Some chemical concepts such as aromaticity, antiaromaticity, and chemical bonding have been evaluated for some molecules based on isotropic magnetic shielding calculations. This includes utilising points as local shielding probes which have been used as a single point or multiple points aligned in one-, two-, or three-dimensional grids. Each of the grids was placed at a specific location around/at the molecular space of the studied molecules.
Nuclear and off-nucleus NMR shielding calculations were performed at different levels of theory using different quantum chemical methods of HF, MP2, and CASSCF with a variety of basis sets. The calculations were performed on some organic and inorganic mono-, bi-, and tricyclic molecules in their ground and, in some cases, low-lying excited states. These molecules are borazine, borazanaphthalene, deltate, squarate, croconate, rhodizonate, disulfur dinitride, naphthalene, anthracene and phenanthrene.
Based on analysing and scanning the changes in the magnetic shielding data of values, 1D curves, 2D contour maps and 3D isosurfaces, the targeted molecular features for the above molecules have been obtained.
The chemical bonding, aromaticity, and antiaromaticity of the molecules are assessed based on the above evaluations. The results show that both borazine and the borazanaphthalene are moderately aromatic. The oxocarbon dianions vary from aromatic deltate, moderately aromatic squarate to antiaromatic croconate and rhodizonate. Also, the vertical excitation of the moderately aromatic ground state disulfur dinitride leads to strongly antiaromatic S1 and moderate antiaromatic T1 states. Naphthalene shows obvious magnetic variations among its different electronic states. In terms of decreasing aromaticity, the naphthalene states follow this order: S2 (strongly aromatic) > S0 (aromatic) > T1 (antiaromatic) > S1 (strongly antiaromatic). Both anthracene and phenanthrene display a strong magnetic behaviour. The central ring of anthracene is more magnetically shielded than the two terminal rings, whereas a contrast shielding profile is found in phenanthrene rings. For all the above molecules, the magnetic shieldings around bonds help in understanding the overall magnetic behaviour and the aromaticity level.
