Comparison of effects of charge delocalization and π-electron delocalization on the stability of monocyclic compounds.

Aromaticities and stabilities of ortho, meta, and para isomers of some derivatives of benzene, CH and CH were studied and compared basis on the NICS index and their relative energies. For the benzene and CH derivatives, the ortho isomers with less stability were more aromatic. This discrepancy was also observed for the molecules with conjugated and non-conjugated π-electrons. However, for the charged conjugated systems, the structures with delocalized charge were more stable. Effect of electron withdrawing (EWGs) and electron donating groups (EDGs) on the electron delocalization and stability of the neutral and charged molecules was investigated. It was observed that the EWDs and EDGs change the stability trend of the neutral systems, while in the case of charged molecules, the isomers with delocalized charge were more stable regardless of the type of substituents. Although both π-electron delocalization and charge delocalization stabilize the aromatic and conjugated systems, effect of charge delocalization on the stability is more than that of π-electron delocalization.