Siaw Ting Ann, Fehr Matthias, Lund Alicia, Latimer Allegra, Walker Shamon A, Edwards Devin T, Han Song-I
Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, USA.
Phys Chem Chem Phys. 2014 Sep 21;16(35):18694-706. doi: 10.1039/c4cp02013h.
For the broadest dissemination of solid-state dynamic nuclear polarization (ssDNP) enhanced NMR as a material characterization tool, the ability to employ generic mono-nitroxide radicals as spin probes is critical. A better understanding of the factors contributing to ssDNP efficiency is needed to rationally optimize the experimental condition for the practically accessible spin probes at hand. This study seeks to advance the mechanistic understanding of ssDNP by examining the effect of electron spin dynamics on ssDNP performance at liquid helium temperatures (4-40 K). The key observation is that bi-radicals and mono-radicals can generate comparable nuclear spin polarization at 4 K and 7 T, which is in contrast to the observation for ssDNP at liquid nitrogen temperatures (80-150 K) that finds bi-radicals to clearly outperform mono-radicals. To rationalize this observation, we analyze the change in the DNP-induced nuclear spin polarization (Pn) and the characteristic ssDNP signal buildup time as a function of electron spin relaxation rates that are modulated by the mono- and bi-radical spin concentration. Changes in Pn are consistent with a systematic variation in the product of the electron spin-lattice relaxation time and the electron spin flip-flop rate that constitutes an integral saturation factor of an inhomogeneously broadened EPR spectrum. We show that the comparable Pn achieved with both radical species can be reconciled with a comparable integral EPR saturation factor. Surprisingly, the largest Pn is observed at an intermediate spin concentration for both mono- and bi-radicals. At the highest radical concentration, the stronger inter-electron spin dipolar coupling favors ssDNP, while oversaturation diminishes Pn, as experimentally verified by the observation of a maximum Pn at an intermediate, not the maximum, microwave (μw) power. At the maximum μw power, oversaturation reduces the electron spin population differential that must be upheld between electron spins that span a frequency difference matching the (1)H NMR frequency-characteristic of the cross effect DNP. This new mechanistic insight allows us to rationalize experimental conditions where generic mono-nitroxide probes can offer competitive ssDNP performance to that of custom designed bi-radicals, and thus helps to vastly expand the application scope of ssDNP for the study of functional materials and solids.
为了将固态动态核极化（ssDNP）增强核磁共振作为一种材料表征工具进行最广泛的传播，使用通用的单硝基自由基作为自旋探针的能力至关重要。需要更好地理解影响ssDNP效率的因素，以便合理优化手头实际可用自旋探针的实验条件。本研究旨在通过研究电子自旋动力学对液氦温度（4 - 40 K）下ssDNP性能的影响，推进对ssDNP机理的理解。关键观察结果是，双自由基和单自由基在4 K和7 T时可产生相当的核自旋极化，这与在液氮温度（80 - 150 K）下ssDNP的观察结果相反，后者发现双自由基明显优于单自由基。为合理解释这一观察结果，我们分析了DNP诱导的核自旋极化（Pn）的变化以及特征性ssDNP信号建立时间随电子自旋弛豫率的变化，而电子自旋弛豫率由单自由基和双自由基自旋浓度调制。Pn的变化与电子自旋 -晶格弛豫时间和电子自旋翻转速率的乘积的系统变化一致，该乘积构成了非均匀展宽EPR谱的积分饱和因子。我们表明，两种自由基物种实现的相当的Pn可以与相当的积分EPR饱和因子相协调。令人惊讶的是，单自由基和双自由基都在中等自旋浓度下观察到最大的Pn。在最高自由基浓度下，更强的电子自旋间偶极耦合有利于ssDNP，而过饱和会降低Pn，这通过在中等而非最大微波（μw）功率下观察到最大Pn得到实验验证。在最大μw功率下，过饱和减少了跨越与（1）H NMR频率匹配的频率差的电子自旋之间必须维持的电子自旋布居差异，这是交叉效应DNP的特征。这种新的机理见解使我们能够合理解释通用单硝基探针可提供与定制设计的双自由基具有竞争力的ssDNP性能的实验条件，从而有助于极大地扩展ssDNP在功能材料和固体研究中的应用范围。
