Easily polarizable hydrogen bonds and solvate structure in aqueous solutions of acids with pKa < 1

Abstract

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.