Although aromaticity is a concept in chemistry, in the last years, special efforts have been carried out in order to propose theoretical strategies to quantify it as a property of molecular rings. Among them, perhaps the computation of nucleus independent chemical shifts (NICSs) is the most commonly used, since it is possible to calculate it in an easy and fast way with most used quantum chemistry software. However, contradicting assignments of aromaticity by NICS and other methods have been reported in the literature, especially in studies concerning inorganic chemistry. In this Article is proposed a new and simple strategy to use the NICS information to assess aromaticity, identifying the point along the axis perpendicular to the molecular plane where the in-plane component of NICS becomes zero; it is called free of in-plane component NICS (FiPC-NICS). This spatial point is proposed as secure to consider NICS as an aromaticity descriptor; this simple proposal is evaluated in borazine and cyclotriphosphazenes. The results are compared with carefully examined aromatic stabilization energies and magnetically induced current-density analysis.

The electron delocalization of benzene (C6H6) and hexafluorobenzene (C6F6) was analyzed in terms of the induced magnetic field, nucleus-independent chemical shift (NICS), and ring current strength (RCS). The computed out-of-plane component of the induced magnetic field at a distance (r) greater than or equal to 1.0 Å above the ring center correlates well (R2>0.99) with the RCS value. According to these criteria, fluorination has two effects on the C6 skeleton; concomitantly, the resonant effects diminish the π electron delocalization and the inductive effects decrease the charge density at the ring center and therefore reduce the magnitude of the paratropic current generated in this region. The equilibrium between both effects decreases aromaticity in the fluorinated benzene derivatives. These results can be extrapolated to determine the aromaticity of any derivative within the series of fluorinated benzene derivatives (C6H(6−n)Fn, where n=1–5).

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In this study we report about the relativistic effects on the aromaticity of the six hexahalogenated compounds (C6H6, C6F6, C6Cl6, C6Br6, C6I6 and C6At6), via a magnetically induced current density method. All-electron density functional theory (DFT) calculations were carried out using the four-component Dirac-Coulomb (DC) Hamiltonian, including scalar and spin-orbit (SO) relativistic effects. Fully relativistic values of the magnetically induced ring currents were obtained by numerical integration over the current flow. These values were compared to the spin-free (SO interaction switched off) and non-relativistic values, in order to assess the corresponding contributions to aromaticity. It was found that in C6I6 and C6At6 there is a strong SO influence, in line with the expected relativistic effects of the heavy elements, iodine and astatine.

Dirac molecular orbital calculations on the octahedral paramagnetic Re6S8Cl63− cluster ion are reported. As the parent diamagnetic Re6S8Cl64− cluster, the calculated relativistic molecular orbitals indicate that the manifold of closely spaced unoccupied energy levels are mainly localized on the octahedral [Re6S8] core, while the cluster highest occupied molecular orbital is largely centered on the terminal chloride ligand. Thus, the probability distribution of the unpaired electron spin in Re6S8Cl63− is 3.5% on each Re187 nuclei, 0.8% on each capping S33 nuclei, and 12.1% on each terminal Cl35 nuclei. The current calculations predicted an isotropic Zeeman interaction, which is in good agreement with single crystal solid state cluster EPR experiments. We also calculated the paramagnetic hyperfine interactions (Ahfi) of the Re187, Cl35, and S33 nuclei allowing us to describe that the metal and apical ligand hyperfine tensors are anisotropic, while the hyperfine tensors of the capping S ligands are small and isotropic. It is postulated that the reversible redox couple [Re6S8Cl64−/Re6S8Cl63−] could constitute a suitable molecular nanocell for applications in molecular electronics.