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Abstract:The aromaticity of all possible cyclopenta-fused pyrene congeners has been investigated at various levels of theory. On the basis of the calculated resonance energies and magnetic properties (δ 1 H data, magnetic susceptibility anisotropies, and NICS values), the overall aromaticity of these compounds is found to decrease gradually with increasing number of externally fused five-membered rings. The relatively small differences (<5 kcal/mol) in thermodynamic stability of the isomeric dicyclopentapyrenes (Etot:, which differs from the aromaticity order based on the magnetic criteria (dicyclopenta [cd,mn], is shown by model calculations to be dominated by σ-strain imposed on the pyrene skeleton by sequential cyclopenta-fusion. This is supported by the computed homodesmotic reaction energies and aromatic stabilization energy (ASEisom) from isodesmic aromatic-nonaromatic isomerization, and by the model calculations on "distorted" cyclopenta [cd]pyrenes. The elusive tetracyclopenta [cd,fg,jk,mn]pyrene is computed to be bowl-shaped; its corresponding planar geometry is the transition state for bowl-bowl interconversion.

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