The Molecular Origin of the “Continuous” Infrared Absorption in Aqueous Solutions of Acids: A Computational Approach

Abstract

dissociation process of the weak acid HA has been the object of numerous experimental and theoretical investigations, which have focused on the chemical nature of the solvated reactants and products and stable or transient intermediate structures. However, the deceptive simplicity of reaction (1) masks a wealth of microscopic behavior that remains largely unexplained. Important insights into the dissociation mechanism can be gleaned from infrared (IR) spectra of aqueous solutions of weak acids. In particular, the IR spectra of these acids exhibit a broad absorption band extending from the lower range of the OH stretch band of water at n= 3000 cm 1 to the HOH bending peak at n= 1500 cm 1 and in some instances all the way down to the onset of the water libration band at n= 1000 cm . This “continuum of absorption” is reminiscent of that seen for solutions of strong acids, where its origin can be explained in terms of frequent protontransfer events between H3O + and H2O units. In the case of weak acids, however, the early transmission spectroscopy investigations of Leuchs and Zundel as well as the more recent attenuated total reflection measurements of Max and Chapados strongly suggest that it is either the unionized hydrated acid HA(aq) or some relatively stable supramolecular