Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel.
J Phys Chem Lett. 2020 Jul 16;11(14):5439-5445. doi: 10.1021/acs.jpclett.0c01527. Epub 2020 Jun 25.
Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing its applications and the quality of NMR spectroscopy as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample, enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low-sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal-ion-based DNP, the polarization agents are distributed in the bulk sample and act as a source of both relaxation and polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted, and the entire sample can be directly polarized. We exploit this finding to measure high-quality NMR spectra of O in the electrode material LiTiO doped with Fe(III) despite its low abundance and long relaxation time.
动态核极化 (DNP) 可显著提高核磁共振 (NMR) 的灵敏度，从而扩展其应用范围，并提高 NMR 光谱作为材料表征工具的质量。核自旋之间的有效自旋扩散被认为对于将超极化扩展到整个样品中至关重要，从而实现大的 DNP 增强。当极化源是外源性自由基时，这种情况主要将低灵敏度核在无机材料中的极化增强限制在表面位置。在基于金属离子的 DNP 中，极化剂分布在整个样品中，并且充当弛豫和极化增强的源。我们发现，只要极化剂是主要的弛豫源，增强就不依赖于核和掺杂剂之间的距离。因此，不再需要有效的自旋扩散，并且可以直接对整个样品进行极化。尽管丰度低且弛豫时间长，但我们利用这一发现测量了掺杂 Fe(III)的 LiTiO 电极材料中 O 的高质量 NMR 谱。
