Factors Influencing Aromaticity: PCA Studies of Monosubstituted Derivatives of Pentafulvene, Benzene and Heptafulvene

The π-electron delocalization of mono-derivatives of benzene as well as penta- and heptafulvenes was analyzed based on quantum chemistry approach in the gas phase and five polar environments of increasing polarity. The most representative aromaticity indices were selected by the aid of the Principal Component Analysis, which reduces the initial set of 32 indices to orthogonal set comprising FLU1/2 index, pEDA and Laplacian estimated at ring critical point (RCP(∇2ρ)). These indices were used for description of the substituent effect in non-polar and polar conditions. A decrease of FLU1/2 values for heptafulvenes and an increase of its values for pentafulvenes was observed with the increasing Hammett σp constant values. For the pi Electron Donor-Acceptor parameter (pEDA) values a decrease was noticed with the rise of σp values. Laplacian estimated at ring critical point (RCP(∇2ρ)) is almost independent on the nature of substituent and polarity of the environment. Interestingly, benzene analogues are not only the least prone to substituent effect but also are the most resistant to polarity of the environment. Among fulvenes, the class of lowest aromaticity, heptafulvenes, is both significantly affected by substituent and this influence is even extended by polar media. This conclusion is consistent for all three measures of aromaticity identified as the most representative ones.