Magnetic shielding in and around benzene and cyclobutadiene: a source of information about aromaticity, antiaromaticity, and chemical bonding.

A detailed picture of the variations in the isotropic shielding σ(iso)(r) in and around the classical examples of aromatic and antiaromatic systems, benzene and square cyclobutadiene, in terms of isosurfaces and contour plots, is obtained by calculating σ(iso)(r) values at fine regular three-dimensional 7 × 7 × 7 Å grids of points with a spacing of 0.05 Å, using π space complete-active-space self-consistent field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). The results demonstrate that the σ(iso)(r) values can be used not only to distinguish between aromatic and antiaromatic systems but also to characterize chemical bonds and investigate the extents to which these bonds are affected by the aromatic or antiaromatic nature of the molecule in which they reside. The strong bonding interactions within the benzene ring are highlighted by the fact that the carbon-carbon and carbon-hydrogen bonds are wrapped up within a doughnut-shaped region of increased shielding, which is especially high along the carbon-carbon bonds, with protrusions marking the carbon-hydrogen bonds. The antiaromatic destabilization in square cyclobutadiene is seen as the consequence of the presence of a markedly deshielded dumbbell-shaped region in the center of the molecule, which disrupts the connections between the shielded regions outlining individual carbon-carbon bonds, decreases the shielding within these regions, and displaces them to off-bond locations outside the ring. The well-known deshielding of aromatic protons and increased shielding of antiaromatic protons are shown to be accompanied by analogous shielding differences along the whole lengths of the carbon-hydrogen bonds, as a result of which the carbon-hydrogen bonds should become slightly weaker in aromatic systems as opposed to the corresponding bonds in antiaromatic systems.