Aromaticity of N‐heterocyclic carbene and its analogues: Magnetically induced ring current perspective

Abstract

The N‐heterocyclic carbene, imidazole‐2‐ylidene, and its main group (13‐15) analogues contain cyclically conjugated 6π electrons. Experimental 1H nuclear magnetic resonance (NMR) spectra suggest an increase in aromaticity along a period from left to right. Whereas the order along a group is as follows: period 2 > period 5 > period 4 > period 3 due to change in structure. To understand the order of aromaticity, the magnetically induced ring currents of the molecules are calculated using aromatic ring current shielding, gauge‐including magnetically induced currents (GIMIC) method and Stanger's σ‐model applying the gauge‐including atomic orbitals NMR technique. It is found that GIMIC best describes the order of aromaticity especially along a group where current‐profile changes on the bivalent atom down a group due to change in electron density. Moreover, the GIMIC provides the visualization of current by sign modulus and the anisotropy of the induced current density plots.

CB6Al0/+: Planar hexacoordinate boron (phB) in the global minimum structure

“…Thus, to gain more insight into aromaticity/antiaromaticity/nonaromaticity the ring current calculations were performed using the GIMIC method. [74][75][76] The clockwise current (positive current) is called the diatropic current, and the anticlockwise current (negative current) is termed the paratropic current. The ring currents were calculated using the Gauss quadrature method in a cross-section area of a molecule.…”

Interplay of Hückel and Möbius Aromaticity in Metal‐Metal Quintuple Bonded Complexes of Cr, Mo, and W with Amidinate Ligand: Ab initio DFT and Multireference Analysis**

“…GIMIC is a compelling method developed by Sundholm and co‐workers [62] to analyze the ring current produce due to the electron flow in a cyclic conjugated system under the influence of an external magnetic field. It has two implications: i) quantitative estimation of current flow employing the numerical integration procedure, and ii) qualitative visualization of current flow and density in two‐ and three‐dimensional space [63] . In this method, a significant positive value of the ring current implies aromaticity while the negative value antiaromaticity.…”

Current density and molecular magnetic properties

“…Calculations have shown that the strength of the ring-current susceptibility of small aromatic molecules correlates with the stabilisation energy due to electron delocalisation in the conjugated chemical bonds. [134][135][136] The correlation between the strength of the ring-current susceptibility and the aromatic stabilisation energy suggests that the ring-current criterion can be used for determining the degree of aromaticity of molecular rings. There is a similar correlation between the strength of the current-density vortex of hydrogen bonds and the strength of the hydrogen bond.…”