Entwicklung einer zuverlässigen Beschreibung der Rotations- und Translationsdynamik in ionischen Flüssigkeiten mittels Field Cycling und Hochfeld-NMR Relaxometrie sowie Molekulardynamischen Simulationen
Zusammenfassung der Projektergebnisse
The primary objective of this project was to investigate the rotational and translational dynamics in ionic liquids (ILs) using high-field (HF) and field-cycling (FC) NMR relaxometry together with classical molecular dynamics (MD) simulations. By synthesizing structurally tailored ILs and employing complementary experimental and computational approaches, the study aimed to access a wide range of Larmor frequencies and temperatures. Our approach enabled a critical evaluation of the classical Bloembergen-Purcell-Pound (BPP) NMR relaxation model for isotropic reorientation being applied to ILs and the development of more advanced models to capture more complex dynamic behavior. The broad liquid range of the selected ILs, extending close to the glass transition temperatures, provided the opportunity to resolve anisotropic and internal molecular motions in considerable detail. Subsequently, we aimed to analyze dipolar relaxation (¹H and ¹⁹F) over broad temperature and frequency ranges to dissect intra- and intermolecular contributions to nuclear magnetic relaxation dispersion (NMRD) profiles. Rotational correlations times obtained from intramolecular contributions should be compared to the reorientation of multiple molecular vectors in cations and anions, while translational diffusion coefficients derived from intermolecular contributions should be compared with those from pulsed field gradient NMR and low-frequency dispersion laws. The use of selectively deuterated ILs and 1H/2H dilution experiments should facilitate the suppression of intermolecular ¹H relaxation, enhancing the reliability of intra- and intermolecular spectral density analysis. Finally, one of the major goals was to employ MD simulations, for selected ILs, to calculate correlation functions, coupling parameters, and relaxation rates. By overlapping experimental and simulation temperature ranges, we wanted to test the validity of our relaxation models and gain molecular-level insights into the structure and dynamics of ILs. Overall, we could give suitable answers to most of the questions. In particular, we were successful in reliably dissecting the intra- and intermolecular contributions to NMRD profiles, thereby highlighting the importance of heteronuclear 1H-19F contributions (P1). We could also present a set of ILs were depending on the shape and the rigidity of the ions different kinds of spectral densities have been used to reliably describe the reorientational motions of the ions (P2). In all works a quite good agreement between experimentally obtained rotational correlations times and self-diffusions coefficients with those obtained by MD simulations could be observed. For the MD computation of the frequency-dependent NMR relaxations rates we first started our approach for the well-studied molecular liquid water. Here, we aimed to present a computational framework for reliably determining the frequency-dependent intermolecular and intramolecular NMR dipole-dipole relaxation rates of spin ½ 1H nuclei (P3). Currently, we are transferring this approach to ILs. We initially focus on the IL [C5Py-d₁₆][NTf2], that is fully deuterated at the cation, so that we are able to focus on the determination of both the frequency-dependent intermolecular and intramolecular 19F NMR relaxation rates of the anion including internal rotation of the CF3 groups (unpublished, see section 3.6). Unfortunately, we could not yet realize the 1H/2H dilution experiments of this project. As indicated already in the proposal, the synthesis of differently deuterated ILs is quite challenging and time-consuming. So far, we synthesized 7 different versions of the IL [C2MIm][DCA], that are either deuterated on the imidazolium ring and/or the methyl group and/or the ethyl group. Due to the decreasing 1H spin density several scans are necessary to capture NMRD profiles in good quality, so that these measurements are way more time-consuming than expected. Even though we did not manage to complete this project within the given time frame, all the ILs have been synthesized and about half of the measurements have already been made. We are therefore confident that we will be able to complete this project successfully in the future, albeit with some delay. (in progress, see section 3.3) For analysing all the experiments, CyFi, a matlab-based evaluation tool was developed during this project. So far, it allows for fitting and dissecting 1H and 19F dipole-dipole relaxation rates as well as 7Li quadrupolar relaxation rates based on different kinds of spectral densities, such as BPP, Cole-Davidson, symmetric top, internal rotation and quadrupolar relaxation (unpublished, see section 3.4). Thanks to an easily accessible graphical user interface (GUI), it is conceivable to use this program not only in research, but also in teaching or for new students and researchers to the field of FFC. The influence of individual parameters to the total and partial relaxation rates can be tracked graphically in real time. This tool is not yet freely available, but we aim to make it freely available by the beginning of 2026.
Projektbezogene Publikationen (Auswahl)
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Computing the frequency-dependent NMR relaxation of 1H nuclei in liquid water. The Journal of Chemical Physics, 160(7).
Paschek, Dietmar; Busch, Johanna; Mock, Eduard; Ludwig, Ralf & Strate, Anne
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Translational Dynamics of Cations and Anions in Ionic Liquids from NMR Field Cycling Relaxometry: Highlighting the Importance of Heteronuclear Contributions. The Journal of Physical Chemistry Letters, 15(41), 10410-10415.
Kruse, Lennart; Chiramel Tony, Angel Mary; Paschek, Dietmar; Stange, Peter; Ludwig, Ralf & Strate, Anne
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Beyond isotropic reorientation: probing anisotropic and internal motions in ionic liquids with fast field cycling NMR relaxometry and MD simulations. Physical Chemistry Chemical Physics, 27(21), 10927-10938.
Kruse, Lennart; van Alphen, Tanja; Busch, Johanna; Paschek, Dietmar; Ludwig, Ralf & Strate, Anne
