Solid effect in magic angle spinning dynamic nuclear polarization
Solid effect in magic angle spinning dynamic nuclear polarization
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DOI：
10.1063/1.4738761
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发表时间：
2012-08-07
影响因子：
4.4
通讯作者：
Griffin, Robert G.
中科院分区：
文献类型：
--
作者：
Corzilius, Bjoern;Smith, Albert A.;Griffin, Robert G.
五十多年来，固体效应（SE）被大量用作执行动态核极化（DNP）的机制。然而，它还没有发现广泛的应用在当代，高磁场DNP实验，因为SE增强显示出一个ω（-2）（0）场的依赖性。特别是，名义上禁止零和双量子SE跃迁被部分允许，它是必要的相邻的核自旋态的混合发生，这导致观察到的字段依赖性。然而，最近我们改进了我们的仪器，并在这里报告了在5 T和80 K下进行的魔角旋转实验中，用有机自由基三苯甲基（OX 063）获得的ε = 91的增强。这比文献中在类似条件下的先前值高6-7倍。由于固体效应强烈地依赖于微波场强度，我们将这种大的增强归因于样品体积内的更大的微波场强度，从而实现了回旋管与样品室的更有效的耦合。此外，我们建立了一个理论模型来解释增强核极化的建立率和稳态增强对微波功率的依赖关系。对于三苯甲基和Gd-DOTA，测量作为H-1浓度的函数的累积时间和增强。结果比较表明，对于三苯甲基来说，初始极化步骤是较慢的速率决定步骤。然而，对于Gd-DOTA，核极化通过homopolyH-1自旋扩散的传播是限速的。最后，我们讨论了固体效应在> 5 T的磁场中的适用性，以及解决固体效应的不利场依赖性的要求。(C)2012年美国物理学会。[http：//dx.doi.org/10.1063/1.4738761]
For over five decades, the solid effect (SE) has been heavily utilized as a mechanism for performing dynamic nuclear polarization (DNP). Nevertheless, it has not found widespread application in contemporary, high magnetic field DNP experiments because SE enhancements display an omega(-2)(0) field dependence. In particular, for nominally forbidden zero and double quantum SE transitions to be partially allowed, it is necessary for mixing of adjacent nuclear spin states to occur, and this leads to the observed field dependence. However, recently we have improved our instrumentation and report here an enhancement of epsilon = 91 obtained with the organic radical trityl (OX063) in magic angle spinning experiments performed at 5 T and 80 K. This is a factor of 6-7 higher than previous values in the literature under similar conditions. Because the solid effect depends strongly on the microwave field strength, we attribute this large enhancement to larger microwave field strengths inside the sample volume, achieved with more efficient coupling of the gyrotron to the sample chamber. In addition, we develop a theoretical model to explain the dependence of the buildup rate of enhanced nuclear polarization and the steady-state enhancement on the microwave power. Buildup times and enhancements were measured as a function of H-1 concentration for both trityl and Gd-DOTA. Comparison of the results indicates that for trityl the initial polarization step is the slower, rate-determining step. However, for Gd-DOTA the spread of nuclear polarization via homonuclear H-1 spin diffusion is rate-limiting. Finally, we discuss the applicability of the solid effect at fields > 5 T and the requirements to address the unfavorable field dependence of the solid effect. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4738761]