GEOMETRY AND BINDING OF ACETYLENE ON CU(111) - AB-INITIO CLUSTER STUDIES

Recent photoelectron diffraction (PED) data show that the adsorption of C2H2 on Cu(111) is combined with major structural changes in the adsorbate while the overall adsorbate-substrate binding is weak. These experimental findings can be explained by details of the C2H2/Cu(111) chemisorption interaction as shown in the present cluster model calculations based on ab initio Hartree-Fock and correlated wave functions. Extended studies on a small Cu7C2H2 cluster confirm that C2H2 stabilizes with its C-C axis parallel to the Cu(111) surface over a bridge site where the two C centers point towards adjacent 3-fold hollow sites as suggested by the PED data. In the calculations the optimized C-C distance of adsorbed C2H2 is increased by 0.16 Angstrom with respect to that of the free molecule which is close to the experimental increase (0.28 +/- 0.10 Angstrom). Further, in the cluster model the C-H axes are found to tilt by 60 degrees with respect to the C-C axis pointing away from the surface (hydrogen positions could not be obtained from PED). As a result, the overall weak C2H2-Cu(111) interaction is determined by a competition between energy required to change the geometry in the adsorbate molecule and energy gained due to local bond formation of the distorted molecule. The latter contribution can be connected with binding mechanisms which are well known from organometallic chemistry. Finally, the present model results suggest strongly that correlation contributions to binding are necessary for a correct evaluation of the energetics of the C2H2/Cu(111) system.