Theoretical analysis of the vibrational shifts of CO chemisorbed on Pd(100)

Abstract

Vibrational modes of CO bridge bonded to the Pd(100) surface are analyzed by means of ab initio cluster wavefunctions. The dependence of the C-O vibrational mode on the height of the CO ligand from the metal surface has been examined. It is shown that the C-O stretch frequency, ω, initially decreases as CO approaches the surface; this is due to the metal to CO 2π* back donation. However, when the metal-CO bond length becomes sufficiently short, the frequency increases. The stretching of the CO molecule against the surface "wall" leads to an increase in the frequency which over-compensates the decrease due to the π back donation. Vibrational ω shifts induced by an uniform electric field, F, have been determined. The computed tuning rate, dω dF = 1.72 × 10-6 ( V cm), is in excellent agreement with the experimental value, 1.55 × 10-6 cm-1 ( V,cm). The Stark tuning rate is not altered significantly by the electronic rearrangement induced by the field; the shifts in ω arise primarily from the field-dipole electrostatic interaction while the chemical effects are small. Although the consequences of the electronic rearrangements which are caused by the applied field F, are small for the change in ω, these chemical effects lead to large changes in the intensity of the infrared, IR, transition. There is a large "ḩemical" change in the dynamic dipole moment, e*, when a uniform electric field is applied which leads to about 20% change in the IR intensity; this is a major origin for the observed intensity change. However, we give evidence that these chemical changes may be within the CO unit and may not involve changes of the charge transfer or dative bonding between the metal and CO. © 1990.