Site-dependent binding of methoxy on Cu(111): Cluster model studies

Abstract

Model calculations on CunOCH3 clusters with n ranging from 1 to 25 are carried out using ab initio Hartree–Fock and configuration interaction (CI) methods. The Cun clusters are chosen to simulate the substrate near the (111) surface and various adsorbate geometries modeling different lateral adsorption sites (top, bridge, fcc hollow, hcp hollow) at the Cu(111) surface are considered. A comparison of the different adsorption sites identifies the face-centered-cubic (fcc) hollow site as energetically preferred for OCH3 binding (D=2.8 eV at the CI level) in agreement with recent experimental results. Correlation contributions account for about 0.9 eV of the binding energies and do not depend on the binding site nor on cluster size. The OCH3–Cu interaction is characterized by metal to methoxy charge transfer filling partially occupied 2e (O 2p lone pair) radical orbitals. This leads to a negatively charged adsorbate such that ionic binding contributions become important. The binding scheme is qualitatively similar to that of other lone pair radical adsorbates like OH and influences the results of adsorbate core and valence ionization in the clusters. The present model results are consistent with experimental data concerning adsorbate geometries and binding as well as photoemission.