C—O stretch frequencies

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

Av(CO) variation of the C - O stretching frequency with respect to that in the gas phase... 

One of the classic examples of an area in which vibrational spectroscopy has contributed to the understanding of the surface chemistry of an adsorbate is that of the molecular adsorption of CO on metallic surfaces. Adsorbed CO usually gives rise to strong absorptions in both the IR and HREELS spectra at the (C-O) stretching frequency. The metal-carbon stretching mode ( 400 cm-1) is usually also accessible to HREELS. 

For the study of mixed oxides, one should characterize the various sites. In this case, the first step is to characterize the CO adsorption at various equilibrium pressures at low temperature, followed by evacuation at increasing temperatures to obtain information about the stabilities of the various species. Although the C—O stretching frequency is the most informative parameter, the data determining the stabilities of the various species can be decisive for the assignment of the bands. Multiple carbonyls adsorbed on the same metal cation are possible, and in order to identify them isotopic mixtures should be used. Sometimes the polycarbonyls are very stable and in this case, if 12CO is adsorbed first and then 13CO introduced, mixed species may not form at ambient temperature. 

Comparison of the C-O stretching frequencies for a series of metal carbonyl complexes can reveal interesting trends. The complexes listed below all obey the 18-electron rule, but with different numbers of CO ligands attached, the metal atoms do not have the same increase in electron density on them because the coordination numbers are different. 

Table 8.2 C-O stretch frequency of different isotopic combinations. ...

Adsorption of 0.05 monolayers (ML) of CO on this surface gives rise to a peak at 2015 cm-1 corresponding to the internal C-0 stretch frequency of the molecule in the on-top adsorption site and one at 470 cm-1 due to the metal-molecule bond. The latter is not easily observable in infrared spectroscopy. Increasing the CO coverage to 0.33 ML enhances the intensity of the HREELS peaks. In addition, the C-O stretch frequency shifts upward because of dipole-dipole coupling [16, 17]. The LEED pattern corresponds to an ordered (V3xV3)R30° overlayer in Wood s notation (see the Appendix) in accordance with the coverage of 0.33 ML. 

Example The C = O stretching frequency is about 1700 cm-1 whereas the C—H stretching frequency is about 3000 cm 1 and both of them are almost independent of the rest of the molecule as depicted in Table 22.1. 

Calculate the fundamental frequency expected in the infrared spectrum for the C — O stretching frequency. The value of the force constant is 5.0 X 105 dyne cm-1. 

Let us consider the system XCO with the mass of the fictitious atom X, mx, changing continously from 1 (X = H) to 3 (X = T). The resulting term energies of the three relevant excited vibrational states are depicted as functions of mx in Fig. 13. For m = 1 the two stretching states (1, 0, 0) and (0, 1, 0) are well separated in energy and the assignment provides no problem. The variation of mx in principle does not affect the C-O stretching frequency and to2 would stay constant (in a diabatic sense). On the other hand, the X-CO frequency scales approximately as 1 A/mx and therefore... 

The effects of substitution in the aromatic ring on the C=C and C—O stretching frequencies (1587 and 1250 cm-1, respectively) in chroman-6-ols have been discussed (59JCS3362). The IR spectra of a number of tocopherols have been reported (48JBC( 173)439, 50JBC(187)83, 56MI22200) and a detailed discussion is available <81HC(36)66). 

Fig. 12. Correlation between C-O stretching frequency and B, ionic state energy ( d orbital ionization energy ) l or LMn(CO)s compounds. (From Ref. 174.)...

It is commonly accepted that chemisorption of CO on transition metals takes place in a way that is quite similar to bond formation in metal carbonyls (4). First experimental evidence for this assumption was obtained from a comparison of the C—O stretching frequencies (5) and was later confirmed by data on the bond strength (6) as well as by valence and core level ionization potentials obtained by photoelectron spectroscopy (7). Recent investigations have in fact shown that polynuclear carbonyl compounds with more than about 3-4 metal atoms exhibit electronic properties that are practically identical to those of corresponding CO chemisorption systems (8, 9), thus supporting the idea that the bond is relatively strongly localized to a small number of metal atoms forming the chemisorption site. 

A further consequence of the back-donation effect consists of a weakening of the C—O bond strength that manifests itself in a lowering of the C—O stretching frequency as compared with the value of free CO (= 2143 cm ). Vibration spectra from CO adsorbed on single crystal surfaces have thus far been obtained with Pd (60), Pt (59, 61-64), and Ru (65). In all cases bands below 2100 cm"1 were observed. Some data for Pd are included in Table I and a more detailed discussion will be presented in the following two sections. 

The C—O stretch frequency of 1823 cm"1 observed at low coverages indicates that the molecules are most probably located in threefold coordinated sites (60) as indicated by the structure model of Fig. 4a. Upon further increasing the coverage the overlayer unit cell is continuously compressed, as indicated by the arrows in Fig. 4a, until at 8 = 0.5 a c4 x 2 structure is reached. This compression is connected with an enormous shift of the IR band that suggests that the CO molecules now move into bridge sites as drawn in Fig. 4b (60). The compression continues until 6 — 0,6. Additional... 

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