Direct Observation of Ion Pairing in Aqueous Nitric Acid Using 2D Infrared Spectroscopy

Ion-ion interactions and ion pairing play an important role in the properties of concentrated electrolyte solutions, yet remain difficult to study due to the heterogeneous and highly dynamic behavior of these systems. In concentrated acid solutions, these questions take on a further level of complexity because the structure of the aqueous proton itself is uncertain and may be influenced by the counterion. Here, we address these questions by studying the IR spectra of nitric acid as a function of concentration in H₂O and comparing these to the spectra of several alkali nitrate salts. We show how the close proximity between cations and NO₃^- ions in solution at high concentration affect the IR spectra and therefore the molecular structures. Using two-dimensional IR spectroscopy, we demonstrate the formation of contracted ion pair configurations in nitric acid solutions between NO₃^- ions and H⁺(aq) via the observation of a distinct anisotropic intermolecular crosspeak between these species. By studying the concentration dependence of this spectral feature, we show that this ion-paired configuration exists in solution at concentrations as low as 2 M and suggests that the structure of H⁺(aq) solvation complex in these ion pairs differs from the structure in bulk solution.