CO INTERACTION WITH ZNO SURFACES - AN MNDO, AM1 AND PM3 THEORETICAL-STUDY WITH LARGE CLUSTER-MODELS

The interaction of a CO molecule with ZnO (0001), (00(0) over bar 1) a nd (1(0) over bar1$ 0) surfaces has been investigated with MNDO, AMI a nd PM3 semiempirical procedures and a (ZnO)(22) cluster model. Optimiz ation of the geometry of adsorbed CO on these surfaces was performed i n order to analyze binding energies, charge transfers, stretching freq uencies and force constants of the different bond sites. Our results i ndicate that there are two main active zinc sites. The interatomic dis tance of the CO molecule decreases when it interacts with the cluster model representation of the surfaces resulting in a charge transfer of 0.17 to 0.58 a.u. from CO to the surface and an increase of CO bond o rder in agreement with ultraviolet photoelectron spectroscopy (UPS) an d X-ray photoelectron spectroscopy (XPS) experimental results. Our res ults are compared with the available experimental data.