Pell Andrew J
Department of Materials and Environmental Chemistry, Stockholm University, Svänte Arrhenius väg 16 C, 106 91 Stockholm, Sweden; Centre de RMN Trés Hauts Champs de Lyon (UMR5082 CNRS/ENS-Lyon/Université Claude Bernard Lyon 1), Université de Lyon, 5 rue de la Doua, 69100 Villeurbanne, France.
J Magn Reson. 2021 May;326:106939. doi: 10.1016/j.jmr.2021.106939. Epub 2021 Feb 19.
For paramagnetic species, it has been long understood that the hyperfine interaction between the unpaired electrons and the nucleus results in a nuclear magnetic resonance (NMR) peak that is shifted by a paramagnetic shift, rather than split by the coupling, due to an averaging of the electronic magnetic moment caused by electronic relaxation that is fast in comparison to the hyperfine coupling constant. However, although this feature of paramagnetic NMR has formed the basis of all theories of the paramagnetic shift, the precise theory and mechanism of the electronic relaxation required to predict this result has never been discussed, nor has the assertion been tested. In this paper, we show that the standard semi-classical Redfield theory of relaxation fails to predict a paramagnetic shift, as does any attempt to correct for the semi-classical theory using modifications such as the inhomogeneous master equation or Levitt-di Bari thermalization. In fact, only the recently-introduced Lindbladian theory of relaxation in magnetic resonance [J.Magn.Reson., 310, 106645 (2019)] is able to correctly predict the paramagnetic shift tensor and relaxation-induced linewidth in pNMR. Furthermore, this new formalism is able to predict the NMR spectra of paramagnetic species outside the high-temperature and weak-order limits, and is therefore also applicable to dynamic nuclear polarization. The formalism is tested by simulations of five case studies, which include Fermi-contact and spin-dipolar hyperfine couplings, g-anisotropy, zero-field splitting, high and low temperatures, and fast and slow electronic relaxation.
对于顺磁物质，长期以来人们都明白，未成对电子与原子核之间的超精细相互作用会导致核磁共振（NMR）峰发生顺磁位移，而不是由于超精细耦合常数相比电子弛豫较快时电子磁矩的平均作用而被耦合分裂。然而，尽管顺磁NMR的这一特征构成了所有顺磁位移理论的基础，但预测这一结果所需的电子弛豫的精确理论和机制从未被讨论过，这一论断也从未得到检验。在本文中，我们表明，标准的半经典雷德菲尔德弛豫理论无法预测顺磁位移，使用诸如非均匀主方程或莱维特 - 迪巴里热化等修正来校正半经典理论的任何尝试也同样无法预测。事实上，只有最近引入的磁共振中的林德布拉德弛豫理论[《磁共振杂志》，310, 106645 (2019)]能够正确预测顺磁NMR中的顺磁位移张量和弛豫诱导线宽。此外，这种新形式能够预测高温和弱序极限之外的顺磁物质的NMR光谱，因此也适用于动态核极化。通过对五个案例研究的模拟对该形式进行了检验，这些案例包括费米接触和自旋 - 偶极超精细耦合、g 各向异性、零场分裂、高温和低温以及快速和慢速电子弛豫。
