Chemical shift tensors: theory and application to molecular structural problems.
Chemical shift tensors: theory and application to molecular structural problems.
复制标题
化学位移张量：分子结构问题的理论与应用。
DOI：
10.1016/j.pnmrs.2010.10.003
复制
发表时间：
2011-05
影响因子：
6.1
通讯作者：
Facelli, Julio C.
中科院分区：
文献类型：
--
作者：
Facelli, Julio C.
也许在成功探测到核磁共振信号之后最重要的发现是观察到核共振频率取决于原子核的化学或电子环境[1，2]，或者正如拉姆齐在他1950年的里程碑式论文[3，4]中所说：“在测量核磁矩时，必须对由外部施加的场引起的分子电子运动产生的磁场进行校正。拉姆齐意识到，仅使用兰姆的抗磁性理论进行修正对分子来说是不够的，因为在分子中存在由二阶顺磁性产生的额外屏蔽贡献。为了解决这个问题，他开发了必要的理论框架来解释并最终计算“化学效应”，这将成为现在通常用于结构解析的化学位移。用微扰理论计算二阶顺磁效应对屏蔽的贡献，五十多年来一直是理论工作者面临的一个挑战。Jameson和de Dios每年都会在《核磁共振光谱学系列年度评论》中报告该领域的进展[5]。在文献中发表的评论文章的完整列表没有在这里列出，但为了参考目的，我们可以注意到最近发表的关于这个主题的评论和书籍[6-9]。磁屏蔽的形式属性在下面讨论，但重要的是要理解，虽然许多科学家认为化学位移或磁屏蔽是与每个共振核相关的数字，实际上屏蔽是一个张量。这意味着分子的电子密度对外部磁场的屏蔽取决于外部磁场和分子的相对取向;因此，屏蔽现象必须用张量而不是标量来描述。当记录固体的NMR谱时，这可以容易地观察到。对于单晶样品，其中分子相对于外部磁场的相对取向可以通过改变晶体相对于外部磁场的取向来宏观地控制，当晶体围绕磁场旋转时，可以在NMR谱的共振线的位置的变化中观察到取向依赖性。在图1中，我们给出了这种行为的一个例子，显示了不同取向的1，3，5-三羟基苯单晶光谱的13 C NMR光谱的巨大差异。
Perhaps the most important discovery after the successful detection of the NMR signal was the observation that nuclear resonance frequencies depend on the chemical or electronic environment of the nuclei [1, 2], or as Ramsey states in his landmark papers [3, 4] of 1950:“In measurements of nuclear magnetic moments, a correction must be made for the magnetic field arising from the motions of the molecular electrons which are induced by the externally applied field.” Ramsey realized that corrections using only Lamb’s diamagnetic theory were inadequate for molecules, because in molecules there are additional shielding contributions arising from the second order paramagnetism. To address this problem he developed the necessary theoretical framework to explain and eventually to calculate the “chemical effect”, which would become the chemical shift commonly used now for structural elucidation. The calculation of the second order paramagnetic contribution to the shielding using perturbation theory has been a challenge to theoreticians for more than fifty years. The progress in this field is reported annually by Jameson and de Dios in the Annual Reviews in NMR Spectroscopy Series [5]. The complete list of review articles published in the literature is not listed here, but for reference purposes we may note recently published reviews and books on the subject [6–9].The formal properties of the magnetic shielding are discussed below, but it is important to understand that while many scientists think about the chemical shift or magnetic shielding as a number associated with each resonant nucleus, in reality the shielding is a tensor quantity. This means that the screening of the external magnetic field by the electron density of the molecule depends on the relative orientation of the external magnetic field and the molecule; therefore, the shielding phenomena has to be described by a tensor instead of a scalar number. This can be easily observed when recording the NMR spectra of solids. For single crystal samples, where the relative orientation of the molecules with respect to the external magnetic field can be macroscopically controlled by changing the orientation of the crystal with respect to the external field, the orientation dependence can be observed in the change of the position of the resonance lines of the NMR spectra as the crystal is rotated around the magnetic field. In Fig. 1, we present an example of this behavior showing the large differences in the 13C NMR spectra of a single crystal spectra of 1, 3, 5-trihydroxybenzene at different orientations.
登录
查看更多内容
影响因子：
4.3
作者：
Bagno, Alessandro;Bonchio, Marcella;Autschbach, Jochen
通讯作者：
Autschbach, Jochen
影响因子：
5.5
作者：
Bagno, A;Casella, G;Saielli, G
通讯作者：
Saielli, G
影响因子：
2
作者：
Bagno, A;Bonchio, M
通讯作者：
Bonchio, M
影响因子：
4.4
作者：
BECKE, AD
通讯作者：
BECKE, AD
影响因子：
15
作者：
BEELER, AJ;ORENDT, AM;KUTZELNIGG, W
通讯作者：
KUTZELNIGG, W