Enhancement Strategies in NMR Spectroscopy
by Eibe Behrend Dücker
Date of Examination:2018-05-05
Date of issue:2018-05-14
Advisor:Prof. Dr. Christian Griesinger
Referee:Prof. Dr. Christian Griesinger
Referee:Prof. Dr. Franc Meyer
Persistent Address:
http://dx.doi.org/10.53846/goediss-6869
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Abstract
English
Hyperpolarization methods have become a powerful assistance technique to NMR spectroscopy in order to improve its sensitivity and consequently reduce the required measurement time to achieve a satisfactory signal-to-noise ratio. In the SABRE (Signal Amplification by Reversible Exchange) polarization technique, an advancement of parahydrogen-induced polarization (PHIP), hyperpolarization is transferred from parahydrogen molecules to heterocyclic small molecules via a metal template without inducing a chemical change to the substrate. Therein, the efficiency of this polarization transfer depends on several factors, such as substrate exchange rate and, most importantly, the magnitude of the magnetic field, at which the hyperpolarization experiment is performed. In the course of this work, the SABRE technique was applied to a variety of N-heterocyclic compounds in order to assess the effect of changes in the substitution pattern and the basicity of the substrate molecule to the attainable proton enhancement. The hyperpolarization experiments were further performed using three different polarization transfer catalysts to validate if the effects caused by these changes are uniform for diverse catalyst systems. It was experimentally determined that the magnetic fields, at which the polarization transfer is most efficient is primarily determined by the employed polarization transfer catalyst and is comparable for a wide variety of substrates. Differences in the substitution pattern of the N-heterocyclic substrates cause a change in the relative enhancement levels of individual substrate protons. This is due to the mechanism of polarization transfer, in which the substrate protons adjacent to the coordination site experience direct transfer of magnetization from the parahydrogen nuclei, while the remaining substrate protons obtain magnetization by an intramolecular redistribution process depending on the scalar couplings within the molecule. A change in the substitution pattern changes this system of coupling, which therefore affects the dynamics of the redistribution process. Furthermore, the effect of a substituent-induced change in basicity on the attainable enhancement was examined. While a strong influence of the employed substituent was observable, the effect was not uniform for the examined catalyst systems and a clear dependence of the enhancement on the basicity of the substrate molecule could not be derived. Lastly, the synthesis of a solid-phase bound N-heterocyclic carbene ligand to be employed in a SABRE catalyst system for heterogeneous transfer of polarization was approached. Due to synthetical difficulties, the target molecule had to be repeatedly reassessed. In the final stage of this work, however, preliminary hyperpolarization experiments with polymer-bound iridium complexes were successfully conducted.
Keywords: SABRE; PHIP; NMR; Parahydrogen; Hyperpolarization; Spectroscopy; Field Dependence