D2NP - New frontiers in electron enhanced high field solid state NMR for interdisciplinary science and technology

当科学家像所有优秀的侦探一样调查问题时，他们需要关于正在发生的事情的线索。对于一系列关键问题，能够揭示原子周围局部环境的技术对于在这一水平上深入了解结构至关重要。核磁共振（NMR）光谱学的重要性已经增加，因为它是一种元素特异性探针，可以区分不同位点周围的非常小的变化（例如，SiO 4单元连接到结构中的角的数量），这在整个科学中变得非常重要。它的主要缺点是由于核能级之间的小的热衍生布居差异而固有地相对较弱的信号。随着交叉极化的实现，固体的NMR发生了革命性的变化，交叉极化将磁化强度从具有高磁矩的核（例如1H）转移到具有较小磁矩的更稀的核（例如13 C），从而使13 C NMR信号强度增加约4倍。今天，人们在广泛的方法上做出了非常大的努力，试图进一步增加NMR信号的大小，并进行了相当大的投资，以实现甚至百分之几十的增加。动态核极化（DNP）是一种使用未成对电子自旋将NMR信号增强多达100，000的技术。虽然这种效应在理论和低磁场下的实验中已经知道了一段时间，但直到现在才能付诸实践，整个实验都是在高磁场下进行的。这现在是可能的，因为可以制造具有足够柔性和鲁棒性的高场磁体，并且在高频下并且具有足够的功率以使DNP在高达395 GHz下工作的微波（类似于微波炉，尽管频率高得多）的生产变得可行。该提案旨在将该技术整合到一种新仪器中，以便在高达14.1 T的磁场下对固体材料进行DNP，并开发在这些磁场中使用连续波和脉冲DNP的技术。灵敏度的巨大增益将来自DNP效应本身，DNP效应本身在热平衡下可以提供电子的旋磁比与核的旋磁比的比率的潜在增强，对于13 C，因子>2500，结合在~ 90 K下的MAS操作，通过热玻尔兹曼因子进一步增加增强。该仪器将产生DNP的NMR频率远远超过那些尚未报道，因此允许现代高分辨率固态NMR实验进行与增益超过常规NMR的100-1000常规预期。四极核（特别是具有非整数自旋的那些），其构成NMR活性核的>75%，已经在很大程度上被排除在DNP之外，因为核共振在当前DNP磁场（B 0）下太宽。这种二阶四极展宽要求使用高Bo，并且这里提出的仪器将具有足够高的Bo以通过DNP打开他们的研究。该仪器的宽频率能力将为高场DNP的物理学提供新的见解，首次允许在这一新兴领域开发最佳技术。该仪器的多功能性意味着，使用相同的设备，人们也可以进行世界领先的脉冲EPR和ENDOR实验。该项目由结构生物学和燃料电池/电化学技术等重要领域的多学科应用驱动。DNP方法将允许考虑NMR，因为迄今为止，由于样本大小和/或感兴趣的自旋数量有限，灵敏度将禁止其使用。这项技术的发展将对英国在一些关键科学技术领域的研究能力产生直接和深远的影响。

When scientists investigate problems like all good detectives they need clues as to what is happening. For a whole range of key problems, techniques that can reveal the local environment around an atom are crucial to provide insight into the structure at this level. Nuclear Magnetic Resonance (NMR) spectroscopy has increased in importance as it is an element specific probe that can distinguish very small changes in the surroundings of different sites (e.g. the number of corners by which an SiO4 unit is connected into a structure) which has become important throughout the sciences. Its major drawback is the intrinsically relatively weak signal due to the small thermally derived population differences between nuclear energy levels. NMR of solids was revolutionised with the implementation of cross-polarisation that transferred magnetisation from nuclei with high magnetic moments (e.g. 1H) to more dilute nuclei with smaller magnetic moments (e.g. 13C) that yielded a factor of ~4 increase in the 13C NMR signal strength. Today there is very significant effort with a wide range of approaches to try and increase the size of the NMR signal still further and considerable investment to achieve even a few tens of percent increase. Dynamic nuclear polarisation (DNP) is a technique that uses unpaired electron spins to boost the NMR signal by as much as 100,000. Although the effect has been known from theory and experiments at low magnetic fields for sometime, it is only now that this can be put into practice, with the whole experiment carried out at high magnetic field. This is possible now because high field magnets of sufficient flexibility and robustness can be manufactured, and the production of microwaves (similar to a microwave oven although much higher frequency) at high frequencies and with sufficient power for DNP to work at up to 395 GHz is becoming feasible. This proposal seeks to bring this technology together in a new instrument to now carry out DNP at magnetic fields up to 14.1 T on solid materials and to develop the technology to use both continuous wave and pulsed DNP at these fields. Huge gains in sensitivity will result from both the DNP effect itself which in thermal equilibrium, could offer potential enhancements of the ratio of the gyromagnetic ratio of the electron to that of the nucleus, a factor of >2500 for 13C, combined with MAS operation at ~90K further increasing the enhancement via the thermal Boltzmann factor. The instrument would produce DNP at NMR frequencies much beyond those yet reported and thus allow modern high resolution solid state NMR experiments to be undertaken with gains over conventional NMR of 100-1000 routinely expected. Quadrupolar nuclei (especially those with non-integer spins), which make up >75% of the NMR-active nuclei, have largely been precluded from DNP because the nuclear resonance is too broad at current DNP magnetic (Bo) fields. This second-order quadrupolar broadening demands the use of high Bo and the instrument proposed here would have sufficiently high Bo to open up their study by DNP. The wide frequency capability of the instrument would provide new insight into the physics of high field DNP allowing, for the first time, an optimum technology to be developed in this emerging field. The versatility of the instrument proposed means that, with the same equipment, one could also carry out world-leading pulsed EPR and ENDOR experiments. The project is driven by the multidisciplinary applications in areas of huge importance as diverse as structural biology and fuel cell/electrochemistry technology. The DNP approach will allow NMR to be considered where hitherto sensitivity would have prohibited its use because of the sample size and/or the number of spins of interest are limited. The development of this technology would have an immediate and profound effect on UK research capability in a number of key areas of science and technology.