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

在过去的二十年中，魔角旋转（MAS）固态NMR光谱已发展成为广泛的有机化合物，无机和混合材料的结构表征的基石技术。然而，NMR的主要缺点是其固有的低灵敏度，阻止了其在材料科学中许多最令人兴奋的领域的使用。动态核极化（DNP）已成为克服NMR灵敏度问题的最有前途的方法。在DNP实验中，未成对电子的大极化在微波辐射时转移到周围的核。为此目的，已经开发了大量的稳定自由基作为极化源，其在5-9.4 T的磁场下提供两个数量级的信号增强。然而，最常用的极化剂，依赖于交叉效应的二氮氧化物，遭受它们对磁场的不利依赖，因为当进入非常高的场时，观察到增强的急剧减少。在快MAS频率下，它们的性能由于去极化过程而进一步衰减，这降低了NMR实验的真实的灵敏度。为了进一步扩大DNP增强固体NMR谱的应用范围，需要克服这些主要瓶颈。本项目致力于开发高磁场和快速MAS下的新DNP MAS NMR方法，用于具有挑战性的材料的结构表征。在第一个任务中，将通过评估一系列新的混合交叉效应自由基的性能来开发用于高场交叉效应DNP NMR的新的极化剂，所述混合交叉效应自由基预期在高场下优于当前的二氮氧化物自由基（18.8 T）。在第二步中，将引入用于奥弗豪瑟效应DNP的新样品制剂，最后，将在高场和快速MAS下的DNP固体NMR方法应用于表面有机金属催化剂。新开发的策略将用于表征负载在氧化铝-二氧化硅载体上的烯烃复分解催化剂中活性中心的详细结构，更具体地说，在分子水平上理解表面在催化活性中的作用。预计DNP MAS NMR本身领域将取得进展，从引入为高场DNP NMR定制的新极化源，涉及样品配方和在DNP条件下实现现有技术的脉冲序列。此外，该项目将提供对氧化铝-二氧化硅表面结构的新见解，并更好地理解它们在活化负载型有机金属催化剂中的关键作用。

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.