Molecular orbital cluster model study of bonding and vibrations of CO adsorbed on MgO surface
Tóm tắt
The interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg2+ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas‐phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field–dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the CO molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ω
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Tài liệu tham khảo
Little L. H., 1966, Infrared Spectroscopy of Adsorbed Species
Hair M. L., 1967, Infrared Spectroscopy in Surface Chemistry
F. B.van Duijeneveldt IBM Res. Report RJ 945 1971(unpublished).
1958, Tables of Interatomic Distances and Configuration in Molecules and Ions
Buenker R. J., 1980, Proceedings of the Workshop on Quantum Chemistry and Molecular Physics
Herzberg G., 1950, Molecular Spectra and Molecular Structure
Bagus P. S., 1989, Quantum Chemistry: Basic Aspects, Actual Trends, 475
If all quantities are expressed in a.u. assuming a Mg‐C distance of 4.90 bohr andQ= +2 the corresponding value ofFis about 0.08 a.u. = 4 × 108V/cm. This field is very large; for comparison in an electrochemical cell when a potential of 1 V is applied the electric field is about 5 × 107V/cm.