, nu = 10(16.7+/-0.8) s(-1), and E(a) 16.9 +/- 0.4 kcal/mol, nu = 10(17.1+/-0.4) s(-1), respectively. Substitution of fluorine for hydrogen in the adsorbed alkyl groups systematically raises the coupling reaction temperature. For example, trifluoropropyl groups self-couple at temperatures similar to 70 K higher than propyl groups on Ag(111). Coadsorbed propyl and trifluoropropyl groups cross; couple at temperatures similar to 10 K higher than the propyl self-coupling reaction. The kinetic parameters evaluated from the results of this study and from results of earlier studies by X.-L. Zhou, J. M. White, and co-workers [Surf. Sci. 1989, 219, 294; Catal. Lett. 1989, 2, 375; J. Phys. Chem. 1991, 95, 5575] are used to plot linear free energy relationships (LFER) which provide insight into the electronic nature of the reaction center. The implication of the LFER plots for the surface alkyl coupling reaction is that the reaction center in the transition state is electron deficient with respect to the initial state.