Solid/liquid intermolecular transfer of dynamic nuclear polarization - enhanced flowing fluid ¹H NMR signals via immobilized spin labels

Abstract

A flow solid/liquid intermolecular transfer (SLIT) ¹H dynamic nuclear polarization (DNP) experiment is described. The SLIT ¹H DNP results are reported for the case of flowing benzene over silica gel surfaces prepared with stable immobilized nitroxide and phenoxy spin labels. The advantages of the flow SLIT DNP experiment in comparison with the recently reported flow liquid/liquid intermolecular transfer (LLIT) ¹H DNP experiment are discussed. The results suggest that the flow SLIT DNP experiment provides a new method for monitoring liquid/surface intermolecular interactions, since the reporter group (flowing benzene) reflects properties of the surface. Potential applications such as: 1) observation of enhanced NMR signals without detrimental line broadening due to the presence of stable free radicals in the NMR detection region, and 2) independent optimization of the "EPR" and "NMR" portions of the DNP experiment, are discussed.

The thrust of this thesis was twofold: 1) to synthesize immobilized free radical systems appropriate for the SLIT DNP measurements, and 2) to establish the factors influencing DNP mechanism(s) for these systems. The resulting ¹H DNP line shapes imply that both Overhauser effect and solid- state effect are effective mechanisms for the DNP enhancements. The results suggest that the relatively low magnitude of the enhancements can be attributed to the longer molecular correlation times of the solid/liquid systems studied. Also, the EPR spectra and the corresponding number of unpaired spins were measured for these immobilized systems.