C—O Stretching Vibrations

The C—O stretching mode is coupled with the adjacent C—C stretching vibration thus in primary alcohols the vibration might better be described as an asymmetric C—C—O stretching vibration. The vibrational mode is further complicated by branching and 

FIGURE 2.16. Benzyl alcohol. O—H stretch intermolecular hydrogen bonded, 3329 cm . C—H stretch aromatic 3100-3000 cm. C—H stretch methylene, 2940-2860 cm Overtone or combination bands, 2000-1667 cm1. C=C ring stretch, 1501,1455 cm-1, overlapped by CH2 scissoring, about 1471 cm-. O—H bend, possibly augmented by C—H in-plane bend, 1209 cm. C—O stretch, primary alcohol (see Table 2.5) 1023 cm. Out-of-plane aromatic C—H bend,745 cm-1. Ring C=C bend,707 cm-1. 

FIGURE 2.17. Phenol (Melt). Broad intermolecular hydrogen bonded, O—H stretch, 3244 cm-1. Aromatic C—H stretch, 3052 cm-1. Overtone or combination bands, 2000-1667 cm 1. C—C ring stretch, 1601,1501,1478 cm 1. Inplane O—H bend, 1378 cm 1. C—O stretch, 1231 cm-1. Out-of-plane C—H bend, 815,753 cm 1. Out-of-plane ring C—C bend, 699 cm 1. (Broad) hydrogen-bonded, out-of-plane O—H bend, about 650 cm 1. 

The absorption ranges of the various types of alcohols appear in Table 2.5. These values are for neat samples of the alcohols. 

pellets, or melts of phenols absorb at 1390-1330 and 1260-1180 cm 1. These bands apparently result from interaction between O—H bending 

---

Section 15 14 The hydroxyl group of an alcohol has its O—H and C—O stretching vibrations at 3200-3650 and 1025-1200 cm respectively The chemical shift of the proton of an O—H group is variable (8 1-5) and depends on concentration temperature and solvent Oxygen deshields both the proton and the carbon of an H—C—O unit Typical... 

The reaction coordinate is the C-O stretch vibration of the CO molecule in its transition state. The rate constant is... 

The bands at 1410 and 1210 cm-1 are due to combined C-O stretching vibration and O-H in-plane deformation of hydroxyl group of niclosamide [22]. 

Further changes in C-O stretching vibrations occur when other ligands are present. For example, the CO stretching band in Cr(CO)6 is found at 2100cm-1, whereas that in Cr(NH3)3(CO)3 is found at approximately 1900 cm-1. The structure of the latter compound is... 

Molecules possess discrete levels of rotational and vibrational energy. Transitions between vibrational levels occur by absorption of photons with frequency v in the mid-infrared range (Table 8.1). The C-O stretch vibration, for example, is at 2143 cm 1. For small deviations of the constituent atoms from their equilibrium positions, the potential energy V(r) can be approximated by that of the harmonic oscillator ... 

Comparing with the normal mode calculation and the experimentally determined value for CO/Pt(lll) below, it seems likely that for the ontop bonded molecules the anharmonic coupling is to the frustrated translation. As expected, d(o is then negative as the C—O stretch vibration frequency decreases when going away from the ontop position. 

C=O Stretching Vibrations The C=O absorption band of saturated aliphatic esters (except formates) is in the 1750-1735 cm-1 region. The C=0 absorption bands of formates, a,/3-unsaturated, and benzoate esters are in the region of 1730-1715 cm-1. Further conjugation has little or no additional effect upon the frequency of the carbonyl absorption. 

C=O Stretching Vibrations Anhydrides display two stretching bands in the carbonyl region. The two bands result from asymmetrical and symmetrical C=0 stretching modes. Saturated acyclic anhydrides absorb near 1818 and 1750 cm-1. Conjugated acyclic anhydrides show absorption near 1775 and 1720 cm-1 the decrease in the frequency of absorption is caused by resonance. The higher frequency band is the more intense. 

C—O Stretching Vibrations Other strong bands appear in the spectra of anhydrides as a result O O... 

The C—O stretching vibration of formaldehyde is reduced from 1 746 cm 1 in the Sq state to 1 183 cm 1 in the S, slate because the strength of the C—O bond decreases when the antibonding tt orbital is populated. 

In the crystalline form of //Y//7s-[(C5H5)Fe(CO) ]2 each molecule lies on a crystallographic center of symmetry the site symmetry is C,. In this site symmetry it should still be true that vibrations of the molecule can be rigorously classified as symmetric and antisymmetric with respect to inversion and that only the antisymmetric ones can be infrared-active. Therefore, according to the site symmetry approximation, only the two antisymmetric C—O stretching vibrations should be observed in the crystal spectrum. As Figure 10.13 shows, however, four C—O stretching bands are actually observed. 

Fig. 15.3 Identification of the synwietnes of the C—O stretching vibrations (represented as vectors) for Fe(CO)j. The reducible representation. T, is derived by counting the number of vectors remaining unmoved during each operation of the point group (see Appendix D for character table). Its irreducible components are obtained by application of Eg. 3.1...

Chromans exhibit a C—O stretching vibration at 1260-1215 cm-1 (64CB682). A detailed analysis of the spectrum of 2,2-dimethylchroman is reported (68JCS(C)1837> and information on variously substituted chromans is available (81HC(36)l). 

---