Dipolar Filtered magic-sandwich-echoes as a tool for probing molecular motions using time domain NMR

TL;DR

This paper introduces a low-field $^1$H NMR method combining a dipolar filter and magic-sandwich-echo to analyze molecular motions, enabling the extraction of dynamic parameters like correlation times and activation energies.

Contribution

The study develops a simple NMR approach using dipolar filtering and magic-sandwich-echoes to probe molecular motions and estimate their parameters from temperature-dependent signal analysis.

Findings

Effective suppression of signals from immobile segments.

Quantitative estimation of molecular motion parameters.

Application of Tikhonov regularization for distribution analysis.

Abstract

We present a simple $^1$H NMR approach for characterizing intermediate to fast regime molecular motions using $^1$H time-domain NMR at low magnetic field. The method is based on a Goldmann Shen dipolar filter (DF) followed by a Mixed Magic Sandwich Echo (MSE). The dipolar filter suppresses the signals arising from molecular segments presenting sub kHz mobility, so only signals from mobile segments are detected. Thus, the temperature dependence of the signal intensities directly evidences the onset of molecular motions with rates higher than kHz. The DF-MSE signal intensity is described by an analytical function based on the Anderson Weiss theory, from where parameters related to the molecular motion (e.g. correlation times and activation energy) can be estimated when performing experiments as function of the temperature. Furthermore, we propose the use of the Tikhonov regularization for estimating the width of the distribution of correlation times.