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NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers

Becher, Manuel ; Lichtinger, Anne ; Minikejew, Rafael ; Vogel, Michael ; Rössler, Ernst A. (2022)
NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers.
In: International Journal of Molecular Sciences, 2022, 23 (9)
doi: 10.26083/tuprints-00022185
Article, Secondary publication, Publisher's Version

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Item Type: Article
Type of entry: Secondary publication
Title: NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers
Language: English
Date: 5 September 2022
Place of Publication: Darmstadt
Year of primary publication: 2022
Publisher: MDPI
Journal or Publication Title: International Journal of Molecular Sciences
Volume of the journal: 23
Issue Number: 9
Collation: 27 Seiten
DOI: 10.26083/tuprints-00022185
Corresponding Links:
Origin: Secondary publication DeepGreen

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of ¹H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole–Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

Uncontrolled Keywords: molecular, ionic and confined liquids, glass transition, nuclear magnetic resonance relaxometry, dielectric spectroscopy
Status: Publisher's Version
URN: urn:nbn:de:tuda-tuprints-221856
Classification DDC: 500 Science and mathematics > 530 Physics
Divisions: 05 Department of Physics > Institute for Condensed Matter Physics
Date Deposited: 05 Sep 2022 13:14
Last Modified: 14 Nov 2023 19:05
SWORD Depositor: Deep Green
URI: https://tuprints.ulb.tu-darmstadt.de/id/eprint/22185
PPN: 498940454
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