Balancing dipolar and exchange coupling in biradicals to maximize cross effect dynamic nuclear polarization
Balancing dipolar and exchange coupling in biradicals to maximize cross effect dynamic nuclear polarization
复制标题
DOI：
10.1039/d0cp02051f
复制
发表时间：
2020-06-28
影响因子：
3.3
通讯作者：
Han, Songi
中科院分区：
文献类型：
--
作者：
Equbal, Asif;Tagami, Kan;Han, Songi
交叉效应（CE）引起的动态核极化（DNP）已经成为固态核磁共振（NMR）光谱学的一个改变者。CE-DNP的效率取决于双自由基极化剂中电子-电子耦合的强度。因此，最近的焦点是设计具有大的净交换（J）和偶极（D）耦合的双自由基。在这项研究中，我们发现，CE-DNP的关键因素不是大的总和，J + D，而是相对大小的J和D，表示为J/D比。我们指出这种干涉的机理基础分别在于J和D耦合的各向同性和各向异性性质。这种干扰可以导致一个小的（有效的）电子-电子耦合的许多方向，即使当J + D是大的，导致非绝热转子事件。当0 <|J/d| <1，CE-DNP效率在大多数取向上衰减，在较高磁场和较快魔角旋转（MAS）频率下观察到较大衰减。本研究中引入的JandD耦合的干扰效应可以解释为什么许多具有高或相当J + D的双自由基仍然表现出显着不同的DNP性能。我们推出J/Das作为设计有效双自由基的重要标准，以在大范围的B（0）场和MAS频率上稳健地执行。
Dynamic nuclear polarization (DNP) by the cross effect (CE) has become a game changer for solid-state nuclear magnetic resonance (NMR) spectroscopy. The efficiency of CE-DNP depends on the strength of the electron-electron coupling in biradical polarizing agents. Hence, the focus lately has been on designing biradicals with a large net exchange (J) and dipolar (D) coupling. In this study, we reveal that the crucial factor for CE-DNP is not the large sum,J+D, but rather the relative magnitude ofJandD, expressed as theJ/Dratio. We show that the mechanistic basis of this interference lies in the isotropicvs.the anisotropic nature of theJandDcouplings, respectively. This interference can lead to a small (effective) electron-electron coupling for many orientations even whenJ+Dis large, resulting in non-adiabatic rotor-events. We find that when 0 < |J/D| < 1 the CE-DNP efficiency is attenuated for the majority of orientations, with greater attenuation observed at higher magnetic fields and faster magic-angle spinning (MAS) frequency. The interference effect ofJandDcoupling introduced in this study can explain why many biradicals with high or comparableJ+Dstill show significantly divergent DNP performances. We debutJ/Das a consequential criteria for designing efficient biradicals to robustly perform across a large range ofB(0)fields and MAS frequencies.