ON THE BONDING MECHANISM OF CO TO PT(111) AND ITS EFFECT ON THE VIBRATIONAL FREQUENCY OF CHEMISORBED CO

The chemisorption of CO on the atop site of Pt(111) has been simulated by a Pt-4 cluster model. Ab initio self consistent held (SCF) and com plete active space self consistent field (CASSCF) cluster model wave f unctions have been obtained for the electronic ground state. Likewise, ab initio SCF wavefunctions have been obtained for two other electron ic states. The optimum geometry and vibrational frequencies of chemiso rbed CO are reported for the three states. The interaction energy and vibrational shift of chemisorbed CO, with respect to free gas phase CO , have been analyzed for the three electronic states. This analysis is carried out by means of the constrained space orbital variation (CSOV ) method. In all cases the bond is found to be dominated by sigma dona tion and pi back-donation, known as Blyholder's mechanism. This mechan ism is further supported by SCF calculations on a larger, Pt-13, clust er model. For both clusters, the CSOV analysis of the vibrational freq uency definitely shows that, contrary to previous recent studies, a ma jor contribution to the experimentally observed vibrational shift come s from the pi back-donation mechanism. However, we found that, contrar y to common belief, sigma donation also acts to lower the CO frequency and not to increase it. Physical reasons for such unexpected behaviou r are given. (C) 1997 Elsevier Science B.V.