Local Aromaticity in Naphtho-Annelated Fluoranthenes:
Can the Five-Membered Rings Be More Aromatic Than the Six-Membered
Rings?
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Abstract
All Kekulé-structure-based
theories predict that the central
five-membered ring in fluoranthene and naphtho-annelated fluoranthenes
is nonaromatic. In the present work, a detailed study of the local
aromaticity in a series of naphtho-annelated fluoranthene derivatives
was performed by means of the following aromaticity indices: the energy
effect (ef), bond resonance energy (BRE), multicenter delocalization
indices (MCI), harmonic oscillator model of aromaticity (HOMA) index,
nucleus-independent chemical shifts (NICS), and ring current maps.
It was found that, according to the ef, BRE, MCI, and HOMA values,
the pentagonal rings in some naphtho-annelated fluoranthenes can be
even more aromatic than some hexagonal rings in the respective molecules.
The magnetic indices do not support the results obtained by the energetic,
electron delocalization, and geometrical aromaticity indices