A Check of Quantum Chemical Molecular Models of Adsorption on Oxides against Experimental Infrared Data

Importance of Madelung potential in quantum chemical modeling of ionic surfaces

“…The value obtained without PCs is in excellent agreement with the experimental data, q21 cm y1 , 67 and with the results of ref. 38. The obvious conclusion would be that the presence of the PCs results in a too-large shift.…”

“…In a recent article, Pelmenschikov et al 38 suggested that molecular models of ionic materials without embedding accurately reproduce the vibrational shifts of adsorbed molecules. For CO adsorbed on MgO, shifts of 10, 21, and 60 cm y1 with respect to the free molecule have been measured and assigned to CO adsorbed on five-, four-, 2q Ž and three-coordinated Mg cations surface, step, .…”

“…In ref. 38, it has been shown that the shifts of CO adsorbed on step and corner sites are quantitatively reproduced by using HF wave functions and small MgO clusters without any representation of the Madelung potential. No attempt has been done, however, to evaluate the importance of the Madelung term in determining the CO vibrational frequency.…”

Can the Three-Coordinated Mg Site of MgO Accommodate More Than One CO Molecule?

“…5,[9][10][11][12]19,20 The employed methodology of calculation of the frequency shift and binding energy of CO using small, bare MgO clusters has been validated and described elsewhere. 9,11,12 The Mg 3c site was modeled by Mg 10 O 10 and Mg 16 O 16 clusters. A combined basis set was used: the 6-31G(d) basis set for CO molecule and the 3-21G basis set for MgO clusters.…”

“…The blue shift of the CO frequency induced by this interaction is commonly accepted as a measure of the acid strength for series of closely related sites: the larger the frequency shift the stronger the Lewis site. Experimental and theoretical studies have shown 7,[9][10][11][12] that the coordination of CO on the Mg 3c site is associated with the largest frequency shift of CO on MgO, namely ∆ν ) 59 cm -1 . An interesting feature of the corresponding 2202 cm -1 band 7 is that its intensity gradually decreases upon increase of the pressure from 10 -1 Torr.…”