A. Paul et Aj. Gellman, FLUORINE SUBSTITUTION EFFECTS ON THE ALKYL COUPLING REACTION ON A AG(111) SURFACE, Journal of the American Chemical Society, 117(35), 1995, pp. 9056-9066
We have investigated fluorine substitution effects on the rate of coup
ling of adsorbed alkyl groups on a Ag(111) surface. Alkyl groups are f
ormed by thermal dissociation of the C-I bond in adsorbed alkyl iodide
s. Variable heating rate temperature programmed reaction (TPR) studies
were used to determine the kinetic parameters for the coupling of eth
yl 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 t
he adsorbed alkyl groups systematically raises the coupling reaction t
emperature. For example, trifluoropropyl groups self-couple at tempera
tures similar to 70 K higher than propyl groups on Ag(111). Coadsorbed
propyl and trifluoropropyl groups cross; couple at temperatures simil
ar to 10 K higher than the propyl self-coupling reaction. The kinetic
parameters evaluated from the results of this study and from results o
f earlier studies by X.-L. Zhou, J. M. White, and co-workers [Surf. Sc
i. 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 defic
ient with respect to the initial state.