Inductive Substituent Effects:  Metal Surfaces versus the Gas Phase

Abstract

The influence of metal surfaces on inductive (or electrostatic field) substituent effect measurements for surface reactions has been evaluated using image charge theory. Substituent effects have been a valuable tool for determining the electronic properties of transition states for reactions in many environments but have not received much use or consideration on metal surfaces. An important mechanism by which substituents can alter the activation barrier of a reaction is through local dipole field effects on a charged reaction center. To evaluate the influence of a nearby metal surface, substituent field effects have been modeled by considering a point charge and a dipole, both positioned above a conductive surface, that interact electrostatically with their corresponding images at equal distances below the surface. It has been found that the magnitude of substituent effects for reactions on metal surfaces will be approximately equal to the gas-phase value for geometries in which the substituent is farther from the surface than the reaction center and both are above the image plane. This model was used to describe -hydride elimination from ethoxides on Cu(111). This reaction has been found to have a substituent effect that correlates with the reaction energetics of gas-phase alcohol dehydrogenation, a result that is in agreement with the image charge model of the electrostatic influence of the metal.

Transition State for β-Elimination of Hydrogen from Alkoxy Groups on Metal Surfaces

“…A question arises as to the importance of screening by the metal as a mechanism for influencing the magnitude of substituent effects. Electrostatic calculations have been made of the influence of conducting metal surfaces on substituent effects for β-hydrogen elimination from ethoxy groups (where the substituent is relatively far from the surface, >3 Å) . These indicate that the substituent effect on Δ E act is not significantly influenced by the conducting nature of the metal surface.…”

The Transition State for Surface-Catalyzed Dehalogenation: C–I Cleavage on Pd(111)

“…They have been chosen to have a wide range of field effect substituent constants (σ F ) as listed in Table 2. The field effect is an empirical measure of the interaction of the electrostatic field of the substituent with the change in charge that develops at the reaction center in the transition state (14,21,22). For example, if the reaction center in the transition state is anionic with respect to the initial state Table I of Hansch et al (14) and adding 0.07 to place it on the same scale as σ F for other similar substituents (CH 2 F, CHF 2 , CF 3 , and CH 2 CF 3 ).…”