Abstract
Of all the theoretical concepts that constitute the rational basis of modern organic chemistry, the concept of aromaticity is one of the most general, but at the same time one of the most vaguely defined. Generally, aromaticity has been defined as the difference between π-electron resonance energies of a non-cyclic π-conjugated compound and the cyclic π-conjugated compound, giving rise to the (4n + 2)π rule in Huckel molecular orbital (HMO) theory.2 Although the definition of aromaticity on the basis of HMO theory can be readily understood, the treatment of complex compounds such as non-planar molecules remains difficulty. Aromaticity has also been treated magnetically, and Schleyer et al. proposed the nucleus-independent chemical shift (NICS) as an index of aromaticity. The values of the NICS are adequate for classification of aromaticity and anti-aromaticity, but measurement is intensive as shown in previous papers. A new criterion of aromaticity for cyclic π-conjugated compounds is proposed based on CiLC (CI/LMO/CASSCF) analysis founded on ab initio molecular orbital methods. The new criterion states that the all bonds should have equivalent electronic states 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 for comparison of the aromaticity of ring-unit compounds.
