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

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Materials Science

Background:

• Proton pulsed field gradient nuclear magnetic resonance (PFG-NMR) is sensitive to water mobility in electrolytes.
• Rapid proton exchange between hydration shells complicates accurate measurement of water diffusion.
• Existing methods struggle to precisely quantify deviations in measured proton diffusivity from true molecular water diffusion.

Purpose of the Study:

• To develop a method for accurately quantifying water diffusion in electrolyte solutions using PFG-NMR.
• To address the challenge of proton exchange obscuring molecular diffusion measurements.
• To establish a reliable reference for correcting proton exchange effects in PFG-NMR studies.

Main Methods:

• Utilized proton PFG-NMR spectroscopy to probe water and DMSO mobility in an Al(OTf)3-H2O-DMSO electrolyte model.
• Employed DMSO methyl protons as a non-labile, first-shell reference for water protons.
• Combined PFG-NMR with proton-proton exchange spectroscopy (EXSY) to analyze exchange dynamics.

Main Results:

• Demonstrated that DMSO methyl protons serve as a local, nonexchangeable reference in the Al3+ solvation shell.
• Revealed a power-law relationship between diffusivity contrast (water vs. DMSO) and the ratio of water proton residence time to PFG-NMR observation time.
• Quantified the condition under which exchange-corrected water proton diffusivity approaches the molecular diffusion limit.

Conclusions:

• A nonexchangeable proton reference, like DMSO methyl protons, effectively corrects for proton exchange artifacts in PFG-NMR.
• Accurate molecular water diffusion can be determined when the water proton residence time is approximately twice the PFG-NMR observation time.
• This approach provides a more precise understanding of water dynamics in complex electrolyte systems.