The behavior of ion pairs is important for selective separations, homogeneous catalysis, formulation of pharmaceuticals, and many other commercially important processes. However, existing methods used to study ion pairing have significant drawbacks. For example, spectroscopic methods such as Raman and NMR are far more sensitive to contact ion pairs than to solvent-separated ion pairs. Dielectric spectroscopy is sensitive to both contact ion pairs and solvent-separated ion pairs, but it is generally not possible to assign the broad peaks to specific species. In order to improve measurements of ion pairing, NIST has initiated the development of an electric-acoustic spectrometer (https://www.nist.gov/programs-projects/electric-acoustic-spectroscopy-intermolecular-interactions-solution). The goal of this new type of spectroscopy is to use acoustic waves to increase the amount of information that can be obtained from dielectric spectroscopy. Developing this new spectroscopic method involves three interrelated activities: (1) designing and building a chip-based electric-acoustic spectrometer, (2) computational modeling of electric-acoustic spectra, and (3) validation of the new method with carefully selected reference systems. Ion pairing behavior in the reference systems will be measured with NMR spectroscopy, dielectric spectroscopy, acoustic spectroscopy, and possibly other methods. This talk will focus on NMR measurements of ion pairing in aqueous solutions of NaF over a range of temperatures, pressures, and concentrations. From the literature, the extent of ion pairing is expected to increase as the temperature increases and as the pressure decreases. Indeed, the peak in the <sup>19</sup>F NMR spectrum shifted downfield (closer to that of “free” fluoride) as the temperature increased or the pressure decreased. A specialized zirconia NMR cell was used to maintain static pressures up to 55 MPa during the NMR experiments. Fitting these peak position data with theoretical relationships allowed us to calculate equilibrium constants for ion pairing, as well as the related thermodynamic properties of Gibbs free energy of ion pairing, enthalpy of ion pairing, and entropy of ion pairing.

Ion pairing of sodium fluoride in water by high-pressure NMR spectroscopy

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ACS Fall 2022

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