Heteroatom effects on aromaticity of five-membered rings in
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.
In addition, it was revealed that in some of the examined molecules the
intensity of cyclic conjugation in the “empty” pentagons is even
stronger than that of some hexagonal rings within the same molecule. The
obtained results refute what one would expect based on ”chemical
intuition”, which is usually strongly rooted to the Kekulé structures.