CO vibrational shifts

In the field of catalysts characterization the use of small unreactive probe molecules to identify coordinatively unsaturated sites is well established [89]. Not always, however, a direct correlation between the CO vibrational frequency, the strength of the interaction, and the surface electric field exists. Recent DPT cluster calculations [90] have shown that CO adsorbed on step sites gives rise to a relatively strong interaction but to a negligible CO vibrational shift this is due to the inhomogeneity in the electric field above a MgO(lOO) step. This study [90] has permitted the complete attribution of the IR spectrum of CO adsorbed on MgO [81,83,91], Table 2. 

Also for cytosine, red shifts are detected in the NH2 stretching and bending modes. Ring stretchings and deformations, involving the CO vibration, suffer little changes with respect to the isolated base (see Fig. 16). In the overall, the computed frequency shifts are in agreement with the... 

Sachtler and coworkers showed evidence of CO adsorption on monoatomic Pt with a CO vibrational frequency at 2123cm Hn H-MOR zeolite [142]. This band is shifted to significantly higher frequencies than the -2070 cm band that is typically observed for CO on multiatomic clusters. They proposed that these electron-... 

The rhodium-hydride vibration disappears upon deuteration of the complex as the rhodium-deuteride vibration appears in the fingerprint region. The large frequency shift of the highest energy absorption is indicative of a trans-CO geometry [40]. In solution IR, the rhodium hydride vibration and the lowest energy CO vibration overlap, which results in only two absorptions. 

Some properties of palladium deposited on different amorphous or zeolitic supports were determined, including catalytic activity per surface metal atom (N) for benzene hydrogenation, number of electron-acceptor sites, and infrared spectra of chemisorbed CO. An increase of the value of N and a shift of CO vibration toward higher frequencies were observed on the supports which possessed electron-acceptor sites. The results are interpreted in terms of the existence of an interaction between the metal and oxidizing sites modifying the electronic state of palladium. 

The obvious decrease in the number of electron-acceptor sites with palladium deposition on silica-alumina strongly suggests an interaction between the metal and these sites. Turkevich (28) first demonstrated that palladium behaves like an electron-donor toward tetracyanoethylene we suppose that it can be the same toward an electron-acceptor site of a solid support. In that hypothesis, palladium should have a partial positive charge on the second class of supports. This is actually observed by the adsorption of CO. This adsorbate can be considered as a detector of the electronic state of palladium. The shift toward higher frequencies of the CO band reflects a decrease in the back donation of electrons from palladium to CO. Thus, palladium on silica-alumina or HY is electron-deficient compared with the silica- or magnesia-supported metal. Moreover, the shift of CO vibration frequency is roughly parallel to the increase of activity thus, these two phenomena are connected. We propose that the high activity of palladium on acidic oxides is related to its partial electron deficiency. 

Time-resolved mid-IR spectra of several photolyzed heme proteins are shown in Fig. 6. The negative-going features correspond to loss of bound CO (A states) and the positive-going features correspond to CO dissociated from the heme (B states). Note the transition frequencies and relative intensities of the A and B states. When CO is bound to heme, back-bonding between the CO -orbitals and the iron d-orbitals weakens the CO bond and enhances its transition moment (42). Compared to free CO, the bound CO vibrational frequency is red shifted about 200 cm 1 and its integrated oscillator strength at 5.5 K is enhanced 21.7 1.6 times (33). 

The zeolite interaction with this compound was evidenced by the low frequency shift experienced by the CO vibration of the face bridged carbonyls. Competition between residual or added water was witnessed suggesting that the solvating properties of the zeolite and water were similar and rather weak. Therefore the stabilization of these zerovalent carbonyls within the zeolite porous structure should be attributed to a cage rather than to a chemical effect. 

Figure 3. The different contribution to the Hartree-Fock vibrational shift of chemisorbed CO. FO stands for the Pauli repulsion or Frozen Orbital, FO, contribution, "pol Pt " and "don Pt " correspond to the substrate polarization and donation from the subtrate to the CO adsorbed molecule (ji-backdonation), "pol CO" and "don CO" stand for to the CO polarization and donation from CO to the surface (o-donation), SCF contains the rest of contributions due to coupling between the different mechanisms.

Moreover, the analysis of the interaction of CO on the different active sites of the low index Pt surfaces shows that the Jt-backdonation contribution to the red-shift is very similar for CO on different sites. Therefore, the n-backdonation cannot be the responsible for the observed difference between the CO vibrational frequency on on-top and bridge sites. The CSOV decomposition has permitted to reveal that the leading term contributing to this difference in vibrational frequency of chemisorbed CO is the initial Pauli repulsion or "wall effect" this was a new, important and unexpected conclusion. 

This apparent contradiction has stimulated a considerable amount of theoretical work from the early 90 s until very recently. A number of more and more refined calculations has been performed by several groups using various methods [72], from simple HF [73,74] to HF with extended inclusion of correlation [75,76], from LDA [77] to gradient corrected DFT [78,79]. However, the most sophisticated methods and calculations lead to the same conclusion CO is bound to the terrace sites of the MgO surface through electrostatic forces, with a very weak binding energy of less then 0.1 eV, and a very small vibrational shift, of -I-IO cm" or less [67,72]. Eventually, the discrepancy between the ab initio calculations and the results on MgO thin films lead to the suggestion that surface defects could be responsible for the observed chemisorption properties of CO/MgO/Mo(l 10). 

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