A Cocoordinated ¹H Internal Reference Quantifies Proton-Exchange Bias in Coordinated-Water Diffusion

Proton PFG-NMR offers high sensitivity for probing water mobility in electrolyte solutions, but rapid proton exchange between distinct hydration environments can obscure the molecular meaning of the measured diffusivity. The measured proton diffusivity may therefore differ from the diffusion of intact water molecules, and the residual deviation after conventional exchange correction remains difficult to quantify. Here, we address this problem using a first-shell, nonexchangeable proton reference. In an Al(OTf)₃-H₂O-DMSO model electrolyte, water and DMSO co-occupy the first solvation shell of Al³⁺. DMSO methyl protons provide a local, nonlabile reference, whereas water protons report exchange-averaged dynamics. Combining proton PFG-NMR with proton-proton EXSY reveals a power-law scaling between the diffusivity contrast of water and DMSO protons and the ratio of the first-shell residence time of water protons to the PFG-NMR diffusion observation time. In this model system, the scaling indicates that exchange-corrected water-proton diffusivity approaches the molecular-water diffusion limit only when the residence time exceeds the observation time by about a factor of 2.