Development of ¹⁹F and ³¹P Dynamic Nuclear Polarization in the Liquid State
by Maik Reinhard
Date of Examination:2025-02-10
Date of issue:2025-12-02
Advisor:Prof. Dr. Marina Bennati
Referee:Prof. Dr. Marina Bennati
Referee:Prof. Dr. Bert De Groot
Persistent Address:
http://dx.doi.org/10.53846/goediss-11656
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Abstract
English
Within the last decades, nuclear magnetic resonance (NMR) spectroscopy in the liquid state has become one of the most powerful techniques allowing for the analysis of structures and dynamics of chemical systems at atomic scale. Regardless of its immense value, NMR is yet vastly limited because of its inherent low sensitivity arising from the small population difference of the nuclear spin states. Overhauser effect dynamic nuclear polarization (OE-DNP) is a hyperpolarization method that aims at enhancing the sensitivity of NMR spectroscopy in the liquid state. Here, the magnetization from higher polarized electron spins is exploited by transfer onto the target nuclei of interest under microwave irradiation at the electron spin Larmor frequency. While the implementation of liquid state OE-DNP for the common ¹H and ¹³C nuclei is ongoing, the focus of this thesis is the development and investigation of ¹⁹F and ³¹P nuclei, that both also find relevant application areas in NMR spectroscopy. It is important to expand the range of target nuclei to reveal the OE-DNP capabilities in the liquid state. Previous insights show that the chemical structure of the polarizing agent and target molecule as well as their interaction dynamics play a decisive role in OE-DNP. Because of this and as large NMR signal enhancements are desired for any magnetic fields, a thorough understanding of the underlying DNP mechanism is necessary. The investigation of the field dependence on the DNP performance is the most reliable way to achieve this goal. Therefore, these target nuclei were investigated at low (¹⁹F and ³¹P) and high magnetic fields (¹⁹F) of 1.2 and 9.4 Tesla with different types of polarizing agents. The obtained results were complemented with reported data from the literature at various magnetic field strengths. With this, the field dependence on the DNP efficiency was fitted showing different performances on the type of target nucleus and employed polarizing agent and the simulations offered insights into the molecular dynamics of the radical-target molecule interaction. The experimental findings were rationalized with the help of density functional theory (DFT) calculations. The results show that weak non-covalent radical-target interactions enable an efficient polarization transfer mediated by lone pair electrons of the ¹⁹F and ³¹P nuclei. Besides the new mechanistic insights into ¹⁹F and ³¹P OE-DNP, considerable ¹⁹F-NMR enhancements on a variety of fluorinated target molecules were obtained at 9.4 T. Moreover, the large generated hyperpolarization on ¹⁹F can be distributed to coupled nuclei in the target molecule in 1D and 2D polarization transfer experiments, paving the way towards possible applications.
Keywords: Hyperpolarization; Overhauser; DNP; ¹⁹F-NMR; ³¹P-NMR; DFT
