{Aromaticity} Why is Caution Needed When Using NICS to Compare Monocyclic Aromaticity?
NICS是现存评判芳香性方法中最流行、靠谱度较高的几个方法之一，然而使用NICS对比不同大小的环系的时候却有潜在隐患，本贴将简述该隐患。 NICS is one of the most popular and reliable methods currently available for evaluating aromaticity. However, there are potential pitfalls when using NICS to compare ring systems of different sizes. This post intends to outline this issue.
引用声明：本文全部内容均来自最后引用文献的Supplementary Materials部分。 Citation statement: All contents of this blog are cited from the Supplementary Materials of the preprint shown at the bottom.
假设一个理想环形势场的环半径为r。那么，它的第一能级是角量子数L=0的，而它的第二、第三能级是角量子数|L|=1的（J. Chem. Educ. 2022, 99, 10, 3497–3501）。
也就是说，对于一个轨道而言，它的角动量（也就是磁矩）是存在上限值的。为方便理解，这里直接假设它的总环电流的磁矩为μ。
那么，环中心点的磁感应强度就是： (, 下载次数 Times of downloads: 0)
Assuming an ideal circular potential field with a ring radius of r, its first energy level corresponds to the angular quantum number L=0, while its second and third energy levels correspond to ∣L∣=1 (J. Chem. Educ. 2022, 99, 10, 3497–3501). This means that for an orbital, its angular momentum (also the magnetic moment) has a theoretical upper limit. For ease of understanding, let us directly assume here that the total magnetic moment of the ring current is μ. Then, the magnetic induction intensity at a point on the centre of the ring is:
This B value is proportional to NICS. In other words, for a consistent magnetic moment of the ring current, the NICS value is approximately inversely proportional to r^3. It can further be demonstrated that the current density is approximately inversely proportional to r^4. A simple but extreme example is benzene and silicobenzene (Si₆H₆). If one calculates the orbital angular momentum of the HOMOs of these two molecules, it can be found that the orbital angular momentum of the HOMO of silicobenzene is even larger than that of benzene. However, using criteria such as NICS and current density, silicobenzene is commonly classified as a weak-aromatic species, because both its ring current and NICS value are too small. The bond length ratio between silicobenzene and benzene is approximately 0.63. It can be estimated that the NICS ratio generated by their HOMOs is 0.25, and the ring current density ratio is 0.16—a very significant difference.
我们或许应该用轨道角动量而不是用磁感应强度与电流强度来衡量芳香性？ Perhaps we should measure aromaticity using orbital angular momentum rather than magnetic induction intensity and current density?
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