Electric field effects on vibrational frequency and bonding mechanism of CO on Pt(111)

In this work the ab initio cluster model approach has been used to describe electric field effects on the on-top interaction of CO with a Pt(111) surface. The effect of a uniform external electric field in the chemisorbed CO geometry and frequency can be understood by using the concept of 'surface wall'. The extent to which an electric field induces chemical changes has been investigated by means of a projection technique. Results from this technique clearly show that, in the absence of an electric field, the sigma-donation and pi-backdonation mechanisms contribute to the chemical bond. When the electric field is explicitly taken into account the projection of 50 and 2 pi* orbitals exhibits substantial changes. In addition, the changes are precisely in the direction that would be predicted according to electronic density response to the electric field. This set of results permits the assessment that electric field effects do in fact change the chemical bonding of CO on Pt(111). (C) 1998 Elsevier Science Ltd. All rights reserved.