Aqueous perchloric acid and trifluoromethane sulphonic acid solutions have been studied as a function of concentration by i.r. spectroscopy. The degree of dissociation was determined from the absorbance of bands characteristic of the acid molecules. Easily polarizable hydrogen bonds are indicated by i.r. continua. In highly concentrated solutions (water : acid ratio n < 1) easily polarizable acid-water hydrogen bondsare formed. The weights of the proton limiting structures demonstrate that the energy surface for these hydrogen bonds is an asymmetric double minimum with the deeper well at the water molecule. The vibrations of the water molecules are strongly influenced by the proton, i.e., they have more or less H30+ character. With increasing n, complete dissociation occurs at n = 1.8 (HC104) or n = 1.6 (CF3S03H). First, the weight of proton limiting structure I1 increases; then H50: is formed and vibrations with H30+ character vanish. The continuum stems from the easily polarizable hydrogen bond in H,Of, and the broad bands originate in the water molecules in these groupings. The absorbance of the H50+2 continuum changes with the interaction of this group with its environment (nature of hydrogen bond acceptors strength of anions or additional water molecules). While the dissociation behaviour of both acids is similar, the hydrogen bond acceptor strength of the anions is much stronger with CF3SO; than with C10;.

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 < l), 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 3OOOcm-1 over the whole range studied (3000-600 cm-l) 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 3000-1750cm-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., H50: is formed. Also in relatively diluted solutions the continuum is caused not only by the easily polarizable hydrogen bonds in H 5 0 i groupings, but also by polarizable acid-water hydrogen bonds which are still present.

Aqueous solutions of fourteen acids in the pKa region 0–4 were studied by ir spectroscopy as a function of concentration. All data are summarized in a table. The degree of dissociation was determined from bands of the anions. The removal of the protons from the anions decreases in the following series of the acids CF3COOH, H2CrO4, (HIO3), CF2HCOOH, HPO(OH)2, HSO4−, (HSeO4−), H2POOH, PO(OH)3, AsO(OH)3 (Table 1, columns 3 and 4). Continua indicate the formation of hydrogen bonds with large proton polarizabilities. In the case of H3PO3, H3PO4, and H3AsO4, the acid–acid hydrogen bonds are still polarizable although they have relatively asymmetrical proton potentials. The acid–water hydrogen bonds (I) [Formula: see text] (II) may show considerable proton polarizability, as indicated by strong continuous absorptions. The proton potential, and thus the proton polarizability of the acid–water hydrogen bonds, are discussed taking into consideration the intensity and the wave number range of the continuum (column 7), as well as the line width of the vibration with δ OH character (column 9). With increasing asymmetry of the hydrogen bonds, the continuum changes to the bands observed with asymmetrical hydrogen bonds. The sequence of acids in the series arranged according to the degree of dissociation is not the same as the sequence according to the intensity of the continuum and proton polarizability. This is explained by the fact that the proton polarizability is mainly determined by the shape of the proton potential, whereas the dissociation equilibrium is also influenced by entropy effects associated with hydrate structure formation.It is shown that the following molecular processes are of significance for the dissociation of the acids: the formation of acid–water hydrogen bonds with proton polarizability, whereby the weight of the polar proton-limiting structure [Formula: see text] increases with increasing addition of water molecules. Besides the shape of the proton potential, the entropy effects mentioned influence the degree of dissociation, too. Finally, the positive charge transfers from the acid–water into water–water hydrogen bonds, i.e., H5O2+ is formed.

Aqueous solutions of 15 acids with pKa < 1 were studied by ir spectroscopy. The ir bands of the acid molecules demonstrate that the proton acceptor strength of the anions increases in the series CF3SO3−, HSO4−, ClO4−, HSeO4−, C6H5SO3−, NO3−. At very high concentrations (number of H2O/acid molecules, n < 1) easily polarizable[Formula: see text]hydrogen bonds are formed causing continuous absorption in the ir spectra. The deeper well of the double minimum energy surface of these hydrogen bonds at the beginning of the above series is present at the water molecule, and at the end of the series at the anion. The H3O+ character of the vibrations of the H+OH2 groups increases with increasing n. For n > 1 the weight of proton limiting structure II increases and more and more protons transfer from the acid–water into the water–water hydrogen bonds, i.e., they are present in H5O2+ groupings which are embedded in the whole hydrate structure network. H3O+ vibrations are no longer found. The easily polarizable hydrogen bond in these groupings causes continuous absorption and the water molecules of these groupings cause broad bands. Of all acids studied, this group is the only cause of continuum in the more diluted solutions. The absorbance of the continuum per H5O2+ group at n = 2 increases in the series HClO4, CF3SO3H, H2SO4, H2SeO4, C6H5SO3H. This is because of the increasing polarity of the environment of the H5O2+ in this series of acids. The position of the water bands in solutions with n = 6 can be used to characterize the hydrogen bond acceptor strength of the anions. The H2O scissor vibration shows that it increases in the series AuCl4−, PtBr62−, PtCl62−, ClO4−, BF4−, CF3SO3−, J−, HSO4−, HSeO4−, C6H5SO3−, C7H7SO3−, Cl−. Finally molecular quantities which determine the pKa values of the acids (proton acceptor strength, hydrogen bond acceptor strength, and special interaction properties of the anions) are discussed.

No abstract