New Criterion of Aromaticity and Implications for the (4n + 2)π Rule

Abstract

A new criterion of aromaticity for cyclic π-conjugated compounds is proposed on the basis of CiLC (CI/LMO/CASSCF) analysis founded on ab initio molecular orbital methods. The new criterion states that the all bonds should have equivalent electronic structures and the difference between weights for the singlet coupling and polarization terms (from CiLC analysis) should be small. The (4n + 2)π rule of aromaticity for cyclic π-conjugated compounds is reexamined using this new criterion. The criterion is applied to CnHn structures with Dnh (n = 4, 6, 8, and 10) symmetries and their equilibrium structures, and it is demonstrated that the proposed criterion reasonably explains the (4n + 2)π rule. An index of deviation from the aromaticity (IDA) is also defined to compare the aromaticity of ring-unit compounds.