Armstrong Brandon D, Han Songi
Department of Physics, University of California, Santa Barbara, California 93106, USA.
J Chem Phys. 2007 Sep 14;127(10):104508. doi: 10.1063/1.2770465.
Nitroxide free radicals are the most commonly used source for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) experiments and are also exclusively employed as spin labels for electron spin resonance (ESR) spectroscopy of diamagnetic molecules and materials. Nitroxide free radicals have been shown to have strong dipolar coupling to (1)H in water, and thus result in large DNP enhancement of (1)H NMR signal via the well known Overhauser effect. The fundamental parameter in a DNP experiment is the coupling factor, since it ultimately determines the maximum NMR signal enhancements which can be achieved. Despite their widespread use, measurements of the coupling factor of nitroxide free radicals have been inconsistent, and current models have failed to successfully explain our experimental data. We found that the inconsistency in determining the coupling factor arises from not taking into account the characteristics of the ESR transitions, which are split into three (or two) lines due to the hyperfine coupling of the electron to the (14)N nuclei (or (15)N) of the nitric oxide radical. Both intermolecular Heisenberg spin exchange interactions as well as intramolecular nitrogen nuclear spin relaxation mix the three (or two) ESR transitions. However, neither effect has been taken into account in any experimental studies on utilizing or quantifying the Overhauser driven DNP effects. The expected effect of Heisenberg spin exchange on Overhauser enhancements has already been theoretically predicted and observed by Bates and Drozdoski [J. Chem. Phys. 67, 4038 (1977)]. Here, we present a new model for quantifying Overhauser enhancements through nitroxide free radicals that includes both effects on mixing the ESR hyperfine states. This model predicts the maximum saturation factor to be considerably higher by the effect of nitrogen nuclear spin relaxation. Because intramolecular nitrogen spin relaxation is independent of the nitroxide concentration, this effect is still significant at low radical concentrations where electron spin exchange is negligible. This implies that the only correct way to determine the coupling factor of nitroxide free radicals is to measure the maximum enhancement at different concentrations and extrapolate the results to infinite concentration. We verify our model with a series of DNP experimental studies on (1)H NMR signal enhancement of water by means of (14)N as well as (15)N isotope enriched nitroxide radicals.
氮氧自由基是动态核极化（DNP）增强核磁共振（NMR）实验中最常用的来源，并且还专门用作抗磁性分子和材料的电子自旋共振（ESR）光谱的自旋标记。已表明氮氧自由基与水中的（1）H有很强的偶极耦合，因此通过众所周知的Overhauser效应导致（1）H NMR信号的大幅DNP增强。DNP实验中的基本参数是耦合因子，因为它最终决定了可以实现的最大NMR信号增强。尽管它们被广泛使用，但氮氧自由基耦合因子的测量结果并不一致，并且当前的模型未能成功解释我们的实验数据。我们发现确定耦合因子时的不一致源于未考虑ESR跃迁的特性，由于电子与一氧化氮自由基的（14）N核（或（15）N）的超精细耦合，ESR跃迁被分裂为三条（或两条）谱线。分子间的海森堡自旋交换相互作用以及分子内氮核自旋弛豫都会混合这三条（或两条）ESR跃迁。然而，在任何关于利用或量化Overhauser驱动的DNP效应的实验研究中，都没有考虑到这两种效应。海森堡自旋交换对Overhauser增强的预期效应已经在理论上被预测，并由Bates和Drozdoski观察到[J. Chem. Phys. 67, 4038 (1977)]。在这里，我们提出了一种通过氮氧自由基量化Overhauser增强效应的新模型，该模型包括了对混合ESR超精细态的两种效应。该模型预测，由于氮核自旋弛豫的影响，最大饱和因子会显著更高。因为分子内氮自旋弛豫与氮氧自由基浓度无关，所以在电子自旋交换可忽略不计的低自由基浓度下，这种效应仍然很显著。这意味着确定氮氧自由基耦合因子的唯一正确方法是测量不同浓度下的最大增强，并将结果外推到无限浓度。我们通过一系列关于利用（14）N以及（15）N同位素富集的氮氧自由基对水的（1）H NMR信号增强的DNP实验研究验证了我们的模型。
