Local aromaticity of the five-membered rings in acenaphthylene derivatives

In this paper, a detailed study of the local aromaticity in a series of cyclopenta-fused linear polyacenes (acenaphthylene derivatives) was performed using several different criteria of aromaticity. Namely, the energy effect (ef), bond resonance energy (BRE), harmonic oscillator model of aromaticity (HOMA) index, multi centre delocalization indices (MCI), electron density at ring critical points ρ(r(C)), nucleus independent chemical shifts (NICS) and ring current maps were employed. All these methods agree that the extent of aromaticity of the five-membered ring in the acenaphthylene molecule is quite small. On the other hand, it was shown that the aromatic character of the five-membered rings depends on the size of the polyacenic part and on the position of the five-membered ring along the polyacenic part of the given acenaphthylene derivatives. According to energetic, electron delocalization and geometrical indices, the aromatic character of the pentagons in some higher acenaphthylene congeners is of the same order of magnitude as the aromatic character of some hexagons in the respective molecules. Ring current maps revealed the existence of a weak paratropic (antiaromatic) circulation in the five-membered rings which, in the case of higher members of the homolog series, becomes weaker. In contrast, the NICS indices showed a significant antiaromatic character of the five-membered rings in higher members of the acenaphthylene series.