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Spatial and temporal evolution of dynamic nuclear polarization on different scales

Abstract : Dynamic Nuclear Polarization (DNP) can help to overcome the intrinsically low sensitivity of magnetic resonance experiments. To reach high nuclear polarization in dissolution DNP (d-DNP), a mixture of water and glycerol containing a high concentration of paramagnetic polarizing agents is required. TEMPOL is one of the most commonly used polarizing agent in DNP. At cryogenic temperatures, the EPR spectrum of TEMPOL is very broad. This considerably complicates the description of the mechanisms that are responsible for the transfer of polarization from electrons to protons. The slow electronic spectral diffusion is not sufficient to average the electron spin-lattice relaxation time over the entire spectrum. In this thesis, the anisotropy of T1e has been included in Abragam’s and Borghini’s thermal mixing (TM) model. Comparisons of experimental 1H polarizations with predictions of the TM model demonstrate that an accurate DNP model requires the consideration of the anisotropy of T1e. On the other hand, it is known that water/glycerol mixtures can undergo a time-dependent phase separation in liquid phase. Such a process can generate variations of ca. 20% of the proton polarization in our DNP samples at 4 K in a static magnetic field of 6.7 T. Combining EPR and cryo-TEM experiments helped us to understand the impact of the ripening of “DNP-juice” on the proton polarization that can be achieved by DNP. In addition to these phenomena, the strong coupling between a high-Q probe and the large magnetization of the 1H spins contained in the “DNP-juice” leads to nonlinear behavior of the nuclear magnetization at short times in the form of NMR MASER bursts, and gives rise to NMR signals that can last more than several tens of seconds. Finally, d-DNP is used to hyperpolarize 31P in inorganic phosphates to get a better understanding of the calcium phosphate clustering that occurs at the early stages of calcification.
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Emmanuelle Weber. Spatial and temporal evolution of dynamic nuclear polarization on different scales. Theoretical and/or physical chemistry. Université Paris sciences et lettres, 2018. English. ⟨NNT : 2018PSLEE083⟩. ⟨tel-03395851⟩

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