The limit of the current aromaticity concept.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Aromaticity Studies

Background:

• Aromaticity is a fundamental concept in chemistry, traditionally defined for cyclic, planar systems with delocalized pi electrons.
• Extending the understanding of aromaticity to systems involving heavier elements (beyond the second row) presents significant theoretical challenges.

Purpose of the Study:

• To investigate and compare different methods for assessing aromaticity in cyclic pi-electron systems.
• To evaluate the applicability and reliability of established aromaticity criteria for systems containing second-, third-, and fourth-row elements.

Main Methods:

• Employed three distinct approaches to evaluate aromaticity: magnetic (NICS variants, current-density analysis), electron-density-based (MCI, MCIπ indices), and energetic (Bond-Length-–Valence (BLW) method).
• Applied these methods to fifteen cyclic 2π-electron systems and nine 6π-electron charged and neutral systems.

Main Results:

• Significant divergence was observed in the aromaticity rankings obtained from the magnetic, electron-density, and energetic criteria.
• No clear correlation was found between the different aromaticity assessment methods.
• Findings suggest that NICS values should be interpreted with caution and not used as a sole determinant of aromaticity.

Conclusions:

• The lack of correlation among various aromaticity measures highlights the intrinsic difficulty in defining and quantifying aromaticity for systems beyond the second row.
• Inconsistencies in applying traditional criteria to heavier congeners raise fundamental questions about the existence of aromaticity in elements beyond the second row.
• Further research is needed to resolve the open question regarding the universality of aromaticity across the periodic table.