Analyses for the C-O stretch frequency shifts of CO chemisorbed on Cu(100) in the absence and presence of an electric field

We have performed cluster model calculations for CO chemisorbed in an on top and assumed bridge site of Cu(100). Detailed analyses are presented for the contributions of the local chemical bonding to the shifts of the C-O stretch frequency upon chemisorption. The repulsion due to the overlap of the free CO and metal charge distributions leads to an increase in ωe. The charge redistributions due to the interaction lead to decreases in ωe which offset the increase due to the repulsion; the major contribution arises from the metal to CO π back-donation. When an electric field is applied, ωe shifts from its value in the absence of a field. Strong evidence is presented which shows that this shift is dominantly a Stark effect which arises from the interaction of the field with the dipole moment of the unperturbed, field free, charge distribution. Field induced changes in the charge distribution give rise to partially cancelling effects so that their net contribution may be quite small. © 1986.