Inductive substituent effects: Metal surfaces versus the gas phase

The influence of metal surfaces on inductive (or electrostatic field) subst ituent effect measurements for surface reactions has been evaluated using i mage charge theory. Substituent effects have been a valuable tool for deter mining the electronic properties of transition states fur reactions in many environments but have not received much use or consideration on metal surf aces. An important mechanism by which substituents can alter the activation barrier of a reaction is through local dipole field effects on a charged r eaction center. To evaluate the influence of a nearby metal surface, substi tuent field effects have been modeled by considering a point charge and a d ipole, both positioned above a conductive surface, that interact electrosta tically with their corresponding images at equal distances below the surfac e. It has been found that the magnitude of substituent effects for reaction s 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 re action center and both are above the image plane. This model was used to de scribe beta-hydride elimination from ethoxides on Cu(111). This reaction ha s been found to have a substituent effect that con elates with the reaction energetics of gas-phase alcohol dehydrogenation, a result that is in agree ment with the image charge model of the electrostatic influence of the meta l.