Overhauser effects in insulating solids
Overhauser effects in insulating solids
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DOI：
10.1063/1.4891866
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发表时间：
2014-08-14
影响因子：
4.4
通讯作者：
Griffin, R. G.
中科院分区：
文献类型：
--
作者：
Can, T. V.;Caporini, M. A.;Griffin, R. G.
我们报告魔角自旋，动态核极化（DNP）实验在9.4 T，14.1 T，和18.8 T的磁场下使用窄线极化剂1，3-bisdiphenylene-2-phenylallyl（BDPA）分散在聚苯乙烯中，和磺化BDPA（SA-BDPA）和三苯甲基OX 063在玻璃甘油/水矩阵。BDPA自由基的H-1 DNP增强场分布表现出显著的DNP Overhauser效应（OE）以及固体效应（SE），尽管这些样品是绝缘固体。相比之下，三苯甲基仅表现出SE增强。数据表明，OE的出现是由于BDPA和SA-BDPA中存在相当强的电子-核超精细耦合，这在三苯甲基和全氘代BDPA（d（21）-BDPA）中不存在。此外，与其他DNP机制（如固体效应或交叉效应）相比，实验数据表明，非导电固体中的OE与磁场成正比，从5 T增加到9.4 T，到14.1 T，再到18.8 T。使用由电子超精细耦合到H-1组成的模型二自旋系统的模拟再现了场分布的基本特征，并表明这些样品中的OE起源于分子内离域未成对电子与其相邻核之间的波动超精细相互作用引起的零和双量子交叉弛豫，并且这些超精细耦合的大小对于增强的幅度是至关重要的。微波功率相关的研究表明，OE饱和在相当低的功率水平比固体效应在相同的样品。我们的研究结果提供了新的见解的Overhauser效应的机制，也提供了一种新的方法来执行DNP实验在化学，生物物理和物理系统在高磁场。(C)2014 AIP Publishing LLC.
We report magic angle spinning, dynamic nuclear polarization (DNP) experiments at magnetic fields of 9.4 T, 14.1 T, and 18.8 T using the narrow line polarizing agents 1,3-bisdiphenylene-2-phenylallyl (BDPA) dispersed in polystyrene, and sulfonated-BDPA (SA-BDPA) and trityl OX063 in glassy glycerol/water matrices. The H-1 DNP enhancement field profiles of the BDPA radicals exhibit a significant DNP Overhauser effect (OE) as well as a solid effect (SE) despite the fact that these samples are insulating solids. In contrast, trityl exhibits only a SE enhancement. Data suggest that the appearance of the OE is due to rather strong electron-nuclear hyperfine couplings present in BDPA and SA-BDPA, which are absent in trityl and perdeuterated BDPA (d(21)-BDPA). In addition, and in contrast to other DNP mechanisms such as the solid effect or cross effect, the experimental data suggest that the OE in non-conducting solids scales favorably with magnetic field, increasing in magnitude in going from 5 T, to 9.4 T, to 14.1 T, and to 18.8 T. Simulations using a model two spin system consisting of an electron hyperfine coupled to a H-1 reproduce the essential features of the field profiles and indicate that the OE in these samples originates from the zero and double quantum cross relaxation induced by fluctuating hyperfine interactions between the intramolecular delocalized unpaired electrons and their neighboring nuclei, and that the size of these hyperfine couplings is crucial to the magnitude of the enhancements. Microwave power dependent studies show that the OE saturates at considerably lower power levels than the solid effect in the same samples. Our results provide new insights into the mechanism of the Overhauser effect, and also provide a new approach to perform DNP experiments in chemical, biophysical, and physical systems at high magnetic fields. (C) 2014 AIP Publishing LLC.