Carboxylic acids infrared absorption band positions

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The most important metal-containing heat stabilizers in PVC can also be identified through IR-spectroscopy (see Section 8.2). Characteristic absorption bands for the salts of carboxylic acids are the bands of the ionized carboxy group in the region from 1590 to 1490 cm and from 1410 to 1370 cm Since the exact position of the absorption bands depends mainly on the metal counterions of the carboxy group, the IR-spectra provide a first identification of these metals. Tin stabilizers also show characteristic bands in these regions of the IR-spectrum. To determine the spectra, one uses pressed pellets made by finely grinding the sample material with potassium bromide. The identification of the metal becomes even more certain if an FT-IR (Fourier Transform/Infrared) instrument is available. With such an instrument, it is possible to subtract the spectrum of an additive-free... 

Some derivatives of both thiazole and benzothiazole have been studied by IR spectroscopy. In particular, an extensive study has been carried out with thiazole-2-carboxylic acids and the corresponding carboxylate ions <88Mi 306-01 >. The infrared spectra of 2,3-disubstituted 1,3-thia-zolidin-4-ones have been studied and the majority of the absorption bands assigned <93PS(78)223>. The tautomerism of thiazoles substituted in 2- and 4-position by amino, thio, and hydroxy groups was examined by infrared spectroscopy (85JPR25l> (see Section 3.06.4.4). 

Conjugation with aromatic rings and donble bonds, and interactions with halogens and cyclic structures, all affect the band positions, as in the mid-infrared. Also, these second overtones are sometimes split into two as there may be two or more small bands in the vicinity of the calculated second overtone. The second overtone of the C=0 of a carboxylic acid appears at about 5260 cm" (1900 nm) and is particnlarly clear in a spectrum of perfluorocaproic acid taken in solution, as this acid has no CH absorptions. It can be seen in Figure 7.2, a comparison between octanoic and octadecanoic acids in carbon tetrachloride. With the shorter-chain acid, the acid carbonyl peak is more prominent. 

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