Formation of acid–water hydrogen bonds with large proton polarizability and molecular processes with dissociation of acids in the pKa range 0–4

Abstract

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.