Radboud University, Institute for Molecules and Materials, Nijmegen, The Netherlands.
Phys Chem Chem Phys. 2011 Oct 21;13(39):17831-40. doi: 10.1039/c1cp22002k. Epub 2011 Sep 6.
Dynamic Nuclear Polarization (DNP) in the liquid state has become the focus of attention to improve the NMR sensitivity of mass limited samples. The Overhauser model predicts a fast reduction in DNP enhancement at high magnetic fields where the Electron Larmor frequency exceeds the typical inverse correlation time of the magnetic interaction between a radical spin and proton spins of the water molecules. Recent experiments have shown that an appreciable DNP enhancement in the liquid state is possible also at magnetic fields of 3 to 9 Tesla. At present it is not clear whether the Overhauser model needs to be adapted to explain these results. In the present paper we aim to resolve this question by a combination of in situ temperature dependent NMR relaxation measurements, EPR and DNP experiments. Enhancement factors of up to -165 are obtained with microwave powers below 500 mW. We conclude that at 3.4 Tesla (95 GHz) the various measurements are consistent with each other and in quantitative agreement with Overhauser theory. Microwave heating of the sample does play an important role to reduce the correlation times and allow a substantial Overhauser DNP. The typical enhancement factors may allow new applications in microfluidic NMR.
动态核极化（DNP）在液相中已成为提高质量有限样品 NMR 灵敏度的研究热点。奥弗豪泽尔模型预测，当电子拉莫尔频率超过自由基自旋与水分子质子自旋之间的磁相互作用的典型逆相关时间时，DNP 增强会在强磁场中迅速降低。最近的实验表明，在磁场为 3 至 9 特斯拉时，在液相中也可以获得相当大的 DNP 增强。目前尚不清楚是否需要调整奥弗豪泽尔模型来解释这些结果。在本文中，我们旨在通过原位温度依赖 NMR 弛豫测量、EPR 和 DNP 实验的结合来解决这个问题。在微波功率低于 500 mW 的情况下，可获得高达-165 的增强因子。我们得出的结论是，在 3.4 特斯拉（95 GHz）下，各种测量结果相互一致，与奥弗豪泽尔理论定量一致。样品的微波加热确实起到了重要作用，可以减少相关时间并允许进行大量的奥弗豪泽尔 DNP。典型的增强因子可能允许在微流控 NMR 中进行新的应用。
