Aromaticity of Benzenoid-Borazinoid Hybrids: Current Maps for Hetero-Graphenes

Abstract

Two isomeric coronene-borazine hybrids C₁₈B₃N₃H₁₂, in which a central or outer C₆-cycle is formally replaced by an alternating B₃N₃ cycle, are assessed for aromaticity on the ring-current criterion. Calculations for the B3LYP/6-31G** optimized structures (of D_3h and C_s symmetry, respectively) at the coupled-Hartree-Fock ipsocentric level for treatment of magnetic response properties show distinct patterns of π-current: central substitution isolates a strongly diatropic C₁₈ perimeter, whereas outer substitution leaves a phenanthrene-like circulation in the carbon-only region of the molecule. Circulations within the B₃N₃ moiety are limited to localized “lone-pair” circulations on the nitrogen centres. In the limit of the replacement strategy, where all C₆ cycles are replaced, as in hexaborazino-coronene (B₁₂N₁₂H₁₂, D_3h), all π-currents become localized on nitrogen centers.

Key Words:

• aromaticity
• borazine
• coronene
• PAH
• ring current