DISTINGUISHING DIRECT AND QUASI-DIRECT MECHANISMS FOR AN ELEY-RIDEAL GAS-SURFACE REACTION - STEREOCHEMISTRY OF H ADDITION TO CYCLOHEXENE ON CU(100)

Previous studies have demonstrated that when H atoms are impinged onto copper surfaces precovered with partial monolayers of physisorbed alkenes at 110 K, addition reactions occur to generate surface alkyl groups. (1,2)Based on the absence of these reactions when alkenes are adsorbed on a copper surface precovered with a partial monolayer of H atoms, it was concluded that these reactions occur by Eley-Rideal mechanisms in which the H reacts with the alkenes prior to thermal accommodation with the surface. The stereochemistry of this Eley-Rideal process has been studied with reference to the addition of hydrogen to cyclohexene on Cu(100). The use of this cyclic alkene eliminates free rotation about the C-C bond in the alkyl product. The stereochemistry of the surface cyclohexyl groups generated by this addition reaction was determined by combining isotope labelling with mass spectrometric detection of cyclohexene that is regenerated as a result of a stereoselective beta-hydride elimination by the surface cyclohexyl groups. To quantify the product yields with high accuracy, a chemical displacement protocol has been applied to remove unreacted alkenes from the monolayer and different isotopic variants of the reaction have been studied to confirm the product distributions. it has been shown that H addition to the C-C double bond (which is oriented parallel to the surface based on IR results) occurs from the top face (vacuum side) of the molecule with > 95% selectivity. This stereoselectivity for the top face is particularly interesting since it is thought that Langmuir-Hinshelwood pathways for analogous processes occur by addition to the bottom face of adsorbed alkenes. Furthermore, since one would expect surface-mediated hot-atom pathways (i.e. quasi-direct Eley-Rideal mechanisms) to show the stereochemistry of the Langmuir-Hinshelwood mechanism, we conclude that H addition to cyclohexene on Cu(100) occurs by a truly direct Eley-Rideal mechanism.