aromaticity indices and particularly well with those defined through the pair density, like FHDD. Moreover, SFLAI is able to correctly detect the progressive
increase of π delocalization on passing from cyclohexane, through cyclohexene and
1,3-cyclohexadiene, to benzene. Eventually, it properly orders the local aromaticity
of different rings in simple PAH and in their partially hydrogenated derivatives
[15]. In summary, the SF descriptor has proved to be a quite interesting tool to
discuss electron delocalization effects, despite its lack of a direct physical link to the
very mechanism of electron delocalization, as it is instead the case of the indices
based on the exchange correlation density like the delocalization indices.
A systematic application of SF analysis on X-ray derived electron densities of
benzene and substituted napthalene crystals has been already discussed some time
ago [36, 37] and related publications have appeared [38] or are currently in
preparation [39].
In the following paragraph, we extend, instead, our electron delocalization SF
study of ab initio EDs to some non-planar and less conventional conjugated
systems.
5.3.1 Non-planar Aromatic Systems
A long-standing tenet of organic chemistry states that aromaticity requires a planar
ring of atoms to be exploited [40]. Accordingly, distortions from planarity imply a
(usually slight) loss of aromatic character [41], even though delocalized π systems
tend in principle to be preserved due to their high stability. The propensity of
aromatic compounds to remain planar arises from the requirement of maximizing
the π-symmetry overlap among formally single-occupied p orbitals on adjacent
nuclei. Au contraire, significant deviations from planarity are usually associated to
antiaromatic systems [42, 43] where the energy cost of achieving an unfavourable
electron configuration in molecular π orbitals can be alleviated by a spontaneous
distortion of the ring that breaks—or reduces—the π overlap. Cycloottatetraene is a
prototypical example of a compound exhibiting such a distortion [44].
Yet, several classes of non-planar aromatic compounds are also known. In most
cases, such compounds contain fused benzene-like ring moieties that are bent
out-of-plane by steric constraints. Molecular belts [45], pyrenophane derivatives
[46, 47] and contracted porphyrinoids [48] are representative examples of these
systems. In general, the study of non-planar aromatic compounds opens new
opportunities for developing molecular devices with potential application in
materials science. For example, porphyrinoids have recently attracted significant
interest, as their curved aromatic structures show concave or convex π surfaces able
to interact in various ways with electroactive compounds [48].
In late 1950s, a novel class of cyclic compounds was discovered, that exhibited
aromatic character from a chemical, thermodynamic and spectroscopic viewpoint
despite the presence of sp
3 -hybridized atoms within the ring [49]. In his pioneering
work, Winstein [49] named such compounds as ‘homoaromatics’ due to their
5 Exploring Chemistry Through the Source Function …
107
increase of π delocalization on passing from cyclohexane, through cyclohexene and
1,3-cyclohexadiene, to benzene. Eventually, it properly orders the local aromaticity
of different rings in simple PAH and in their partially hydrogenated derivatives
[15]. In summary, the SF descriptor has proved to be a quite interesting tool to
discuss electron delocalization effects, despite its lack of a direct physical link to the
very mechanism of electron delocalization, as it is instead the case of the indices
based on the exchange correlation density like the delocalization indices.
A systematic application of SF analysis on X-ray derived electron densities of
benzene and substituted napthalene crystals has been already discussed some time
ago [36, 37] and related publications have appeared [38] or are currently in
preparation [39].
In the following paragraph, we extend, instead, our electron delocalization SF
study of ab initio EDs to some non-planar and less conventional conjugated
systems.
5.3.1 Non-planar Aromatic Systems
A long-standing tenet of organic chemistry states that aromaticity requires a planar
ring of atoms to be exploited [40]. Accordingly, distortions from planarity imply a
(usually slight) loss of aromatic character [41], even though delocalized π systems
tend in principle to be preserved due to their high stability. The propensity of
aromatic compounds to remain planar arises from the requirement of maximizing
the π-symmetry overlap among formally single-occupied p orbitals on adjacent
nuclei. Au contraire, significant deviations from planarity are usually associated to
antiaromatic systems [42, 43] where the energy cost of achieving an unfavourable
electron configuration in molecular π orbitals can be alleviated by a spontaneous
distortion of the ring that breaks—or reduces—the π overlap. Cycloottatetraene is a
prototypical example of a compound exhibiting such a distortion [44].
Yet, several classes of non-planar aromatic compounds are also known. In most
cases, such compounds contain fused benzene-like ring moieties that are bent
out-of-plane by steric constraints. Molecular belts [45], pyrenophane derivatives
[46, 47] and contracted porphyrinoids [48] are representative examples of these
systems. In general, the study of non-planar aromatic compounds opens new
opportunities for developing molecular devices with potential application in
materials science. For example, porphyrinoids have recently attracted significant
interest, as their curved aromatic structures show concave or convex π surfaces able
to interact in various ways with electroactive compounds [48].
In late 1950s, a novel class of cyclic compounds was discovered, that exhibited
aromatic character from a chemical, thermodynamic and spectroscopic viewpoint
despite the presence of sp
3 -hybridized atoms within the ring [49]. In his pioneering
work, Winstein [49] named such compounds as ‘homoaromatics’ due to their
5 Exploring Chemistry Through the Source Function …
107