Molecular orbital cluster model study of bonding and vibrations of CO adsorbed on MgO surface

International Journal of Quantum Chemistry - Tập 42 Số 5 - Trang 1115-1139 - 1992
Gianfranco Pacchioni1, Giuseppe Cogliandro1, Eugene S. Ilton2
1Dipartimento di Chimica Inorganica e Metallorganica, Centro CNR, Università di Milano, via Veneziano 21, 20133 Milano, Italy
2IBM Research, Almaden Research Center, 650 Harry Road, San Jose, California 95012-6099

Tóm tắt

AbstractThe 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 CO 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 ωe, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal–CO bond as arising from σ donation and π back donation.

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International Journal of Quantum Chemistry

Tập 42 Số 5

1115-1139

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