Stretch, asymmetric carboxylate

The carbonate can also be compared with adsorbed formate species prepared by reacting methanol with the alumina surface at 350 C ( ). The spectrum for adsorbed formate. Figure 4d, shows the asymmetric carboxylate stretches at 1565 and 1440 cm l respectively, the CH stretch at 2832 cm , and the CH bending mode at 1505 cm . The Al-OC stretching mode is seen at 1060 cm", and the out-of-plane deformation at 750 cm . The signal to noise ratio in the low frequency end of the spectrum is insufficient to see the planar deformation, which should occur around 630 cm . It should be noted that the carbonate and formate species are very similar, the main distinction being the vibrations associated with the CH bond. 

To these two sets of results we can contrast the IR spectrum reported for adsorption from several torr of acetic acid gas onto oxidized indium films. This spectrum consists of two broad peaks at 1590 and 1455 cm-1, attributed to the asymmetric and symmetric carboxylate stretch. Both peaks are of roughly similar intensity. 

Figure 3. External reflection IR spectrum of an 10-A film of poly (acrylic acid) on native-oxide-covered, evaporated aluminum. The jagged line is the unsmoothed spectrum at 2 cm 1 resolution. The major peak assignments are 1740 cm 1, unionized carboxylic acid, C = O stretch and 1620 cm 1, the carboxylate ion asymmetric stretch 1475 cm 1, CH2 bending. (Reproduced, with permission, from Ref.

The asymmetric carboxylate stretch appears at about 1560 cm in basic solution (top spectra. Figure 4). The C-0 stretch of COOH can be seen at about 1230... 

These eompounds exist as zwitterions (internal salts) and exhibit spectra that are combinations of carboxylate and primary amine salts. Amino aeids show NH3" stretch (very broad), N—H bend (asymmetrie/synunetric), and COO stretch (asymmetric/symmetric). 

The formation of the latter complexes can be followed by infrared spectroscopy, as it involves a proton transfer from the acid of poly (acrylic acid) to the tertiary amine of the mesogens. Complexation results in a significant reduction, or almost complete elimination, for the equimolar complexes, of the acid carbonyl band near 1700 cm and the appearance of a new band near 1550cm attributed to the asymmetric carboxylate stretch. The residual carbonyl absorption that remains in the equimolar mixtures indicates the presence of a small... 

As coverage is increased in the first layer, new peaks appear, particularly, a strong vibration at 1626 cm associated with the asymmetric carboxylate stretch,... 

Broadly speaking, in the study of AB cements derived from poly-carboxylic acids, the band of interest falls in the region 1550-1620 cm (Mehrota Bohra, 1983 Bellamy, 1975). This band is the asymmetric stretch of the carboxylate group and its exact position depends on both the nature of the bonding involved (i.e. whether purely ionic or partially covalent), and the nature of any chelation by the carboxylate group (Bellamy, 1975). 

IR spectroscopy is often used for distinguishing between unidentate and bidentate coordination of carboxylate (02CR) ligands. For monomeric carboxylate derivatives the separation between the symmetric and asymmetric C02 stretching bands, At = [t asym(C02) - t syJCOa)], provides a useful indication of the coordination mode complexes which exhibit values of greater than 200 cm-1 invariably possess unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

FTIR has been mainly used to obtain structural details of films and to monitor intercalation of metal ions into the film structure and the subsequent reactions of the films with dihydrogen chalcogenides. Both transmission (FTIR-T) and reflection-absorbance (FTIR-RA) modes have been utilized. For the most part these studies have involved films of fatty acids with divalent metal ions. The key features of the FTIR spectra of these films include the asymmetric and symmetric stretching modes of the carboxylate group vs(C02-) and va(C02 ), associated with the M2+/carboxylate complex, and the carbonyl stretching mode v(C=0) of the proton-ated fatty acid. The disappearance of the v(C02 ) (1500-1600 cm-1) and appearance of the v(C=0) bands (—1700 cm-1), concurrent with the formation of the metal chalcogenide and regeneration of the fatty acid, have been used to evaluate... 

The carboxylic dimer has a center of symmetry only the asymmetrical C=0 stretching mode absorbs in the IR. Hydrogen bonding and resonance weaken the C=0 bond, resulting in absorption at a lower frequency than the monomer. The C=0 group in dimerized saturated aliphatic acids absorbs in the region of 1720-1706 cm1. 

The carboxylate ion gives rise to two bands a strong asymmetrical stretching band near 1650-1550 cm-1 and a weaker, symmetrical stretching band near 1400 cm-1. 

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