Investigation of DNP mechanisms : the solid effect

Abstract

Dynamic Nuclear Polarization (DNP) enhances signal to noise in NMR experiments, by transferring the large electron Boltzmann polarization to nuclear polarization, via application of pulsed or continuous-wave microwave irradiation. This results in increases in NMR sensitivity of 2-3 orders of magnitude. DNP greatly reduces experimental times and makes some experiments possible that are otherwise unfeasible due to lack of sensitivity. DNP methods have undergone vast improvements in recent years. However, continued advancement of DNP methods will rely on having a clear understanding of the underlying mechanisms. We develop instrumentation and software intended for the study of DNP mechanisms. This includes a three-channel (e-, 13C, 1H) probe for observing both electrons and nuclei, and a 140 GHz pulsed-EPR spectrometer. We also have developed DNPsim, a program designed for easy quantum-mechanical simulation of basic DNP experiments, combined with the flexibility to customize simulations for more advanced experiments and mechanistic studies. Using these tools, we develop a theoretical framework for the solid effect DNP mechanism, which considers the roles of quantum mechanical and relaxation processes in many-spin systems. NMR experiments under static conditions that monitor nuclear polarization buildup were fit to models of electronnuclear polarization transfer; the results show that nuclei near the electron and the observed (bulk) nuclei compete for electron polarization. Therefore bulk nuclear enhancements are reduced, since nuclei near the electron deplete electron polarization. This result is also reproduced for magic angle spinning NMR experiments. EPR experiments that monitor electron polarization as a function of microwave frequency can be used to measure DNP 'matching conditions'. Experiments utilizing the solid effect show DNP matching conditions that are a result of polarization transfer through many spin, high-order coherences. Previously, it was thought that transfers involving highorder coherences should be highly forbidden, whereas these experiments present strong evidence of their presence. Simulations using DNPsim also show that high-order coherences can play a significant role in DNP polarization transfers in strongly coupled, many-spin systems.