Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy at Very High Field and Fast MAS

点击翻译按钮获取中文摘要

Over the last two decades, magic angle spinning (MAS) solid-state NMR spectroscopy has evolved into a cornerstone technique for the structural characterization of a broad range of organic compounds, inorganic and hybrid materials. However, the main weakness of NMR is its intrinsically low sensitivity, preventing its use in many of the most exciting areas in materials science. Dynamic Nuclear Polarization (DNP) has emerged as the most promising approach to overcome the sensitivity issue of NMR. In a DNP experiment, the large polarization of unpaired electrons is transferred upon microwave irradiation to surrounding nuclei. For this purpose, a significant number of stable radicals as polarization sources has been developed which provide signal enhancements of two orders of magnitude at magnetic fields of 5–9.4 T. The most commonly used polarizing agents however, binitroxides relying on the Cross Effect, suffer from their unfavorable dependence on the magnetic field, as drastic reductions in enhancements are observed when going to very high field. At fast MAS frequencies, their performance is further attenuated due to depolarization processes, which decrease the real sensitivity of the NMR experiment. These major bottlenecks need to be overcome to further extend the application range of DNP enhanced solid-state NMR spectroscopy.This project is dedicated to the development of new DNP MAS NMR approaches at high magnetic field and fast MAS for the structural characterization of challenging materials. In a first task, new polarizing agents for high field Cross Effect DNP NMR will be developed by evaluating the performance of a new series of mixed Cross Effect radicals which are expected to outperform current binitroxide radicals at high field (18.8 T).In a second step, new sample formulations for Overhauser Effect DNP will be introduced, including the investigation of BDPA derivatives designed for this polarizing scheme as well as the optimization of the glassy matrices.Finally, DNP solid-state NMR methods at high field and fast MAS will be applied to surface organometallic catalysts. The newly developed strategies will be employed to characterize the detailed structure of active sites in alkene-metathesis catalysts supported on alumina-silica supports and more specifically, understand at a molecular level the role of the surface in the catalytic activity.Advances are expected in the field of DNP MAS NMR itself, from the introduction of new polarization sources tailored for high field DNP NMR, to sample formulation and to the implementation of state of the art pulse sequences under DNP conditions. In addition, the project shall provide new insights into the structure of alumina-silica surfaces and a better understanding of their key role in the activation of supported organometallic catalysts.