Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot, Israel.
Phys Rev E. 2023 Mar;107(3-2):035303. doi: 10.1103/PhysRevE.107.035303.
J-driven dynamic nuclear polarization (JDNP) was recently proposed for enhancing the sensitivity of solution-state nuclear magnetic resonance (NMR), while bypassing the limitations faced by conventional (Overhauser) DNP at magnetic fields of interest in analytical applications. Like Overhauser DNP, JDNP also requires saturating the electronic polarization using high-frequency microwaves known to have poor penetration and associated heating effects in most liquids. The present microwave-free JDNP (MF-JDNP) proposal seeks to enhance solution NMR's sensitivity by shuttling the sample between higher and lower magnetic fields, with one of these fields providing an electron Larmor frequency that matches the interelectron exchange coupling J_{ex}. If spins cross this so-called JDNP condition sufficiently fast, we predict that a sizable nuclear polarization will be created without microwave irradiation. This MF-JDNP proposal requires radicals whose singlet-triplet self-relaxation rates are dominated by dipolar hyperfine relaxation, and shuttling times that can compete with these electron relaxation processes. This paper discusses the theory behind the MF-JDNP, as well as proposals for radicals and conditions that could enable this new approach to NMR sensitivity enhancement.
J 驱动动态核极化（JDNP）最近被提出用于提高溶液核磁共振（NMR）的灵敏度，同时绕过在分析应用中感兴趣的磁场下传统（Overhauser）DNP 面临的限制。与 Overhauser DNP 一样，JDNP 也需要使用高频微波来饱和电子极化，而这些微波在大多数液体中的穿透性差且存在相关的加热效应。本研究提出的无微波 JDNP（MF-JDNP）方案旨在通过在较高和较低磁场之间来回移动样品来提高溶液 NMR 的灵敏度，其中一个磁场提供与电子间交换耦合 J ex 相匹配的电子拉莫尔频率。如果自旋以足够快的速度穿过这个所谓的 JDNP 条件，我们预测将在不进行微波辐照的情况下产生可观的核极化。MF-JDNP 方案需要自由基，其单重态-三重态自弛豫率由偶极超精细弛豫主导，并且迁移时间能够与这些电子弛豫过程竞争。本文讨论了 MF-JDNP 的理论基础，以及可能实现这种新 NMR 灵敏度增强方法的自由基和条件的建议。
