Can the three-coordinated Mg site of MgO accommodate more than one CO molecule?

A new mechanism of CO adsorption at the three-coordinated (3c) Mg sites of MgO is proposed on the basis of a quantum chemical study of the phenomenon of gradual diminution of the 2202 cm(-1) band of the OC Mg-3c species upon increase of the CO pressure above 10(-1) Torr [Coluccia, S., et al. Spectro chim. Acta 1993, 49A, 1289]. The MgO surface is modeled by bare Mg10O10 and Ng(16)O(16) clusters at the HF and DFT (B3LYP) levels of theory. We examin e the hypotheses that the intensity decrease may be due to (i) the static d ipole-dipole or/and inductive interactions with the CO molecules at the nei ghboring Mg-4c sites and (ii) the coadsorption of two or three CO molecules at the Mg-3c sites. The computed frequency shifts and binding energies sug gest that the discussed phenomenon is due to the coadsorption of two CO mol ecules at the Mg-3c sites.