National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Dr, Tallahassee, FL 32310, USA.
Phys Chem Chem Phys. 2020 Feb 14;22(6):3643-3652. doi: 10.1039/c9cp06201g. Epub 2020 Jan 30.
Nitroxide biradicals are common polarizing agents used to enhance the sensitivity of solid-state NMR experiments via Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP). These biradicals are used to increase the polarization of protons through the cross-effect mechanism, which requires two unpaired electrons with a Larmor frequency difference greater than that of the protons. From their early conception, the relative orientation of the nitroxide rings has been identified as a critical factor determining their MAS-DNP performance. However, the MAS leads to a complex DNP mechanism with time dependent energy level anti-crossings making it difficult to pinpoint the role of relative g-tensor orientation. In this article, a single parameter called "g-tensors' distance" is introduced to characterize the relative orientation's impact on the MAS-DNP field profiles. It is demonstrated for the first time how the g-tensors' distance determines the nuclear hyperpolarization and depolarization properties of a given biradical. This provides a new critical parameter that paves the way for more efficient bis-nitroxides for MAS-DNP.
氮氧自由基双自由基是常见的极化剂，通过魔角旋转动态核极化（MAS-DNP）用于提高固态 NMR 实验的灵敏度。这些双自由基通过交叉效应机制用于增加质子的极化，该机制需要两个不成对电子，其拉莫尔频率差大于质子的拉莫尔频率。从最初的构想开始，氮氧自由基环的相对取向就被确定为决定其 MAS-DNP 性能的关键因素。然而，MAS 导致具有时间相关能级反交叉的复杂 DNP 机制，使得难以确定相对 g 张量取向的作用。在本文中，引入了一个称为“g 张量距离”的单一参数来描述相对取向对 MAS-DNP 场分布的影响。首次证明了 g 张量距离如何决定给定双自由基的核超极化和去极化特性。这为 MAS-DNP 提供了一个新的关键参数，为更有效的双氮氧自由基铺平了道路。
