Spectral signatures and molecular origin of acid dissociation intermediates

The existence of a broad, mid-infrared absorption ranging from 1000 to 3000 cm(-1) is usually interpreted as a signature for the existence of protonated water networks. Herein, we use cryogenic mixtures of water and hydrogen fluoride (HF) and show experimental and computational evidence that similarly wide absorptions can be generated by a broad distribution of proton-shared and ion pair complexes. In the present case, we demonstrate that the broadening is mainly inhomogeneous, reflecting the fact that the topology of the first solvation shell determines the local degree of ionization and the shared-proton asymmetric stretching frequency within H2O x HF complexes. The extreme sensitivity of the proton transfer potential energy hypersurface to local hydrogen bonding topologies modulates its vibrational frequency from 2800 down to approximately 1300 cm(-1), the latter value being characteristic of solvation geometries that yield similar condensed-phase proton affinities for H2O and fluoride. By linking the local degree of ionization to the solvation pattern, we are able to propose a mechanism of ionization for HF in aqueous solutions and to explain some of their unusual properties at large concentrations. However, an important conclusion of broad scientific interest is our prediction that spectral signatures that are normally attributed to protonated water networks could also reveal the presence of strong hydrogen bonds between un-ionized acids and water molecules, with important consequences to spectroscopic investigations of biologically relevant proton channels and pumps.