EFFECT OF THE MADELUNG POTENTIAL VALUE AND SYMMETRY ON THE ADSORPTIONPROPERTIES OF ADSORBATE OXIDE SYSTEMS/

An ab initio Hartree-Fock self-consistent field cluster model approach has been used to analyze the effect of surface irregularities located at long distances of NO and CO adsorbed on a monocoordinated position of the Cu2O(111) surface. To perform this study different surface clu ster models were used. One of these clusters has C-3v symmetry as in t he perfect surface, In the two other cluster models the adsorption sit e local symmetry is still C-3v but to simulate surface irregularities the array of point charges used to include the Madelung potential has C-s symmetry. The NO/Cu+ interaction is always dominated by the electr ostatic effects without important contributions from charge transfer m echanisms. The symmetry change between these two kinds of models does not practically alter the geometrical and vibrational parameters of th e bond but affects the strength of the NO/Cu2O interaction in a non-ne gligible way. On the other hand the CO/Cu+ interaction is much larger with substantial contributions from sigma-donation and pi-backdonation . In this case the symmetry change does not have noticeable effects. T he present study shows that, on adsorbate/oxide systems with electrost atic bonds, the energy of interaction may depend not only on the local electrostatic field strength felt by the adsorbate but also on the de gree of perfection of the oxide surface. The implications of this resu lt for the analysis of experimental data are discussed.