Is delocalization a prerequisite for stability of ring systems? A case study of some inorganic rings.

A combined DFT, AIM and ELF study has been carried out on borazine and its heavier analogs containing both the pnictogens and chalcogens as the ring constituent. Compared to the pnictogen substituted rings, chalcogen substituted rings are found to be less aromatic. Except for a few systems, the computed aromatic stabilization energies (ASE) do not correlate with the NICS values. For these ring systems, NICS and bond length equalization are found to be better indicators of aromaticity than ASE. It was found that bulky electronegative substituents at the metal atom dramatically increases the stability and aromaticity of these molecules. AIM and ELF analysis predicts that boron and gallium based heterocycles are moderately aromatic while the aluminium analogs are significantly less aromatic.