Carboxylic acid, characteristic frequencies

The shift to lower frequencies of the A-H stretch (ws) band is very often taken as the primary characteristic of a hydrogen bond. The maximal shifts of Vj bands may be as much as 3000 cm , as observed in some acid salts of carboxylic acids. The frequency shift results from the anharmonicity increase of the A-H stretching potential itself, and the anhar-monic coupling of the Vs and v modes. The influence of anharmonicity on the IR spectral changes of A-H stretching bands has been studied by Sandorfy and co-workers. Their results show that the strength of the hydrogen bond determines the amount of anharmonicity. 

Poly(acrylic acid) (PAA), a MIP film candidate, has been shown to bind to the bare CdSe surface from methanol solution with considerable affinity [13]. Placement of drops of a PAA-methanol solution on the surface of CdSe and evaporation of the solvent leaves a PAA film on the semiconductor surface. Once coated with this PAA film, CdSe shows no change in PL intensity in the presence of amines. Despite the lack of a PL change, the deprotonation of the carboxylic acid could be observed by the shifting of the infrared (IR) carboxylic acid peak to lower frequencies characteristic of the carboxylate anion upon amine binding, as shown in Fig. 5. The reaction chemistry is ... 

The vibrational spectrum of 4-pyridine-carboxylic acid on alumina in Fig. 4d is equivalent to an infrared or Raman spectrum and can provide a great deal of information about the structure and bonding characteristics of the molecular layer on the oxide surface. For example, the absence of the characteristic > q mode at 1680 cm 1 and the presence of the symmetric and anti-symmetric O-C-O stretching frequencies at 1380 and 1550 cm indicate that 4-pyridine-carboxylic acid loses a proton and bonds to the aluminum oxide as a carboxylate ion. 

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. 

The use of aliphatic and aromatic alcohols, amines, and carboxylic acids as proton donor molecules with DTBN reveals that whereas the X-H protons are shifted to low frequency by the radical the C-H protons, other than X-H, are moved to high frequency (223) (Table X). These high frequency shifts are shown to be characteristic of protic molecules and demonstrate conformational or geometrical dependences. Thus, protons lying on a zig-zag path from the -OH or -NH... 

In the anions of the carboxylic acids the characteristic carbonyl frequency is absent although a characteristic frequency of 1,350 to 1,400 cm , present in the acids is retained (see Table AX/). This is evidently due to... 

Similar intense bands at approximately the same frequencies are observed when other molecules of comparable proton affinity are adsorbed in HZSM-5 e.g.water [41],dimethylether [38,40,41], acetone and various carboxylic acids [41]. Pelmenschikov et al. [42,43] pointed out that these bands are very similar to the so-called A,B,C triplet of OH bands characteristic of strong molecular hydrogen bonded complexes in liquid or solid phases. The most widely accepted explanation for the A,B,C triplet in hydrogen bonded systems is that due to Claydon and Sheppard [44], who suggested that the A,B,C triplet are in fact pseudobands caused by the superposition onto a very broad single (OH) band of two so-called Evans transmission windows caused by Fermi resonance between the (OH) mode and the first overtones of in-plane ( 2 5(OH) = 2600 cm l) and out of plane ( 2 y(OH) = 1900 cm l) bending... 

Carboxylic acid azides have been analysed and again, the characteristic asymmetric stretching frequency is 2140 cm . Substitution alpha to the carbonyl group has little effect d . A review of the infrared characteristics of carbamoyl azides has appeared which cites 2150 cm as the characteristic frequency for the azido group. 

Phenols (ArOH) also show both these bands, but the C 0 stretching appears at somewhat higher frequencies. Ethers show C—O stretching, but the 0-—H band is absent. Carboxylic acids and esters show C—O stretching, but give absorption characteristic of the carbonyl group, C O, as well. (For a comparison of certain oxygen compounds, see Table 20.3, p. 689.)... 

A portion of the organic by-products was dissolved in carbon tetrachloride or chloroform (spectroscopic grade) for infra-red cuialysis. The absorption peaks corresponded to nitrophenols, nitrocresols, dinitrophenols, dinitrocresols, traces of trinitro conpounds euid nitro-hydroxy carboxylic acids. The presence of individual compounds wais confirmed by comparison with the absorption spectrum of the pure compounds euid by thin-layer chromotography. Quantitative cuialyses of the concentrations of several components of the by-products were made. If a component did not have an absorption bcuid free from interference by euiother constituent of the mixture, queuititative cUialysis could still be achieved by mectsurement of the extinction coefficients of the interfering components at severcLL different frequencies, together with the absorbance of the mixture at these frequencies. Characteristic absorption bands used are shown in Tetble I. 

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