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

Recent photoelectron diffraction (PED) data show that the adsorption o f C2H2 on Cu(111) is combined with major structural changes in the ads orbate while the overall adsorbate-substrate binding is weak. These ex perimental findings can be explained by details of the C2H2/Cu(111) ch emisorption interaction as shown in the present cluster model calculat ions based on ab initio Hartree-Fock and correlated wave functions. Ex tended 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 dista nce of adsorbed C2H2 is increased by 0.16 Angstrom with respect to tha t 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 t he distorted molecule. The latter contribution can be connected with b inding mechanisms which are well known from organometallic chemistry. Finally, the present model results suggest strongly that correlation c ontributions to binding are necessary for a correct evaluation of the energetics of the C2H2/Cu(111) system.