Gershoni-Poranne Renana, Stanger Amnon
Schulich Faculty of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000 (Israel).
Chemistry. 2014 May 5;20(19):5673-88. doi: 10.1002/chem.201304307. Epub 2014 Mar 27.
Nucleus-independent chemical shift (NICS)-based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi-ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS-based method for the determination of local and global ring currents in conjugated multi-ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICSπZZ , but it is shown that at a height of 1.7 Å above the molecular plane, NICSZZ provides the same qualitative picture. This method, namely the NICS-XY-scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.
基于核独立化学位移（NICS）的方法在确定外磁场下的感应磁场方面非常流行。这些方法主要（但不限于）用于定性和定量地确定分子和离子的芳香性和反芳香性。作为NICS探针的虚拟原子感知感应磁场并以NMR化学位移的形式报告它。然而，NICS无法确定磁场的来源。因此，在一个可能包含多个感应电流回路（因此有多个感应磁场源）的多环系统中，NICS值可能代表许多感应磁场的总和。这可能导致对所研究系统的芳香性（和反芳香性）的错误归属。在本文中，我们提出了一种基于NICS的方法来确定共轭多环系统中的局部和全局环电流。该方法包括将NICS探针沿X轴放置，如有需要，沿Y轴放置，在研究系统上方的恒定高度处。跟踪沿这些轴的感应场的变化可以识别全局和局部感应电流。用于这些扫描的最佳NICS类型是NICSπZZ，但结果表明，在分子平面上方1.7 Å的高度处，NICSZZ提供相同的定性图像。这种方法，即NICS-XY扫描，提供的信息与通过电流密度分析方法获得的信息相当，并且在某些情况下，提供了更多细节。
