KINETICS AND ENERGETICS OF BETA-HYDRIDE ELIMINATION ON CU(100) - DETERMINING THE COPPER-ALKYL BOND-ENERGY

Recent studies have shown that adsorbed alkyl groups can be generated and isolated on metal surfaces by a variety of methods, including the thermal dissociation of alkyl iodides. For copper surfaces, the primar y reaction pathway is beta-hydride elimination at similar to 250 K to produce the corresponding olefin and adsorbed hydrogen atoms. We show here that, on a Cu(100) surface, 1-10% of the product olefin is rehydr ogenated to the alkyl group. This reversibility has been detected and quantified by deuterium isotope labeling in temperature-programmed rea ction experiments. In addition, partially deuterated ethyl groups have been synthesized on the surface by a novel hydrogenation reaction, an d these selectively deuterated alkyls have been used to measure the de uterium isotope effect for beta-hydride elimination which is 10.3 +/- 0.7 at 255 K. From these results, the enthalpy change for beta-hydride elimination and the metal-ethyl bond energy on Cu(100) have been dete rmined to be 6.5 +/- 4 kcal/mol and 33 +/- 6 kcal/mol, respectively. T hese values are compared with those for alkyl groups on other metal su rfaces, for alkyl groups in metal compounds, and for alkyl groups bond ed to metal atoms in the gas phase.