Magnetic shielding paints an accurate and easy-to-visualize portrait of aromaticity.

Brett VanVeller,Peter B. Karadakov

doi:10.1039/d1cc03701c

Abstract

Chemists are trained to recognize aromaticity semi-intuitively, using pictures of resonance structures and Frost-Musulin diagrams, or simple electron-counting rules such as Hückel's 4n + 2/4n rule. To quantify aromaticity one can use various aromaticity indices, each of which is a number reflecting some experimentally measured or calculated molecular property, or some feature of the molecular wavefunction, which often has no visual interpretation or may not have direct chemical relevance. We show that computed isotropic magnetic shielding isosurfaces and contour plots provide a feature-rich picture of aromaticity and chemical bonding which is both quantitative and easy-to-visualize and interpret. These isosurfaces and contour plots make good chemical sense as at atomic positions they are pinned to the nuclear shieldings which are experimentally measurable through chemical shifts. As examples we discuss the archetypal aromatic and antiaromatic molecules of benzene and square cyclobutadiene, followed by modern visual interpretations of Clar's aromatic sextet theory, the aromaticity of corannulene and heteroaromaticity.

Highlights

These isosurfaces and contour plots make good chemical sense as at atomic positions they are pinned to the nuclear shieldings which are experimentally measurable through chemical shifts
To enable comparison of the results obtained with the current approach to those coming from methods utilizing the out-of-plane zz-component of the off-nucleus shielding tensor, szz(r), such as NICSzz(0), NICSzz(1), nucleus-independent chemical shift (NICS)-X-scans and NICS-XY-scans, as well as to ring current plots, in Fig. 4 we show szz(r) contour plots for phenanthrene and coronene in the respective molecular planes and in planes 1 Å above the molecular planes
For planar conjugated mono and polycyclic hydrocarbons and heterocyclic compounds the extent of isotropic shielding delocalization in the contour plots in a plane 1 Å above the molecular plane can be viewed as a highly sensitive visual two-dimensional aromaticity criterion which, while costlier to obtain computationally, has clear advantages over single-point NICS and one-dimensional aromatic ring chemical shielding (ARCS),[54] NICS-X-scan and NICS-XY-scan plots.[35,36]

Summary

Introduction

Clar’s aromatic sextet theory based on isotropic shielding contour plots.[21] There is an important difference between siso(r) isosurfaces and contour plots and NICS that needs to be emphasized: We treat increased shielding as a bondenhancing factor and decreased shielding as a bond-disrupting factor; this is not the case with NICS, for which the sign inversion (vide supra) facilitates comparison to proton chemical shifts. The aim of this feature article is to explain, through easy-tovisualize examples, the essential features of the isotropic shielding analysis of aromaticity, antiaromaticity and chemical bonding. The isotropic shielding isosurfaces and contour plots calculated using ab initio and density functional theory (DFT) methods provide a much more capable, reliable and theoretically sound approach for describing aromaticity in a wide range of molecules

Benzene and cyclobutadiene

Making aromaticity ‘‘klar’’

Heteroaromaticity

Conclusions