GEOMETRY OF ACETYLENE AND ETHYLENE ADSORBED ON CU(111) - THEORETICAL CLUSTER STUDIES

Recent experiments on acetylene and ethylene adsorbed at metal surface s have shown that the adsorbates undergo major changes in their geomet ric structures while the overall adsorbate-substrate binding is weak. These findings can be explained by competitive adsorbate-substrate bin ding as evidenced in ab initio DFT and Hartree-Fock cluster studies us ing Cu-7(4,3)Ad, Ad = C2H2, C2H4, clusters to simulate adsorption on C u(111). The calculations confirm the increased C-C distances of both a dsorbates found in the experiment and predict bending of the adsorbate CH and CH2 ends respectively near the surface which has not been obse rved so far. The distortion of the adsorbates is connected with rehybr idization resulting in C-C bond weakening and increased adsorbate-subs trate coupling. The latter can be characterized for both adsorbates by a Dewar-Chatt-Duncanson type donation scheme which is well known from organometallic chemistry.