Electron-driven spin diffusion supports crossing the diffusion barrier in MAS DNP.
Electron-driven spin diffusion supports crossing the diffusion barrier in MAS DNP.
复制标题
DOI：
10.1039/c8cp00265g
复制
发表时间：
2018-04
期刊：
影响因子：
--
通讯作者：
J. J. Wittmann-J.;M. Eckardt;W. Harneit;B. Corzilius
中科院分区：
文献类型：
--
作者：
J. J. Wittmann-J.;M. Eckardt;W. Harneit;B. Corzilius
动态核极化(DNP)可用于将固体核磁共振实验的灵敏度提高几个数量级，这是因为微波驱动的自旋极化从未成对的电子转移到原子核。虽然在微观层面上已经充分了解了基本的量子力学方面，但对于通常涉及每个电子数百到数千个核自旋的大规模自旋动力学的准确描述仍然缺乏共识。通常认为，只有在没有受到未成对电子强烈影响的原子核上才能观察到核超极化，从而显著地从顺磁极化剂中移除。同时，DNP转移需要足够强的超精细相互作用。因此，有效的核自旋从强相互作用核扩散到核磁共振可观测到的块体被认为是有效的核超极化的关键。根据在C_(60)中稀疏稀释的面内富勒烯N@C_(60)作为偏振剂的实验结果，我们讨论了自旋扩散势垒的作用。我们介绍了电子驱动的自旋扩散(EDSD)作为一种在电子自旋附近进行核极化转移的新机制，这在魔角自旋(MAS)DNP条件下尤其重要。
Dynamic nuclear polarization (DNP) can be applied to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude due to microwave-driven transfer of spin polarization from unpaired electrons to nuclei. While the underlying quantum mechanical aspects are sufficiently well understood on a microscopic level, the exact description of the large-scale spin dynamics, usually involving hundreds to thousands of nuclear spins per electron, is still lacking consensus. Generally, it is assumed that nuclear hyperpolarization can only be observed on nuclei which do not experience strong influence of the unpaired electrons and thus being significantly removed from the paramagnetic polarizing agents. At the same time, sufficiently strong hyperfine interaction is required for DNP transfer. Therefore, efficient nuclear spin diffusion from the strongly-interacting nuclei to the NMR-observable bulk is considered to be essential for efficient nuclear hyperpolarization. Based on experimental results obtained on the endohedral fullerene N@C60 as a polarizing agent sparsely diluted in C60, we discuss the effect of the spin-diffusion barrier. We introduce electron-driven spin diffusion (EDSD) as a novel mechanism for nuclear polarization transfer in the proximity of an electron spin which is particularly relevant under magic-angle spinning (MAS) DNP conditions.