3D Visualisation of Chemical Shielding Tensors (VIST) to Elucidate Aromaticity and Antiaromaticity

Aromaticity is a central concept in chemistry, pervading areas from biochemistry to materials science. Recently, synthetic chemists started to exploit more intricate phenomena such as the interplay of local and global (anti)aromaticity as well as 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, its local variations and anisotropy. To overcome these challenges, a method for the visualisation of chemical shielding tensors (VIST) is developed here. The VIST method is based on nucleus-independent chemical shifts but, in contrast to other methods, allows for a 3D visualisation with quantitative information about the local variations and anisotropy of the chemical shielding. The VIST method is exemplified in benzene to show its main properties, in phenanthrene to highlight various degrees of local aromaticity, and in cyclobuta[l]phenanthrene to illustrate the interplay between local aromaticity and antiaromaticity in its singlet ground state and Baird aromaticity in its triplet excited state.
Subsequently, the interplay of local and global aromaticity is investigated in two non-planar macrocycles, paracyclophanetetraene and [8]cycloparaphenylene, exemplifying the unique benefits of the VIST method for studying (anti)aromaticity in molecules with competing $\pi$-conjugated systems aligned in different planes.
Finally, a stacked norcorrole dimer is studied, showing clear evidence of through-space aromaticity. In summary, we believe that the VIST method will be a highly valuable addition to the computational toolbox of chemists studying (anti)aromaticity or considering it in their molecular design.