Visualisation of Chemical Shielding Tensors (VIST) to Elucidate Aromaticity and Antiaromaticity.

Felix Plasser,Florian Glöcklhofer

doi:10.1002/ejoc.202100352

Felix Plasser, Florian Glöcklhofer

Open Access

https://doi.org/10.1002/ejoc.202100352

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Abstract

Aromaticity is a central concept in chemistry, pervading areas from biochemistry to materials science. Recently, chemists also started to exploit intricate phenomena such as the interplay of local and global (anti)aromaticity or aromaticity in non‐planar systems and three dimensions. These phenomena pose new challenges in terms of our fundamental understanding and the practical visualisation of aromaticity. To overcome these challenges, a method for the visualisation of chemical shielding tensors (VIST) is developed here that allows for a 3D visualisation with quantitative information about the local variations and anisotropy of the chemical shielding. After exemplifying the method in different planar hydrocarbons, we study two non‐planar macrocycles to show the unique benefits of the VIST method for molecules with competing π‐conjugated systems and conclude with a norcorrole dimer showing clear evidence of through‐space aromaticity. We believe that the VIST method will be a highly valuable addition to the computational toolbox.

Highlights

The concept of aromaticity has intrigued chemists for over 200 years[1] and is a fundamental ingredient in our understanding of the properties and reactivity of molecules
Shielding tensors for benzene were computed at various positions around the molecule to examine how the shielding varies with the position, illustrating the basics of the visualisation of chemical shielding tensors (VIST) method (Figure 2)
Moving to the nucleus-independent chemical shifts (NICS)(1) tensors in Figure 4c we find that antiaromaticity is still clearly visible but that the magnitude of the deshielding is strongly reduced, a result obtained for an isolated CBD molecule.[77,80]

Summary

Introduction

The concept of aromaticity has intrigued chemists for over 200 years[1] and is a fundamental ingredient in our understanding of the properties and reactivity of molecules. Aromatic molecules play central roles in many areas of chemistry, such as organic chemistry, biochemistry, photochemistry, and molecular materials science. Chemists started to consider local aromatic effects and antiaromaticity in the design of organic materials offering promising applications based on fascinating chemistry.

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