The adsorption of methyl on Rh-10 clusters: selective C-H bond weakening

The adsorption of methyl on an Rh-10 cluster was studied by ab initio densi ty functional calculations as a model for the Rh(1/1/1) surface. The surfac e 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 onef old site of the Ph. Crystal orbital overlap populations were calculated and showed that the tilted methyl was energetically preferred because of a bon ding interaction between the d states of the Ph and one of the C-H antibond ing orbitals in the methyl. Electron donation from the d states to this C-H orbital causes a selective weakening of the bond, possibly explaining expe rimental vibrational data in which a softened C-H stretch mode is observed. A discussion of the advantages and the limitations of the cluster approxim ation is also presented. (C) 1999 Elsevier Science B.V. All rights reserved .