Easily polarizable hydrogen bonds in aqueous solutions of acids. Perchloric acid and trifluoromethane sulphonic acid

Abstract

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 bonds AH [graphic omitted] OH₂⇌ A^– [graphic omitted] H⁺OH₂ are 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 H₃O⁺ character. With increasing n, complete dissociation occurs at n= 1.8 (HCIO₄) or n= 1.6 (CF₃SO₃H). First, the weight of proton limiting structure II increases; then H₅O⁺₂ is formed and vibrations with H₃O⁺ character vanish. The continuum stems from the easily polarizable hydrogen bond in H₅O⁺₂ and the broad bands originate in the water molecules in these groupings. The absorbance of the H₅O⁺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^–₃ than with ClO^–₄.