Reversal of Paramagnetic Effects by Electron Spin Saturation
Reversal of Paramagnetic Effects by Electron Spin Saturation
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DOI：
10.1021/acs.jpcc.8b00312
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发表时间：
2018-03-15
影响因子：
3.7
通讯作者：
Han, Songi
中科院分区：
文献类型：
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作者：
Jain, Sheetal K.;Siaw, Ting A.;Han, Songi
我们提出了一项研究，其中既显着的动态核极化（DNP）增强的Li-7 NMR和顺磁效应（PE）的逆转是通过微波（μ W）辐照诱导的氮氧自由基的电子自旋饱和在液氦温度下实现的。PE的反转表现在Li-7 NMR谱线的显著变窄和DNP条件下顺磁化学位移的反转。PE的程度被发现减少与增加的饱和度的电子顺磁共振线，调制的微波（μ W）功率，频率，照射时间，和门控时间之间的μ W照射和NMR检测的函数。定义的观察是缩短的电子相位记忆时间，T-m，激发观察者自旋与增加μ W照射和并发电子自旋饱和的电子自旋浴。这和一系列的确证研究揭示了NMR谱线变窄的起源是顺磁弛豫增强（PRE）的逆转，这使我们首次提出了电子自旋饱和（REPRESSION）对PRE的抑制（REPRESSION）一词。任何顺磁系统的电子T-m的缩短作为电子自旋饱和度的函数，迄今为止还没有报道，使得REPRESSION成为这项研究的发现。顺磁偶极位移的反转是由于电子自旋有序度的降低，也由电子自旋饱和促进。这项研究提供了新的基本见解PE在DNP条件下和一种方法来检测和识别NMR信号近端的顺磁性网站减少或最小的线增宽。
We present a study in which both significant dynamic nuclear polarization (DNP) enhancement of Li-7 NMR and reversal of the paramagnetic effects (PEs) are achieved by microwave (mu w) irradiation-induced electron spin saturation of nitroxide radicals at liquid-helium temperatures. The reversal of the PE was manifested in significant narrowing of the Li-7 NMR line and reversal of the paramagnetic chemical shift under DNP conditions. The extent of the PE was found to decrease with increased saturation of the electron paramagnetic resonance line, modulated as a function of microwave (mu w) power, frequency, duration of irradiation, and gating time between mu w irradiation and NMR detection. The defining observation was the shortening of the electron phase memory time, T-m, of the excited observer spins with increasing mu w irradiation and concurrent electron spin saturation of the electron spin bath. This and a series of corroborating studies reveal the origin of the NMR line narrowing to be the reversal of paramagnetic relaxation enhancement (PRE), leading us to debut the term REversal of PRE by electron Spin SaturatION (REPRESSION). The shortening of electron T-m of any paramagnetic system as a function of electron spin saturation has not been reported to date, making REPRESSION a discovery of this study. The reversal of the paramagnetic dipolar shift is due to the decrease in electron spin order, also facilitated by electron spin saturation. This study offers new fundamental insights into PE under DNP conditions and a method to detect and identify NMR signal proximal to paramagnetic sites with reduced or minimal line broadening.