Spectral Signatures and Molecular Origin of Acid Dissociation Intermediates

Abstract

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.

Dynamics of Proton Transfer and Vibrational Relaxation in Dilute Hydrofluoric Acid

“…For instance, the large anharmonic shift of HF stretching frequency is seen in the experiment of HF in pyridine 30 and in the CPMD simulation. 9 Moreover, the equilibrium ACF of kinetic energy in Figure 8d decays much faster than the nonequilibrium response in Figure 8b, which underpins the above picture.…”

HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study

“…It is characteristic of the hydronium ion 39 and systematically observed in acid hydrates. 17,[41][42][43] The band at 2130 cm À1 is assigned to the HCl stretch in the self-solvating HCl : Cl À configuration; 27 it is the origin of the molecular line observed in XPS. The H 3 O + (1740 cm À1 ) and the HCl : Cl À (2130 cm À1 ) band intensities have a Langmuir behavior, as expected for a surface reaction.…”

Perspective: How good is DFT for water?

“…It also provides a way of investigating water systems completely independent of force-fields, so that it gives the possibility of fruitful dialog between the approaches. Although our review is restricted to pure water systems, the development of improved XC approximations reviewed here is highly relevant to the DFT description of more general aqueous systems, including solutions and acid-base systems [36][37][38][39][40][41][42][43][44][45][46], as well as confined water [47][48][49] and water adsorbed on surfaces [50][51][52][53][54][55][56]. The crucial role of H-bonding in the cohesion of water systems [57,58] makes them an outstanding paradigm of this bonding mechanism, which is so widespread in many other molecular systems, including those important in biology [59].…”