Heteroatom effects on aromaticity of five-membered rings in acenaphthylene analogs.

The pattern of cyclic conjugation was thoroughly studied in the series of N- and P-acenaphthylene derivatives using several different aromaticity indices: the energy effect (ef), multicenter delocalization index (MCI), harmonic oscillator model of aromaticity (HOMA) index, and nucleus independent chemical shifts (NICS). The Kekulé-structure-based reasoning predicts that there would be no cyclic conjugation in the "empty" five-membered heteroatom-containing rings in the studied molecules. It was found that according to the ef, MCI, and HOMA values, the extent of cyclic conjugation in the pentagonal rings is strongly influenced by the number and mutual arrangement of the hexagonal rings.

Local aromaticity in naphtho-annelated fluoranthenes: can the five-membered rings be more aromatic than the six-membered rings?

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.