The NICS‐XY‐Scan: Identification of Local and Global Ring Currents in Multi‐Ring Systems

Abstract

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.

Is (benzene)Cr(CO)3 really more aromatic than benzene?

“…The NICS-X-scans values of the annulated benzene moieties (∼-17 to -18.5) are practically identical to that of free benzene. 13 This conclusion is also supported by the NICS(1) π,ZZ values of the annulated benzene rings (Table S1), which show only minor changes relative to the changes in the 14-annulene. D r a f t one to that of the aromatic annulation is preferred.…”

“…(2) Local vs. global ring currents in benzene-and cyclobutadiene-annulated dihydropyranes: The NICS-XY-scan method 13 was used for the investigation of the currents. The individual scans of each system, including the respective trajectories of the calculation through the annulated moieties are given in the Supporting Information.…”

“…NICS(1) π,ZZ values for identifying local induced magnetic field at specific points were obtained from the NICS-scan 12 and σ-only model. 9 The NICS-XY-scan method 13 was used for the assessment of local and global ring currents. All NICS calculations were carried out at the GIAO-B3LYP/6-311+G(d) computational level, using the Aroma program.…”

Adaptive aromaticity in S0 and T1 states of pentalene incorporating 16 valence electron osmium

“…In line with the BLAs, the harmonic oscillator model of aromaticity (HOMA Aromaticity analyses based on magnetic properties. To further evaluate the (anti)aromaticity of these osmacycles, we performed the nucleus-independent chemical shift (NICS) calculations in both their S 0 and the T 1 states 36,37 , two-dimensional NICS grids are capable to display the magnetic shielding in aromatic rings and de-shielding in antiaromatic rings 38,39 . Compared with other NICS indices, the NICS(1) zz is chosen here as it can be easily computed and also highly effective in evaluating π aromaticity in both the S 0 and the T 1 states 40,41 .…”

Which NICS method is most consistent with ring current analysis? Assessment in simple monocycles

“…34,[37][38][39][40][41] It is also important to highlight that the quantitative relationship between shielding and ring current strengths has been rigorously analyzed in the past. [42][43][44][45] In these works, it is assumed that the wire carrying the ring current is circular and innitely thin; thus, the induced magnetic eld is easily determined from classical electrodynamics, using the Biot-Savart's law. 46 These studies conclude that the relationship between these two properties is not linear, due to the distance dependence of the induced magnetic eld.…”