Polarizable acid–water hydrogen bonds with aqueous solutions of carboxylic acids

Abstract

Trifluoroacetic, difluoroacetic and formic acids have been studied, pure and in aqueous solutions, by i.r. spectroscopy. The formation of monomers from the dimers and dissociation of the acids with increasing dilution is demonstrated by bands in the spectra; the polarizability of hydrogen bonds formed is indicated by regions of continuous absorption. At very high concentrations (n= number of water molecules per acid molecule < 1), acid–water hydrogen bonds are formed. With trifluoroacetic acid a double minimum energy surface (with a deeper well at the anion) is present in these hydrogen bonds. They cause an i.r. continuum extending from 3000 cm^–1 over the whole range studied (3000-600 cm^–1) indicating that these hydrogen bonds are easily polarizable. With difluoroacetic acid, the degree of asymmetry of these bonds is larger, but they are still polarizable, as indicated by a continuum in the range 3000-1750 cm^–1. With formic acid, these acid–water hydrogen bonds are largely asymmetric. In the case of trifluoro- and difluoroacetic acids, with addition of more water the absorbance of the continuum continues to increase between n= 1 and n= 3, since the degree of asymmetry of the acid–water hydrogen bonds decreases because of the influence of these water molecules. An additional reason for this intensity increase may be the coupling of transitions in the polarizable hydrogen bonds with vibrations in the environment. On further dilution, the absorbance of the continuum decreases. With increasing dilution, trifluoroacetic acid protons transfer into water–water hydrogen bonds, i.e., H₅O⁺₂ is formed. Also in relatively diluted solutions the continuum is caused not only by the easily polarizable hydrogen bonds in H₅O⁺₂ groupings, but also by polarizable acid–water hydrogen bonds which are still present.