5 research outputs found
Ring Currents in Polycyclic Sodium Clusters
In the recent work by Khatua et al. (Khatua, S.; Roy, D. R.; Bultinck, P.; Bhattacharjee, M.; Chattaraj, P. K. <i>Phys. Chem. Chem. Phys.</i> <b>2008</b>, <i>10</i>, 2461–2474) the synthesis and structure of a <i>fac</i>-trioxo molybdenum metalloligand and its sodium complex containing 1D hexagonal chains of sodium ions was reported. In the same paper, the aromaticity of hexagonal Na clusters was quantified by means of the nucleus-independent chemical shift and electronic multicenter indices. It was shown that the aromaticity of hexagonal Na-clusters is of the same order as the aromaticity of analogous benzenoid hydrocarbons. In the present study current density maps are used to rationalize the aromaticity of polycyclic Na clusters. It is shown that although polycyclic Na systems sustain a diatropic ring current, the induced current density is several times weaker than in analogous benzenoid hydrocarbons. A detailed analysis indicates that the current density in hexagonal Na systems is almost completely determined by four HOMO σ electrons
Aromaticity of Nonplanar Fully Benzenoid Hydrocarbons
The
Clar aromatic sextet theory can provide a qualitative description
of the dominant modes of cyclic π-electron conjugation in benzenoid
molecules and of the relative stability among a series of isomeric
benzenoid systems. In a series of nonplanar fully benzenoid hydrocarbons,
the predictions of the Clar theory were tested by means of several
different theoretical approaches: topological resonance energy (TRE),
energy effect (ef), harmonic oscillator model of aromaticity (HOMA)
index, six center delocalization index (SCI), and nucleus-independent
chemical shifts (NICS). To assess deviations from planarity in the
examined molecules, four different planarity descriptors were employed.
It was shown how the planarity indices can be used to quantify the
effect of nonplanarity on the local and global aromaticity of the
studied systems
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.
The magnetic indices do not support the results obtained by the energetic,
electron delocalization, and geometrical aromaticity indices
Effect of Benzo-Annelation on Local Aromaticity in Heterocyclic Conjugated Compounds
The effect of benzo-annelation on
the local aromaticity of the central ring of acridine (<b>1</b>), 9<i>H</i>-carbazole (<b>2</b>), dibenzofuran (<b>3</b>), and dibenzothiophene (<b>4</b>) was analyzed by
means of the energy effects (ef), pairwise energy effects (pef), multicenter
delocalization index (MCI), electron density at ring critical points
(ρ(<i>r</i><sub>C</sub>)), harmonic oscillator model
of aromaticity (HOMA), and nucleus independent chemical shifts (NICS).
According to energetic, electron delocalization, and geometrical indices,
angular benzo-annelation increases, whereas linear benzo-annelation
decreases, the extent of the local aromaticity of the central ring
containing heteroatoms. The local aromaticity of the central heterocyclic
ring in the examined molecules can significantly vary by applying
different modes of benzo-annelation. The NICS values do not always
support the results obtained by the other aromaticity indices and,
in some cases, lead to completely opposite conclusions
Aromaticity of Closed-Shell Charged Polybenzenoid Hydrocarbons
The aromatic stabilization of closed-shell
charged polybenzenoid
hydrocarbons (PBHs) has been scrutinized by means of energetic and
magnetic aromaticity criteria and by direct measures of electron delocalization.
Thus, topological resonance energies and their circuit contributions,
ring current maps, and multicenter delocalization indices have been
calculated for a series of 18 polybenzenoid cations containing from
3 to 10 benzene rings. All calculations indicate that the closed-shell
cations have a similar degree of aromaticity compared to that of the
corresponding closed-shell neutral PBHs. All cations investigated
display a large degree of electronic delocalization in the ring, accompanied
by significant aromatic stabilization and a strong diatropic peripheral
electron current. Graph theoretical models describe perfectly the
aromatic features of these hydrocarbon fragments, showing how they
can be understood as a superposition of specific neutral PBHs. The
large aromatic character of these systems suggests they may be relatively
stable upon formation at combustion conditions, like those given in
the interstellar medium. It has been postulated that closed-shell
fragments of PBHs may play an important role in the photoluminescent
phenomenon known as extended red emission