Microstructural Geometry Revealed by NMR Lineshape Analysis

TL;DR

This paper presents a novel NMR lineshape analysis technique that reveals microstructural geometry at angstrom-scale resolution using weak magnetic field gradients, surpassing traditional methods reliant on strong gradients.

Contribution

The authors introduce a new NMR analysis approach that achieves high-resolution microstructural imaging with weak gradients, expanding capabilities beyond existing q-space PFG NMR methods.

Findings

Simulated gas diffusion in carbon nanotubes shows confinement effects in NMR lineshapes.

The method achieves angstrom-scale resolution in structural analysis.

Coupling MD simulations with Langevin equations enables accurate transport property estimation.

Abstract

We introduce a technique for extracting microstructural geometry from NMR lineshape analysis in porous materials at angstrom-scale resolution with the use of weak magnetic field gradients. Diverging from the generally held view of FID signals undergoing simple exponential decay, we show that a detailed analysis of the line shape can unravel structural geometry on much smaller scales than previously thought. While the original q-space PFG NMR relies on strong magnetic field gradients in order to achieve high spatial resolution, our current approach reaches comparable or higher resolution using much weaker gradients. As a model system, we simulated gas diffusion for xenon confined within carbon nanotubes over a range of temperatures and nanotube diameters in order to unveil manifestations of confinement in the diffusion behavior. We report a multiscale scheme that couples the above MD simulations with the generalized Langevin equation to estimate the transport properties of interest for this problem, such as diffusivity coefficients and NMR lineshapes, using the Green-Kubo correlation function to correctly evaluate time-dependent diffusion. Our results highlight how NMR methodologies can be adapted as effective means towards structural investigation at very small scales when dealing with complicated geometries. This method is expected to find applications in materials science, catalysis, biomedicine and other areas.