Stretch, symmetric carboxylate

In addition to the VCD from the methine C H stretching vibration, which alone gives rise to a strong positive bias in the CH stretching region, the CH stretching VCD of amino acids contains contributions from two other sources. Minor features can be attributed to combination bands of the very intense antisymmetric carboxylate stretch at 1610 cm with the symmetric carboxylate... 

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. 

If the reacting molecules are already in contact, the rate is expected to be similar to that found for the recombination of H30+ and OH in ice, i.e. similar to the rate of vibration of a proton in a hydroxyl group. Under certain specialised conditions, the rates of recombination of H30 + and A in contact may be calculated from the width of lines in the Raman spectrum of a mixture of the acid and its conjugate base [15]. The trifluoracetate ion has a fairly narrow Raman peak at 1435 cm 1, attributed to the symmetrical carboxylate stretching frequency, which does not appear in acid solutions of trifluoracetic acid. Trifluoracetate— trifluoracetic acid buffers show a lower, wider peak than that for the trifluoracetate ion, and the width of this peak is inversely proportional to the time a proton spends on the carboxylate group. The variation of the width of the line with acid and anion concentrations gives a measure of the rate of transfer of a proton from an adjacent H30 + to the anion, and a rate coefficient of about 101 2 lmole 1 sec 1 has been calculated. 

Figure 4.7. These diagrams are designed to emphasize the reproducibility of various physical parameters in monolayer assemblies of different thicknesses a) reciprocal capacitance per unit area versus number of monolayers of cad-mium arachidate on an aluminum substrate (see r erence 38), (Jb) absorption intensity versus number of monolayers for the symmetric carboxylate stretching mode of cadmium arachidate at 1432 cm (see r erence 39), (c) count rate of rays versus number of layers of barium stearate labeled with (see reference 40), and d) X-ray photoelectron signal (XPS) intensity versus number of layers of cadmium dimethylarcwhidate on silver (see reference 28).

Cylindrical internal reflectance spectra were obtained on size-fractionated materials isolated from a small glaciated bog. Band assignments were made on the basis of their behavior under changing pH and comparison to model compounds. The bands appearing at about 1630 and 1390-1340 cm were assigned to the asymmetric and symmetric stretches of carboxylates bound to metal cations. The increased intensities for these bands in the smaller size fractions were found to be strongly correlated with increased cation content, consistent with this interpretation of the infrared spectra. 

The bands are broad and overlap with each other and with the CH bands. A broad band at 2100 cm often appears in these spectra. The spectrum of valine, Fig. 4.52, shows the broad NH stretch overlapping the CH and OH bands (2600-3200 cm ) trying to sort out the methyl and CH bands from the OH and NH bands is not practical. The carbonyl stretch is typical of the carboxylate ion, shifted to a lower wavenumber than a typical carbonyl stretch, broader and overlapped by the NH bend. The symmetric NH bend and symmetric carboxylate ion stretch can be seen at 1500 and... 

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... 

Carboxylate ions —COj Most types 1610-1550 Antisymmetrical and symmetrical stretching. 

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. 

Other than the chloro derivative, 2, the drum and ladder compounds are prepared in high yield, > 70%. All are soluble in organic solvents and show characteristic infrared spectra. The drum compounds and A exhibit a symmetrical doublet for the carboxylate stretching frequency, Vqqq, centered near 1550 cm 1 and a single Sn-0 stretch, v Q, near 600 cm-1. In contrast, the open-drum structures, 2 and 3, show an unsymmetrical vcoo doublet in the same region as that for the drums and the presence of two Sn-0 stretches near 600 cm. ... 

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