Ion-Pair Speciation in Aqueous Alkali Fluorides from Nuclear Magnetic Resonance of ¹⁹F

We demonstrate that a combination of multiscale modeling and experimental ¹⁹F NMR can be used to examine the molecular details of how ion-pair speciation changes with temperature and molality. Excellent agreement with experimental ¹⁹F NMR chemical shifts is attained through the pairing of the theoretical ion-pair chemical shift profile with speciation populations estimated with molecular dynamics simulations and thermodynamic equilibrium constants. Clear periodic trends down Group I metal cations show that the ion-pair chemical shift profile is dominated by the shielding effects of pairing-induced dehydration for smaller cations (Li⁺ and Na⁺) that are overwhelmed by short-range deshielding effects for heavier cations (K⁺, Rb⁺, and Cs⁺). The experimental ¹⁹F NMR chemical shift trends with increasing temperature and molality are connected explicitly to changes in ion-pair populations relative to the free-ion state. This work provides a general approach to model ion pairing and solvation within the framework of NMR, which has implications for understanding ion-pairing phenomena in various systems.