1.1 A brief account of the chemical shift The chemical shift of a nucleus, i, in a molecule arises from the nuclear shielding effect of an applied magnetic field, caused by an induced magnetic field resulting from circulation of surrounding electrons [1–6]. The magnitude of such an induced magnetic field is proportional to the strength of the applied external magnetic field B0, so that the effective field Beff at the nucleus is given as Beff=B0(1−σi) (1) where σi is the second-rank nuclear shielding tensor and 1 is the unit matrix. In normal NMR experiments B0 is a uniform field along the z-axis; therefore, σi= σizz. The resonance NMR frequency, νi, of a given nucleus in a molecule is thus related to its gyromagnetic ratio, γi, as given by

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化学位移张量——核磁共振的核心：深入了解蛋白质的生物学方面

1.1 化学位移的简要说明 分子中原子核 i 的化学位移源于外加磁场的核屏蔽效应，由周围电子循环产生的感应磁场引起 [1-6]。这种感应磁场的大小与施加的外部磁场 B0 的强度成正比，因此核处的有效场 Beff 为 Beff=B0(1−σi) (1) 其中 σi 是第二个- rank 核屏蔽张量，1 是单位矩阵。在常规 NMR 实验中，B0 是沿 z 轴的均匀场；因此，σi=σizz。因此，分子中给定核的共振 NMR 频率 νi 与其旋磁比 γi 相关，如下式所示