Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2012 Aug 7;137(5):054201. doi: 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 ω(0)(-2) 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 ε = 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 (1)H 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 (1)H 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.
五十多年来，固体效应（SE）一直被广泛用作进行动态核极化（DNP）的机制。然而，它并没有在当代高磁场 DNP 实验中得到广泛应用，因为 SE 增强显示出ω(0)(-2)场依赖性。特别是，为了使相邻核自旋态的混合发生，从而允许部分允许名义上禁止的零和双量子 SE 跃迁，需要发生这种混合，这导致了观察到的场依赖性。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，使用有机自由基三苯甲（OX063）获得的ε=91 的增强。这比文献中类似条件下的先前值高 6-7 倍。由于固体效应强烈依赖于微波场强度，我们将这种大的增强归因于样品体积内更大的微波场强度，这是通过更有效地将回旋管与样品室耦合实现的。此外，我们开发了一个理论模型来解释增强核极化的建立速率和稳态增强对微波功率的依赖性。作为（1）H 浓度的函数，测量了 trityl 和 Gd-DOTA 的建立时间和增强。结果的比较表明，对于 trityl，初始极化步骤是较慢的、速率决定步骤。然而，对于 Gd-DOTA，通过同核（1）H 自旋扩散传播核极化是限速步骤。最后，我们讨论了在>5 T 场中应用固体效应的适用性以及解决固体效应不利场依赖性的要求。
