The adsorption of methyl on Rh10 clusters:: selective C-H bond weakening

The adsorption of methyl on an Rh-10 cluster was studied by ab initio density functional calculations as a model for the Rh(1/1/1) surface. The surface was modeled by a three-layer, 10 atom Rh cluster. The most stable binding site was found to be the threefold site, and the most stable geometry was a methyl species tilted 20 degrees from the surface normal towards the onefold site of the Ph. Crystal orbital overlap populations were calculated and showed that the tilted methyl was energetically preferred because of a bonding interaction between the d states of the Ph and one of the C-H antibonding orbitals in the methyl. Electron donation from the d states to this C-H orbital causes a selective weakening of the bond, possibly explaining experimental vibrational data in which a softened C-H stretch mode is observed. A discussion of the advantages and the limitations of the cluster approximation is also presented. (C) 1999 Elsevier Science B.V. All rights reserved.