Bonding and vibrations of CO molecules adsorbed at transition metal impurity sites on the MgO (001) surface. A density functional model cluster study

First principles density functional cluster investigations of adsorption at the (001) surface of pure and doped magnesium oxide are carried out to characterize and compare the interaction of CO molecules with main group (Mg²⁺) and d metal (Co²⁺, Ni²⁺, Cu²⁺) surface cationic centers of the ionic substrate. The geometry of the adsorption complexes, the binding mechanism and spectroscopic manifestations of the surface species are analyzed. Special attention is payed to vibrational frequencies and intensities. The calculations qualitatively reproduce observed trends in the adsorption-induced frequency shifts for the series of the surface aggregates Mg_5cCO→Ni_5cCO→CO_5cCO and the corresponding change of the infrared intensities of the CO vibrational mode. For the transition metal impurity sites these results are rationalized in terms of a small, but notable Md_πCOπ interaction.