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Instrument and Method development for High Field Dynamic Nuclear Polarization with Magic Angle Spinning Spectroscopy at 25 K


The aim of the work presented here has been to optimize the sensitivity of nuclear magnetic resonance (NMR) through the use of the technique of dynamic nuclear polarization (DNP) and magic angle spinning (MAS). DNP is a technique where the polarization from free electron “DNP agent” can be transferred to hyperfine coupled nuclei through microwave irradiation of the sample. The greatest nuclear polarizations are to be expected when the starting thermal electron and nuclear polarization is maximized by operating MAS at a helium cryogenic temperatures. Through a collaboration with Revolution NMR LLC and Bruker Biospin we have development a Cryo DNP-MAS probe operation at a temperature of 25 K and spin rates of 8 kHz. We have measured the microwave beam transmission through each component of the Cryo DNP-MAS probe and optimized the NMR coil and rotor material to maximize the microwave transmission to the sample. We have found that changing the geometry of the radio frequency NMR coil allowing for greater transmission of microwaves doubled the resulting nuclear signal enhancement.

Much of the current development in DNP has focused on tethered nitroxide radicals as the DNP agents, but the design of potent radicals for DNP, in particular under magic angle spinning (MAS) conditions, is still debated and relies on empirical trial and error as the contributing factors for MAS DNP enhancement are not entirely understood. Significant instrumental effort is needed to measure the electron paramagnetic resonance EPR parameters at magnetic fields of 7 T or greater. This work presents the development of an EPR spectrometer at 7 T in order to measure the electron spin dynamics contribution factors to DNP. We have found that the nuclear depolarization induced by MAS is determined by the spin-lattice relaxation time of the nitroxide DNP agent, and once this depolarization is accounted for different tethered nitroxide radical designs have the same DNP signal enhancement. Finally the signal enhancement capabilities of the Han labs home built DNP system is demonstrated through direct enhancement of aluminum spins on the surface of a mesoporous material targeting catalytically active aluminum spins on the surface. When the nitroxide radical is tailored to have a favorable electrostatic interaction with the surface species, the aluminum NMR signal enhancement can be up to 10 fold.

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