Hyperpolarization with parahydrogen in NMR

• Hyperpolarisation mit Parawasserstoff in der NMR

Lehmkuhl, Sören; Blümich, Bernhard (Thesis advisor); Appelt, Stephan (Thesis advisor); Theis, Thomas (Thesis advisor)

Aachen (2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

Parahydrogen based hyperpolarization can drastically boost NMR signals and merges many disciplines from science and technology. Especially, the relatively new SABRE approach is promising as it enables easy, affordable and repetitive hyperpolarization of suitable target molecules. Despite many successful efforts towards broad applicability, especially in the context of biocompatibility have already been taken, it is still a growing topic with vast untapped potential. To advance and broaden this field, four hitherto not accomplished steps were chosen for this work: Heterogeneous catalysts allow for convenient recovery from reaction mixtures and reuse compared to the often toxic homogeneous catalysts. For this purpose, homogeneous PHIP catalysts were anchored on Al2O3 using a heteropolyacid as a linker. This modular system, known for efficient and enantioselective hydrogenations at negligible leaching, is evaluated for its potential in parahydrogen experiments. The activation, experimental procedures and operating conditions for the immobilized catalysts were optimized in a model system and extended to various exemplary substrates and catalysts. Additionally, a suitable choice of the nature and particle size of the support material turned out to be essential. Only PHIP mechanisms based on p-H2 addition and replacement in target substrates were observed so far. Thus, experiments with alternate immobilization approaches with a focus on heterogeneous SABRE and water based systems were part of a sidetrack.Moving towards biomedical applications, water and L-histidine were hyperpolarized with parahydrogen for the first time. The impact of various experimental conditions on the water polarization was studied using a water soluble SABRE catalyst. A special focus is laid on the polarization transfer field, crucial for the polarization of both water and histidine. Based on these results, the underlying spin order transfer mechanisms are elaborated. In addition, a setup for hyperpolarization during continuous flow was introduced. A membrane reactor at its core brought the parahydrogen into solution. Important experimental parameters such as pressure and flow rate of this system are evaluated in proof of principle experiments under continuous flow SABRE over several hours. Finally, a parahydrogen fueled RASER has been discovered by us. Based on the protons of organic molecules and continuously fed by SABRE pumping, this system emits coherent radiation similar to LASERs and MASERs. Its initial nonlinear dynamics is studied by simulations and experiments with various substrates. The impact of heteronuclei such as 13C and 15N on proton RASERs is elaborated. The first self oscillating system based heteronuclear two-spin order is presented, which as a result self-compensates distant dipolar fields. Most excitingly, its multi mode operation, based on spin eigenstates of RASER active molecules, envisions precise chemical structure elucidation and therefore applications not only in the fundamental physics, but in various fields of science and technology.