Carbonyl group infrared absorption frequencies

Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

Table 1.2 Infrared absorption frequencies of substituted carbonyl groups...

In order to determine the conformational equilibrium of a-halocyclohexanones, Corey used infrared spectroscopy, since the substitution of one a-hydrogen in a cyclohexanone with a halogen produced a frequency shift in the absorption of the carbonyl group, where the frequency shift magnitudes depended upon whether or not the a-halogen atom was axial or equatorial (Table 1.1). 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

The role of specific interactions in the plasticization of PVC has been proposed from work on specific interactions of esters in solvents (eg, hydrogenated chlorocarbons) (13), work on blends of polyesters with PVC (14—19), and work on plasticized PVC itself (20—23). Modes of iateraction between the carbonyl functionaHty of the plasticizer ester or polyester were proposed, mostly on the basis of results from Fourier transform infrared spectroscopy (ftir). Shifts in the absorption frequency of the carbonyl group of the plasticizer ester to lower wave number, indicative of a reduction in polarity (ie, some iateraction between this functionaHty and the polymer) have been reported (20—22). Work performed with dibutyl phthalate (22) suggests an optimum concentration at which such iateractions are maximized. Spectral shifts are in the range 3—8 cm . Similar shifts have also been reported in blends of PVC with polyesters (14—20), again showing a concentration dependence of the shift to lower wave number of the ester carbonyl absorption frequency. 

The UV-spectra of azolides have already been discussed in the context of hydrolysis kinetics in Chapter 1. Specific infrared absorptions of azolides were mentioned there as well increased reactivity of azolides in nucleophilic reactions involving the carbonyl group is paralleled by a marked shift in the infrared absorption of the corresponding carbonyl bond toward shorter wavelength. For example, for the highly reactive N-acetyl-tetrazole this absorption is found in a frequency range (1780 cm-1) that is very unusual for amides obviously the effect is due to electron attraction by the heterocyclic sys-tem.[40] As mentioned previously in the context of hydrolysis kinetics of both imidazo-... 

One of tire distinguishing physical characteristics of the cephalosporins is the infrared stretching frequency of the /i-lactam carbonyl. This absorption occurs at higher frequencies (1770-1815 cm-1) than those of either normal secondary amides (1504-1695 cm-1) or ester carbonyl groups (1720-1780 cm-1). 

All these frequencies are in the region of other heteroaromatic compounds and of azulenes. Infrared absorption spectra for several derivatives of the following pseudoazulene systems have been reported 26,56 28,77 2985 86 33 96.uio 35,165 39,"3114 42, 23 49,135 136 and 56.143144-146 The key frequencies for substituents at positions 1 or 3 in the five-membered ring are shifted to lower wavelengths in a typical manner. This is especially pronounced in the case of carbonyl groups. 

Infrared spectral studies on molybdenum hexacarbonyl-alumina were reported by Davie, Whan, and Kemball 78). Without any activation procedure they obtained a sharp carbonyl frequency corresponding to unchanged hexacarbon-yl on the support. This material was not active for disproportionation. After treatment for one hour under vacuum at 373 °K the catalyst had lost the sharp carbonyl band but showed two wider and broader bands and was active for dis-proportionating propylene. The authors stated that the active catalyst clearly had a lower symmetry than the hexacarbonyl and must have lost one or more of the carbonyl groups. After exposure of the activated catalyst to air, the catalyst was inactive and showed no absorption in the carbonyl region. 

The most obvious feature in the infrared spectrum of a carboxylic acid is the intense carbonyl stretching absorption. In a saturated acid, this vibration occurs around 1710 cm-1, often broadened by hydrogen bonding involving the carbonyl group. In conjugated acids, the carbonyl stretching frequency is lowered to about 1690 cm-1. 

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm-1 for simple ketones and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

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