Contribution of rotational diffusion to pulsed field gradient diffusion measurements

Area of Science:

• Physical Chemistry
• Analytical Chemistry
• Materials Science

Background:

• Nuclear Magnetic Resonance (NMR) diffusion experiments with pulsed field gradients are standard for measuring translational diffusion of nuclear spins.
• Conventional analysis assumes diffusion is purely translational, potentially overlooking other motional contributions.

Purpose of the Study:

• To demonstrate that rotational motion significantly impacts NMR diffusion measurements under specific conditions.
• To show how this motional contribution can lead to anomalously large apparent diffusion coefficients.
• To establish a method for characterizing species dimensions using the dependence of the effective diffusion coefficient on diffusion delay.

Main Methods:

• Utilized NMR diffusion experiments with pulsed field gradients.
• Analyzed experimental data under conditions where species dimensions exceed the root mean square displacement of translational diffusion.
• Investigated the relationship between the effective diffusion coefficient and the duration of the diffusion delay.

Main Results:

• Demonstrated significant contribution of rotational motion to NMR diffusion data when species dimensions are large relative to translational displacement.
• Observed anomalously large apparent diffusion coefficients when conventional analytical methods are applied in these cases.
• Found that the effective diffusion coefficient is dependent on the diffusion delay time.

Conclusions:

• Rotational motion must be considered in NMR diffusion experiments, especially for larger species.
• The dependence of the effective diffusion coefficient on diffusion delay provides a novel method for dimensional characterization.
• This approach enhances the utility of NMR diffusion for probing molecular and supramolecular structure.