similarity with homo-conjugated alkenes. According to the IUPAC Compendium of
Chemical Terminology [50], “…whereas in an aromatic molecule there is continuous overlap of p-orbitals over a cyclic array of atoms, in an homoaromatic
molecule there is a formal discontinuity in this overlap resulting from the presence
of a single sp
3 hybridized atom at one or several positions within the ring; p-orbital
overlap apparently bridges these sp
3 centres, and features associated with aromaticity are manifest in the properties of the compound”.
Great efforts were devoted in the last decades to gain insights into the
homoaromatic character, and several new homoaromatic compounds were discovered since then. Some authors have recently claimed to have found neutral
homoaromatic molecules [51], even though homoaromatic character in neutral
compounds was largely questioned and debated in the past [52]. In fact,
homoaromaticity is usually associated to charged moieties exhibiting homoconjugate π network, i.e. a formally conjugated system with an interposed non-conjugate
group or a sp
3 -hybridized C atom. From a structural viewpoint, this implies that
homoaromaticity is associated with the union of a cyclopropane-like system with an
unsaturated linear segment [53–58]. From the ED perspective, homoconjugate
patterns may be either distinguished as through-bond (if a bcp is present between
the sp
2 carbon atoms bonded to the off-plane sp
3 group) or as through-space (if a
bcp is lacking). Both kinds of interactions have been thoroughly characterized by
the topological analysis of the ED scalar field by Childs, Cremer and co-workers
[59, 60]. Barzaghi and Gatti [22] have studied the structures, energetics and ED
topological features of a series of supposedly 6π electron homoaromatic systems,
which differ by the number of basal carbon atoms (from 4 to 7) and the formal
charge (−2 to +1), i.e. C n+3 H n+4
(n−4) for n = 2−5. They actually found that Möbius
aromaticity is preferred over through-bond homoaromaticity, in stabilizing all these
structures. In the former, stability is achieved by involving the basis orbitals of the
external methylene group, thus effectively ruling out transmission of conjugation
along the ring-fused bond. The electron delocalization largely involves the off-plane
sp
3 group, leading to a strong enhancement and to a gain of partial π-character of
the bonds linking the polyenic fragment with the methylene group. This situation
chiefly characterizes the case with n = 5, discussed through the SF descriptor in the
next paragraph.
5.3.2 Homotropylium Cation
Homotropylium cation (cyclooctatrienylium, (I), C 8 H 9
+ , Scheme 5.1), also called
homotropenylium, is the archetype of homoaromatic compounds [61] and belongs
to the category of through-space homoconjugate systems [60, 62]. It adopts a
boat-shaped conformation, with just the sp
3 C2 atom being significantly off-plane.
This system has been thoroughly studied, both recently and in the past [63–65],
with the aim of investigating the interplay among its structural, energetics and
induced magnetic properties [60, 66] as well as of evaluating energy barriers
108
C. Gatti et al.
Chemical Terminology [50], “…whereas in an aromatic molecule there is continuous overlap of p-orbitals over a cyclic array of atoms, in an homoaromatic
molecule there is a formal discontinuity in this overlap resulting from the presence
of a single sp
3 hybridized atom at one or several positions within the ring; p-orbital
overlap apparently bridges these sp
3 centres, and features associated with aromaticity are manifest in the properties of the compound”.
Great efforts were devoted in the last decades to gain insights into the
homoaromatic character, and several new homoaromatic compounds were discovered since then. Some authors have recently claimed to have found neutral
homoaromatic molecules [51], even though homoaromatic character in neutral
compounds was largely questioned and debated in the past [52]. In fact,
homoaromaticity is usually associated to charged moieties exhibiting homoconjugate π network, i.e. a formally conjugated system with an interposed non-conjugate
group or a sp
3 -hybridized C atom. From a structural viewpoint, this implies that
homoaromaticity is associated with the union of a cyclopropane-like system with an
unsaturated linear segment [53–58]. From the ED perspective, homoconjugate
patterns may be either distinguished as through-bond (if a bcp is present between
the sp
2 carbon atoms bonded to the off-plane sp
3 group) or as through-space (if a
bcp is lacking). Both kinds of interactions have been thoroughly characterized by
the topological analysis of the ED scalar field by Childs, Cremer and co-workers
[59, 60]. Barzaghi and Gatti [22] have studied the structures, energetics and ED
topological features of a series of supposedly 6π electron homoaromatic systems,
which differ by the number of basal carbon atoms (from 4 to 7) and the formal
charge (−2 to +1), i.e. C n+3 H n+4
(n−4) for n = 2−5. They actually found that Möbius
aromaticity is preferred over through-bond homoaromaticity, in stabilizing all these
structures. In the former, stability is achieved by involving the basis orbitals of the
external methylene group, thus effectively ruling out transmission of conjugation
along the ring-fused bond. The electron delocalization largely involves the off-plane
sp
3 group, leading to a strong enhancement and to a gain of partial π-character of
the bonds linking the polyenic fragment with the methylene group. This situation
chiefly characterizes the case with n = 5, discussed through the SF descriptor in the
next paragraph.
5.3.2 Homotropylium Cation
Homotropylium cation (cyclooctatrienylium, (I), C 8 H 9
+ , Scheme 5.1), also called
homotropenylium, is the archetype of homoaromatic compounds [61] and belongs
to the category of through-space homoconjugate systems [60, 62]. It adopts a
boat-shaped conformation, with just the sp
3 C2 atom being significantly off-plane.
This system has been thoroughly studied, both recently and in the past [63–65],
with the aim of investigating the interplay among its structural, energetics and
induced magnetic properties [60, 66] as well as of evaluating energy barriers
108
C. Gatti et al.