Origin of the vibrational shift of CO chemisorbed on Pt(111)

Ab initio self-consistent field and complete active space self-consistent field cluster-model wave functions have been obtained for a CO-${\mathrm{Pt}}_4$ cluster model simulating the atop interaction of CO on Pt(111). The origin of the vibrational shift between free and chemisorbed CO has been investigated by means of the constrained space orbital variation method. This analysis shows that the vibrational shift is the result of several effects. First, there is a large positive shift due to Pauli repulsion, and second various negative contributions; these are substrate polarization, σ donation, and π back donation, respectively. This theoretical analysis shows that the mechanism suggested by Blyholder is, in fact, the one responsible for the observed vibrational shift.