Mechanisms and Optimization Methods of 13C Dynamic Nuclear Polarization
Niedbalski, Peter James
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Nuclear magnetic resonance (NMR) is a technique that probes the microscopic environment of molecules by investigating the interaction between nuclear magnetic moments and the magnetic field in which they reside. Nuclear magnetic moments, however, are very weak, causing the Boltzmann polarization of nuclei and hence the NMR signal to be very small. This may be resolved through the use of dynamic nuclear polarization (DNP), a process by which high electron polarization is transferred to nuclei through microwave irradiation near the electron resonance. In this work, primary focus is given to dissolution DNP in which nuclei are highly polarized at cryogenic temperature and then rapidly dissolved with a superheated solvent. This process results in a liquid sample whose NMR signal is enhanced many thousand-fold over thermal equilibrium. While applications of this technique are abundant, there are many unanswered questions surrounding the underlying physics and methods of optimization of the DNP process. In this work, some of these open questions are investigated through development of instrumentation and exploration of DNP free radicals. In particular, the construction of two high magnetic field DNP polarizers are discussed, one of which is the first of a new generation of cryogen-free polarizers. Furthermore, the water soluble DNP free radicals TEMPO and trityl are thoroughly investigated with a specific emphasis on the addition of paramagnetic ion complexes. Over the course of these experiments, several paramagnetic agents were tested for the first time and proven to be effective to a similar degree as the field standard gadolinium. Additionally, the link between shortened electronic T1 and improved DNP efficiency was con firmed through the study of transition metal complex dopants to DNP. A number of supporting experiments are also discussed, including Earth's field NMR and classification of free radicals by UV-Vis spectrophotometry and electron paramagnetic resonance. Finally, several basic molecular imaging applications of dissolution DNP are presented highlighting one of the many possible uses of hyperpolarized 13C NMR spectroscopy. Ultimately, this dissertation presents and discusses a number of novel methods by which 13C dynamic nuclear polarization may be optimized, paving the way for further study into the physics and applications of this technique.