Carboxylic acids Raman frequencies

The carboxylic acid carbonyl frequency is shifted to lower frequency by 1 to 6 percent in carboxylic acid dimers, which is much less than the relative shifts of The mechanical splitting of the symmetric and antisymmetric carbonyl stretching modes is evident in the difference between the Raman and IR spectra of formic acid. Of course the average of these two is of interest, but often only one is measured. 

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. 

Unfortunately these vibrations are difficult to study because they are so low in frequency, and they may be diffuse or complex because of thermal excitation. There are few IR data available. There is quite a bit of pertinent Raman work, much of it performed by Russian workers, notably Batuev, Chulanovskii, Gross, Raskin, Sechkarev, Simova, Skiripov, and Val kov. The most thoroughly studied compounds are the carboxylic acids and water. 

Other Carboxylic Acids, The low frequency modes of acetic acid and higher carboxylic acids are not as well understood as those of formic acid, and will not be discussed in detail. Raman shifts are reported for solid benzoic acid at 190 and 400 cm (1693, 1694) and for solid tartaric acid at 52, 80, 101, 115, 144, and 164 cm (1891, 1695). Gross and Val kov conclude that the frequency of the O—H 0 vibration is near 200 cm and is unchanged either by deuterium substitution or by increase in the mass of the attached alkyl groups (830). Batuev s studies seem to be in disagreement with this conclusion (162, 163). 

The distinction between cyclic dimers (M-OH)2 and infinite chains (M-OH) is based in the mutual exclusion rule for centrosymmetric cyclic dimers, i.e. infrared and Raman frequencies should not coincide. In the case of carboxylic acids R-COOH, the C=0 stretching mode is particularly sensitive and the difference between infrared and Raman frequencies can be as much as 60-70 cm . ... 

In the case of methanol, the gas phase dimer has not been observed experimentally, thus the configuration found in the crystal structure [31] is taken to be close to that expected for the free dimer at OK. Further, the far infrared and raman spectra are known only for the crystalline [49] state, and the symmetric stretch frequencies used here as guides are experimentally somewhat perturbed by crystal packing effects and thus are allowed to deviate somewhat from the observed values. The experimental dimerization data for methanol [20] is also not as well defined as it is for the carboxylic acids, and an average of several experimental values is used. 

Of these bands, the former is very much more characteristic, as it is generally more constant in frequency whilst many other skeletal vibrations occur in the wide range 1400—1300 cm". Lecomte s general finding has been confirmed by later workers, and, as will be seen later, a somewhat similar state of affairs has been found to occur with amino-acids in which the zwitterion form permits resonance in the same way. Raman data [40] on numbers of carboxylic acids also indicate that salt formation results in the disappearance of the C=0 absorption and its replacement by a band near 1430 cm". The fact that in this case the group is identified by the symmetric vibration is in accordance with theory which requires this mode to be strong in the Raman and weak in the infra-red. The reverse is true of the asymmetric frequency, and Ehrlich [80] estimates the intensity ratio for polymeric acids in the infra-red as about 7.6 1. 

Table 5 lists the basic frequencies for the carbonyl group in some of its more commonly encountered chemical forms. It can be seen that the carboxylic acid frequency observed depends on the technique. This is because they are almost always (in liquid and solid states) dimerized (see [5]) so that the IR band observed is from the coupled asymmetric mode while that in the Raman is from the symmetric one. 

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. 

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