Separation of the Energetic and Geometric Contributions to Aromaticity. 2. Analysis of the Aromatic Character of Benzene Rings in Their Various Topological Environments in the Benzenoid Hydrocarbons. Crystal and Molecular Structure of Coronene

Abstract

Statistical analysis of the aromatic character and its geometric and energetic contributions of 167 benzene rings embedded in various topological environments in 26 benzenoid hydrocarbons leads to the following conclusions: the aromatic character of benzene rings with three or fewer fused rings is due mostly to geometric contributions, whereas in other cases energetic contribution is decisive. Aromaticity indices for individual rings (local aromaticity) depend strongly on the kind of topological environment. Terminal rings always exhibit a strong aromatic character, whereas those fused to many rings are often weakly aromatic. The study is based on precisely solved X-ray or neutron crystal structure determination retrieved from Cambridge Structural Database supplemented by our own precise determination of coronene.