Relativistic effects on the aromaticity of E3M3H3 (E = C–Pb; M = N–Bi) benzene analogues

Abstract

The relativistic effects on the aromaticity of a set of benzene analogues, E₃M₃H₃ (E = C–Pb; M = N–Bi) heterocycles, using magnetically induced current density (MICD) and the NICS_zz component of the conventional nucleus independent chemical shift (NICS), is hereby examined. The relativistic effects were evaluated by means of four-component relativistic MICD, and two-component NMR relativistic shielding tensor methods. MICD and NICS were also computed in a non-relativistic fashion, to assess the influence of scalar-relativistic and spin–orbit effects. Most of the studied compounds exhibit a net diatropic ring current (aromatic), excluding the nitrogen-containing compounds which are non-aromatic (except for C₃N₃H₃), in agreement with their higher E–N electronegativity difference. Only in the case of bismuth compounds, E₃Bi₃H₃, aromaticity is substantially decreased when relativistic effects are included (mainly due to the spin–orbit contribution). The larger the mass of the system, the larger the magnitude of this change, in line with the expected relativistic effects for heavier elements. The analysis based on the NICS_zz computations agrees with that of the MICD, thus supporting both the magnetic behavior and the aromatic character of these compounds.