Bonding and vibration of CO molecules adsorbed on low-coordinated surface sites of MgO: a LCGTO-LDF cluster investigation

The adsorption of CO molecules on low-coordinated edge and corner cationic centers of a MgO surface is investigated theoretically by the LCGTO-LDF cluster method. Internuclear distances as well as frequencies and absolute IR intensities of the MgO-CO and C-O vibrational modes are calculated for model clusters embedded in large arrays of point charges. The MgO-CO bond is found to be predominantly electrostatic. But there is also a minor redistribution of the surface electronic charge similar to that occurring for the adsorption at Mg²⁺ sites of a perfect MgO(001) surface. For the C-end-bound CO the main contribution to the increased blue shift of the stretching frequency at the more open sites comes from a change of the electrostatic field near the center of coordination. For CO molecules bound via the oxygen atom to the same sites a red shift is calculated which should provide a facile way to distinguish O-end bound species from the moiety coordinated via carbon. Surface relaxation is found to have only a negligibly small effect on the MgO/CO vibrational frequencies and intensities, even for the most relaxed corner site. The cluster models predict an approximate doubling of the C-O absolute IR intensity upon the CO adsorption C-end down. The intensity shows only a moderate dependence on the coordination of the adsorption center. This enhancement derives to a large extent from a change of the π component of the CO dynamical dipole moment due to interactions between the adsorbate at the cation-atop position and the nearest-neighbor surface anions. For the CO molecule adsorbed via the oxygen atom the intensity enhancement is mainly of electrostatic origin. The LCGTO-LDF characteristics are compared to the available data obtained at the Hartree-Fock level of theory for non-stoichiometric cluster models of MgO. The effect of the composition and charge of the model cluster on calculated vibrational parameters and on the adsorption energy is also analyzed.