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，他们需要线索来了解正在发生的事情。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个水平上深入了解结构至关重要。核磁共振（NMR）光谱学的重要性日益增加，因为它是一种元素特异性探针，可以区分不同位置周围的非常小的变化（例如，SiO4单元连接到结构中的角的数量），这在整个科学中已经变得重要。它的主要缺点是本质上相对较弱的信号，由于小热衍生的人口差异之间的核能水平。随着交叉极化的实施，固体的核磁共振发生了革命性的变化，交叉极化将磁化从具有高磁矩（例如1H）的原子核转移到具有较小磁矩（例如13C）的更稀的原子核，从而产生了13C核磁共振信号强度增加约4倍。今天，人们正在努力尝试各种各样的方法来进一步增加核磁共振信号的大小，并投入大量资金来实现几十个百分点的增长。动态核极化（DNP）是一种利用未成对电子自旋将核磁共振信号提高10万倍的技术。虽然这种效应在低磁场下已经从理论和实验中认识了一段时间，但直到现在才能够付诸实践，整个实验都是在高磁场下进行的。这是可能的，因为现在可以制造出足够灵活和坚固的高磁场磁铁，并且在高频率下生产微波（类似于微波炉，但频率要高得多），并且具有足够的功率使DNP在高达395千兆赫的频率下工作正在变得可行。该提案旨在将该技术整合到一种新仪器中，以在高达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。灵敏度的巨大提高将来自DNP效应本身，在热平衡中，DNP效应本身可以提供电子与原子核的回旋磁比的潜在增强，13C时为bbb2500的因子，再加上MAS在~90K下的操作，通过热玻尔兹曼因子进一步增加了增强。该仪器产生的DNP频率将远远超过已报道的频率，从而允许进行现代高分辨率固态核磁共振实验，其增益超过常规核磁共振100-1000的常规预期。四极核（特别是具有非整数自旋的核）占核磁共振活性核的75%，由于核共振在当前的DNP磁场（Bo）下太宽，在很大程度上被排除在DNP之外。这种二阶四极展宽要求使用高Bo，而这里提出的仪器将有足够高的Bo来开启DNP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，从而首次在这一新兴领域开发出最佳技术。该仪器的多功能性意味着，使用相同的设备，也可以进行世界领先的脉冲EPR和ENDOR实验。该项目是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的。由于样本量和/或感兴趣的自旋数有限，迄今为止，由于灵敏度的限制，核磁共振将被考虑使用DNP方法。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而深远的影响。

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.