Other Characteristic Carboxylic Acid Vibrations

On the other hand, other workers studying saturated fatty acids and those in which any double bonds are well removed from the carboxyl group [42, 43, 44] all find this absorption very close to 1290 cm , and Hadzi and Sheppard [49] quote the relatively narrow range of j300 15 cm for this absorption in all the acids which they have examined. Freeman [44] has pointed out that in the fatty acids series this band is invariably accompanied by a second absorption at lower frequency. These two bands are remarkably consistent in their frequencies (1285 5 cm and 1235 5 cm ) 

O Connor et al. [24] also report a band in the range 1170— 1150 cm in the spectra of a large number of even-numbered carbon atom fatty acids which is not present in the corresponding ester spectra. The lower frequency suggests, however, that this is probably due to some other cause and that it is not directly related to the bands under discussion. 

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Characteristic carboxyl acid peak appears at 1730 cm with two other distinct bands at 1650 and 1550 cm located in the zone related to the (-CONH-), corresponding respectively to the (C=0) stretching band and to the (-NH) bending vibration band [101]. Indeed, as already reported by several authors [18, 102-103], the carboxylic acid band is well separated from the amide group (-CONH-) bands at 1650 and 1550 cm. The FTIR analysis clearly shows that an amide bond has been formed during the grafting reaction and a correlation between degrees of conversion and the intensity of characteristic peaks is indicated. 

Sodium salts of amino acids show the normal N—H stretching vibrations at 3400-3200 cm-1 common to other amines. The characteristic carboxylate ion bands appear near 1600-1590 cm-1 and near 1400 cm-1. 

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. 

Amino acids. a-Amino acids (and other amino acids also) normally exist as zwitterionic salts (e.g. (1)), and therefore show bands characteristic of the ionised carboxyl group and an amine salt. Hence there is no absorption corresponding to the normal stretching vibrations as exhibited by an amine, but instead a complex series of bands is observed between 3130 and 2500 cm - and this is also the... 

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