SOME NEW INSIGHTS INTO THE DYNAMIC NUCLEAR POLARIZATION MECHANISM IN SOLIDS

Dynamic Nuclear Polarization has become an integral part of Magnetic Resonance spectroscopy. This sensitivity enhancement mechanism has been implemented in NMR and MRI experiments for the study of structure and folding of protein, molecular composition of surfaces, metabolic pathways, differentiation between tissues and much more. Introduced more than sixty years ago, DNP relies on our ability to transfer electron polarization to nuclear polarization by microwave saturation of the transitions of unpaired electrons that are coupled to the nuclei of interest. The basic theoretical models explaining the DNP phenomena were established already forty years ago and most experimental observations were interpreted using these models. Only in the last decade or so with the significant increase of NMR and MRI applications involving DNP it has become apparent that the basic theoretical models are not sufficient to account for all experimental observations.

Using our hybrid EPR-NMR spectrometer we have been following the frequency dependence of the electron and nuclear polarizations during MW irradiation on samples containing nitroxide TEMPOL and trityl radicals. Detecting frequency swept ELDOR and DNP enhancement profiles at low temperatures, we showed that the standard Thermal Mixing model for describing DNP is not able to describe our experimental observations.

In this lecture we will present a comprehensive description of the different mechanisms contributing to the DNP enhancement in radical containing amorphous solids, including the indirect and heteronuclear Cross Effects mechanisms. In addition we will report on the influence of sample rotation during Magic Angle Spinning on these processes. All explanations will be accompanied by experimental DNP results.