Theoretical Analysis for Aromaticity

Abstract

The concept of aromaticity, usually associated with Huckel's 4n + 2 rule, has proven to be extremely fruitfuiil in organic chemistry. Schleyer and co-workers proposed the nucleus-independent chemical shifts (NICS) for the index of aromaticity. The values of NICS indicate the nature of aromaticity and/or anti-aromaticity for compounds. However, the values of NICS are not sensitive to the same nature of aromaticity. We defined the aromaticity property from a different point of view. The two structures, Kekule's (D3h symmetry) and aromatic (D6h symmetry) structures, were analyzed by a CiLC ( CI/LMO/CASSCF) method. The CiLC analysis showed the alternation of C-C bonds for Kekule's structure and the same electronic states of six C-C bonds for aromatic structure. Especially for aromatic structure, the weights of singlet coupling term are same to those of polarization (ionic) term for each C-C bond. These hypothesis for aromaticity were applied for other compounds.