Publication
NMR spectroscopy is a very powerful tool for investigating chemical systems at the atomic level because of its ability to provide information on the local environment and dynamics of the nuclei. Because of this, NMR has become one of the most popular analytical methods for both liquid-state and solid-state samples characterization. However, the application of NMR is hampered by its intrinsic low sensitivity, resulting from the small energy difference between the nuclear spin quantum states.
Hyperpolarization methods can enhance the NMR signal sensitivity by creating a population imbalance of nuclear spin state greater than that at thermal equilibrium. Dynamic nuclear polarization (DNP) is a successful hyperpolarization method that exploits the relatively large polarization of electron spins to hyperpolarize nuclear spins upon microwave irradiation. Thanks to the theory and hardware development, DNP has become a growing topic in NMR and DNP systems have become commercially available.
Although DNP has been proven being successful in enhancing the NMR sensitivity, it still faces several barriers. For example, the instability of the polarizing agent â the source of electron spin polarizationâ hampers the application of DNP in reducing environments such as in cells. Moreover, DNP in liquids is less developed than in solids mainly because at high magnetic fields the Overhauser effect, the only DNP mechanism active in liquids, is efficient only for nuclei in some special environments. The overall object of this thesis is to improve the DNP performances in both solids and liquids by developing DNP methods and novel polarizing agents.
In the first part, the design principle of a Gd(III)-based polarizing agent is discussed. The relation between the structure of Gd(III) complexes and their DNP performances is explored by characterization of their DNP and electron spin properties. Strategies to improve the solubility of Gd(III) complexes in organic solvents are also presented, as well as the possible use of other metal ions. Given that Gd(III) complexes are in general more bioresistant than conventional DNP polarizing agents, we show the preliminary but convincing results of in-cell DNP using Gd(III)-based polarizing agents.
In the second part, applications of Overhauser effect (OE) DNP in liquids are analyzed. Because OE DNP at high magnetic fields is efficient for some 13C nuclear sites but inefficient for 1H spins, we show the feasibility of transferring 13C hyperpolarization generated by OE DNP to adjacent 1H spins via J-couplings. With this method, 1H NMR signals can be enhanced by a factor of 50, which is unachievable by direct DNP. Afterward, by exploiting chemical exchange in solution, we also show that hyperpolarization of triphenylphosphine can be transferred from its free forms to its coordinated form in metal complexes. Enhancement of two orders of magnitude on coordinated triphenylphosphine sites can be obtained by this method.
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