The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

Area of Science:

• Physical Chemistry
• Analytical Chemistry
• Materials Science

Background:

• Nuclear Magnetic Resonance (NMR) techniques enable routine measurement of molecular self-diffusion coefficients (D).
• Quantitative interpretation of diffusion NMR data, especially for small molecules, remains a significant challenge.
• The Stokes-Einstein equation often fails to accurately predict diffusion coefficients for small molecules.

Purpose of the Study:

• To explain the limitations of the Stokes-Einstein equation in predicting small molecule diffusion.
• To present and compare three complementary methods for the quantitative interpretation of diffusion NMR data.
• To demonstrate the broad applicability of diffusion NMR across various chemical systems.

Main Methods:

• Review of the theoretical basis for the failure of the Stokes-Einstein equation.
• Description of two power-law-based methods using different reference molecules.
• Explanation of a third method that directly addresses Stokes-Einstein uncertainties.

Main Results:

• The Stokes-Einstein equation's inadequacy for small molecule diffusion is detailed.
• Three distinct, complementary approaches for quantitative diffusion analysis are presented.
• Examples illustrate the application of these methods across diverse chemical systems.

Conclusions:

• Quantitative interpretation of diffusion NMR data for small molecules requires methods beyond the standard Stokes-Einstein equation.
• The reviewed methods offer robust alternatives for obtaining accurate diffusion coefficients.
• Diffusion NMR, when analyzed with appropriate models, is a powerful tool for chemical research.