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Development of methods and theory of DNP enhanced NMR relaxometry for the study of complex fluids and porous media

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Analytical Chemistry
Experimental Condensed Matter Physics
Experimental and Theoretical Physics of Polymers
Term from 2017 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 384109209
 
Final Report Year 2024

Final Report Abstract

Combining NMR relaxometry with Dynamic Nuclear Polarization (DNP), i.e. magnetization transfer from electrons to nuclear spins, is not only employed to achieve up to a thousandfold increase of measurement sensitivity, but more importantly for a better understanding of the mobility of small and large molecules. This approach extends the conventional application spectrum of commercial DNP-NMR solutions by the frequency dependence: relaxometry with variable magnetic fields typically measures a frequency range of 103 to 108 Hz, but taking the behavior of the electron spins in added stable radicals also into account will extend this range by a factor of 1000 at the upper end. Suitable experiments therefore do not only measure the spectrum of motions of the target molecule itself, but also the timescale of interaction of the rigid probe molecule with an unpaired electron. DNP relaxometry is based on the construction of a double-resonant receiver that is sensitive to both the Larmor frequencies of the electrons and the different nuclear isotopes involved, it therefore must allow the generation of standing microwaves as well as the variation of an NMR receiver coil with matching characteristic frequency. Sampling the desired information in a frequency spectrum covering several orders of magnitude further requires integration into a magnet with rapidly variable field strength in order to measure spin lifetimes that are often on the scale of milliseconds. The presented project contains, on the one hand, design and construction of such an apparatus, on the other hand its application to current topics of molecular dynamics which cannot, or not yet, be solved satisfactorily by MD computer simulations alone. Two liquids being fully miscible under normal conditions appear to undergo a phase separation when placed inside of mesopores, i.e. structures measuring a few nanometers in size – DNP employing radicals is able to address the question whether this separation is indeed complete, and which liquid will preferentially be accumulated at the interface. This topic is of outstanding importance for understanding the behavior of two liquids inside a catalyst pellet and for optimizing reactor efficiency. Similar questions arise for ionic liquids which are gaining importance in reaction and battery technology: how strong is the interaction between adjacent anions and cations, does clustering occur and how does this influence the electric conductivity of the medium? DNP provides information on a molecular scale but also when it comes to macroscopic diffusion properties within a membrane. One further, of potentially many, applications looks at large molecules, either biologically relevant proteins or synthetic polymers: DNP relaxometry is able, by a suitable choice of the interaction of a series of radical containing molecules, to dock to different positions within the macromolecule and selectively probe order and mobility in the immediate vicinity. It therefore expands our understanding not only of the structure, but of dynamics/function relationships of macromolecules of any kind.

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