Heteronuclear DNP of 1H and 19F nuclei using BDPA as a polarizing agent.
Heteronuclear DNP of 1H and 19F nuclei using BDPA as a polarizing agent.
复制标题
DOI：
10.1039/d0cp00892c
复制
发表时间：
2020-04
期刊：
影响因子：
--
通讯作者：
Antonio Gennaro;A. Karabanov;A. Potapov;W. Köckenberger
中科院分区：
文献类型：
--
作者：
Antonio Gennaro;A. Karabanov;A. Potapov;W. Köckenberger
本文研究了以1，3-二苯基-2-苯基烯丙基(BDPA)为极化剂的25/75(%v/v)氟苯/甲苯样品中1H和19F核的动态核极化。以前，研究DNP中的异核效应是通过分析DNP谱的形状，或者通过观察不同类型原子核之间的交叉弛豫来进行的。在这项工作中，我们报告了一个相当特殊的DNP谱，其中1H和19F核在微波(MW)辐射下获得了符号相反的极化。为了解释这一现象，我们引入了一种新的机制，称为异核热混合(hn-TM)。在这个机制中，相反符号的光谱可以由于四自旋系统的存在而得到解释，其中包括一对偶极耦合电子自旋和1H和19F的超精细耦合核自旋，从而满足与它们的拉莫尔频率|ω1E-ω2E|≈ωH-ωF有关的条件。在这种情况下，电子态和核态会发生强烈的混合，从而实现同时的四自旋触发器。用微波辐照电子自旋跃迁，然后用这种四自旋触发器产生|αHβF和|βHαF态的非平衡布居，从而导致1H和19F的相反符号的增强。信号增强、建立时间和DNP光谱作为微波功率和极化剂浓度的函数，都为将观察到的DNP机制指定为hn-TM并将其与其他可能的机制区分开来提供了额外的支持。我们还发展了一个基于自旋哈密顿量平均的HN-TM量子力学模型。基于该模型的模拟结果与实验数据具有很好的定性一致性。此外，该系统在1H和19F之间表现出由BDPA引起的交叉弛豫，这是通过用一系列射频脉冲测量1H核饱和时的19F信号建立来检测的。我们证明，这种交叉弛豫很可能源于四自旋系统中相同的电子态和核态的混合。
This work explores the dynamic nuclear polarization (DNP) of 1H and 19F nuclei in a sample of 25/75 (% v/v) fluorobenzene/toluene containing the radical 1,3-bisphenylene-2-phenylallyl radical (BDPA) as a polarizing agent. Previously, heteronuclear effects in DNP were studied by analysing the shapes of DNP spectra, or by observing cross-relaxation between nuclei of different types. In this work, we report a rather specific DNP spectrum, where 1H and 19F nuclei obtain polarizations of opposite signs upon microwave (MW) irradiation. In order to explain this observation, we introduce a novel mechanism called heteronuclear thermal mixing (hn-TM). Within this mechanism the spectra of opposite signs can then be explained due to the presence of four-spin systems, involving a pair of dipolar coupled electron spins and hyperfine coupled nuclear spins of 1H and 19F, such that a condition relating their Larmor frequencies |ω1e - ω2e| ≈ ωH - ωF is satisfied. Under this condition, a strong mixing of electron and nuclear states takes place, enabling simultaneous four-spin flip-flops. Irradiation of electron spin transitions with MW followed by such four-spin flip-flops produces non-equilibrium populations of |αHβF and |βHαF states, thus leading to the enhancements of opposite signs for 1H and 19F. Signal enhancements, build-up times and DNP-spectra as a function of MW power and polarizing agent concentration, all provide additional support for assigning the observed DNP mechanism as hn-TM and distinguishing it from other possible mechanisms. We also develop a quantum mechanical model of hn-TM based on averaging of spin Hamiltonians. Simulations based on this model show very good qualitative agreement with experimental data. In addition, the system exhibits cross-relaxation between 1H and 19F induced by the presence of BDPA, which was detected by measuring the 19F signal build-up upon saturation of 1H nuclei with a train of radio-frequency pulses. We demonstrate that such cross-relaxation most likely originates due to the same electron and nuclear states mixing in the four-spin systems.