The Ionised Carboxyl Group Salts

Lecomte and his co-workers have made a very extensive study of the infra-red spectra of the salts of organic acids and have examined almost a thousand such materials [32—35]. When ionisation occurs, giving the COO group, resonance is possible between the two C—O bonds. In consequence the characteristic carbonyl absorption vanishes and is replaced by two bands between 1610 cm and 1550 cm and between 1400 cm and 1300 cm which correspond to the anti-symmetrical and symmetrical vibrations of the COO structure. 

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. 

In the solid state the two frequencies show minor variations with the nature of the metallic ion and also with the nature of the group to which the ionised carboxyl group is attached. Kagarise [83] has shown that for mono- and di-valent elements there is a linear relationship between the electro-negativity of the element and the asymmetric stretching frequency of the salt, and Stimpson [84] has also noted variations of this type. Changes in the nature of the 

In general, the spectra of fatty acid salts in the sohd state show more distinctive differences than do the original acids, and Childers and Struthers [88] have made use of this in the analysis of acid mixtures of various types. 

The carbonyl frequencies of ionised acids have been studied in solution by Gore, Barnes and Petersen [36], and by Dunn and McDonald [102] who have used solutions in water and in heavy water, in silver chloride cells to scan the whole spectral range. In a typical case sodium acetate was found to show the COO absorptions near 1560 cm and 1410 cm Using heavy water which avoids the obscuring of the carbonyl region by the water bands, it is possible to observe the reappearance of the unionised acid, and consequently of the carbonyl group, on adding DCl, when a 

---