High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals
High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals
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DOI：
10.1063/1.2429658
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发表时间：
2007-01-28
影响因子：
4.4
通讯作者：
Griffin, Robert G.
中科院分区：
文献类型：
--
作者：
Hu, Kan-Nian;Bajaj, Vikram S.;Griffin, Robert G.
在以前的通信[胡，J。Soc.126，10844（2004）]，证明了一种提高在高场动态核极化（DNP）实验中采用的交叉效应极化机制的效率的方法。具体地，表明束缚两个克里思（2，2，6，6-四甲基哌啶-1-氧基）自由基将电子-电子偶极耦合从单体克里思溶液中的类似于1 MHz增加到束缚的双自由基中的类似于25 MHz。较大的耦合导致DNP增强增加了近似3-4倍，从45-50增加到近似165。这里，描述了提高极化过程效率的第二种方法，其涉及近似满足平行于ω（2 e）-ω（1 e）的匹配条件，其中ω（2 e）和ω（1 e）是电子顺磁共振（EPR）谱中的两个频率，ω（n）是被极化的核自旋的拉莫尔频率。具体地，在克里思和三苯甲基[三（8-羧基-2，2，6，6-四甲基（-d3）-苯并[1，2d：4，5-d ']双（1，3）二硫醇-4-基）甲基]自由基的混合物中，这两种物质的EPR谱中的强度最大值大致由H-1 NMR频率分开。在这种情况下，克里思的g（yy）值与三苯甲基的窄伪各向同性g值之间的频率差类似于224 MHz，而H-1拉莫尔频率为211 MHz。发现使用混合物的DNP的最佳磁场与三苯甲基EPR共振一致。在90 K和5 T下，20 mM克里思和20 mM三苯甲基的混合物将H-1极化增强了约160倍，比40 mM克里思的增强约50倍有所改善。改进的原因进行了讨论，并提出证据表明，DNP增强可以进一步提高拴克里思和三苯甲基或两个类似的自由基。(c)2007年，美国物理学会。
In a previous communication [Hu , J. Am. Chem. Soc. 126, 10844 (2004)], an approach was demonstrated that improves the efficiency of the cross-effect polarization mechanism employed in high field dynamic nuclear polarization (DNP) experiments. Specifically, it was shown that tethering two TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) radicals increases the electron-electron dipole coupling from similar to 1 MHz in solutions of monomeric TEMPO to similar to 25 MHz in a tethered biradical. The larger coupling resulted in an increase in the DNP enhancements by a factor of similar to 3-4, from 45-50 to similar to 165. Here, a second approach to improving the efficiency of the polarization process is described that involves approximately satisfying the matching condition parallel to omega(2e)-omega(1e)parallel to=omega(n), where omega(2e) and omega(1e) are two frequencies in the electron paramagnetic resonance (EPR) spectrum and omega(n) is the Larmor frequency of the nuclear spins being polarized. Specifically, in a mixture of TEMPO and trityl [tris (8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) methyl] radicals, the intensity maxima in the EPR spectra of these two species are approximately separated by the H-1 NMR frequency. In this case the frequency difference between the g(yy) value of TEMPO and the narrow pseudo-isotropic g-value of trityl is similar to 224 MHz and the H-1 Larmor frequency is 211 MHz. The optimal magnetic field for DNP using the mixtures was found to coincide with the trityl EPR resonance. At 90 K and 5 T, a mixture of 20 mM TEMPO and 20 mM trityl enhanced the H-1 polarization by a factor of similar to 160, an improvement over the enhancement of similar to 50 with 40 mM TEMPO. The reasons for the improvement are discussed and evidence is presented suggesting that DNP enhancement can be improved further by tethering TEMPO and trityl or two similar radicals. (c) 2007 American Institute of Physics.