Detailed Visualization of Aromaticity Using Isotropic Magnetic Shielding

Abstract

For many years, Clars aromatic sextet theory has served as a qualitative method for assessing the aromatic character of polycyclic aromatic hydrocarbons. A new approach, based on the calculation of isotropic magnetic shielding (IMS) contour plots, is shown to provide a feature-rich picture of aromaticity that is both quantitative yet still easily interpreted. Chemists are visual creatures who are adept at discerning reactivity and chemical behavior from molecular structures. To quote Roald Hoffmann, "People like pictures. Chemists live off them." Thus, the detailed image analysis we present simultaneously provides quantitative assessment of electronic structure, which is still easy-to-understand through visual inspection, embedded in an aesthetically appealing and intuitive picture that draws the reader in. We provide novel computed IMS contour plots for a representative selection of aromatic molecules. Where Clars static drawings capture only a partial sketch of the electronic properties of a molecule, IMS contour plots present a detailed, global landscape of a molecule that sums all possible resonance structures. This novel analysis allows us to correct certain drawbacks of Clars analysis with respect to polycyclic aromatics and quantitatively assess the bonding and electronic structure of acene hydrocarbons.

Aromaticity and Magnetic Behavior in Benzenoids: Unraveling Ring Current Combinations

“…Isotropic magnetic shielding contour plots used in aromaticity analyses suggest that the local aromaticity of terminal rings decreases as the acene length increases [10b] . Moreover, these studies indicate greater aromaticity within acene‘s central rings than in the peripheral ones, inferring an amplified influence of migrating sextets within the inner rings, aligning with previous NICS studies [38] .…”

Simple and efficient visualization of aromaticity: bond currents calculated from NICS values

“…We note that, in general, there is a continued interest in visualizing abstract chemical phenomena, as is demonstrated by the recent publications detailing such methods for, e.g., dispersion forces, 25 strain 26 and Clar sextets in polybenzenoid hydrocarbons. 27 In the study of aromaticity, despite the clear advantages of pictorial depiction, p-electron current density visualization methods are not the most widely used. This is because, for a visualization to be valuable, it should be an accurate and clear representation of the chemical information and, ideally, also easy to generate and analyze.…”

How Different are the Diamagnetic and Paramagnetic Contributions to Off‐Nucleus Shielding in Aromatic and Antiaromatic Rings?

“…Similarly to NICS(1) and NICS(1) zz , these plots show mainly shielding effects associated with the π electrons. The $${{\sigma }_{{\rm i}{\rm s}{\rm o}}^{{\rm G}{\rm I}{\rm A}{\rm O}}\left({\bf r}\right)}$$ contour plots in planes parallel to and 1 Å above the molecular planes of polycyclic aromatic hydrocarbons (PAHs) have been shown [14,35] to capture the essence of the popular Clar aromatic sextets with high accuracy, providing visual and yet quantitative vindication of Clar's ideas. It should be noted that, at positions in the plane parallel to and 1 Å above the molecular plane different from that used to calculate NICS(1), at least two of the components $${{\sigma }_{xz}\left({\bf r}\right)}$$ , $${{\sigma }_{yz}\left({\bf r}\right)}$$ , $${{\sigma }_{zx}\left({\bf r}\right)}$$ and $${{\sigma }_{zy}\left({\bf r}\right)}$$ are non‐zero which makes the use of the zz components of the shielding tensor and its diamagnetic and paramagnetic contributions calculated in that plane less well‐justified than the use of the respective quantities calculated in the molecular plane.…”