Wittmann Johannes J, Eckardt Michael, Harneit Wolfgang, Corzilius Björn
Institute of Physical and Theoretical Chemistry, Institute of Biophysical Chemistry, and Biomolecular Magnetic Resonance Center (BMRZ), Goethe University Frankfurt, Max-von-Laue-Str. 7-9, 60438 Frankfurt, Germany.
Phys Chem Chem Phys. 2018 Apr 25;20(16):11418-11429. doi: 10.1039/c8cp00265g.
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.
动态核极化（DNP）可用于将固态核磁共振实验的灵敏度提高几个数量级，这是由于微波驱动自旋极化从不成对电子转移到原子核。虽然在微观层面上，其潜在的量子力学方面已得到充分理解，但对于大规模自旋动力学的确切描述仍未达成共识，这种大规模自旋动力学通常每个电子涉及数百到数千个核自旋。一般认为，核超极化只能在未受到不成对电子强烈影响且因此与顺磁极化剂有显著距离的原子核上观察到。同时，DNP转移需要足够强的超精细相互作用。因此，从强相互作用的原子核到可进行核磁共振观测的主体的有效核自旋扩散被认为是实现有效核超极化的关键。基于以作为在C60中稀疏稀释的极化剂的内嵌富勒烯N@C60所获得的实验结果，我们讨论了自旋扩散势垒的影响。我们引入电子驱动自旋扩散（EDSD）作为在电子自旋附近核极化转移的一种新机制，这在魔角旋转（MAS）DNP条件下尤为相关。
