below (trans-C2) the pseudo-7MR plane.
1 On the other hand, for the C2–C3 bond,
the SF contributions are clearly different whether they are evaluated above or below
the plane. More in detail, SF nn % and SF others % are greater when the rp is chosen in
trans- with respect to the C2 atom, that is by moving the rp towards the interior
rather than to the exterior of the ring and thus to a more symmetric position relative
to the atoms of the ring. Thanks to the degree of freedom ensured by the choice of
the rp, the SF descriptor may be a very efficient sensor of electron delocalization
magnitude asymmetries.
In conclusion, even for a homoconjugated system, like the homotropylium
cation (I), the SF picture nicely fits with that provided by more sophisticated
instruments like the delocalization indices. On top of this, due to its mixed
local/integral nature (choice of the rp and summation of local sources within a
region of space), the SF descriptor may reveal relevant asymmetries in the electron
delocalization processes. These may be related to different magnitudes of the
electron delocalization effects when travelling in opposite directions along a
sequence of bonds, or to different magnitudes of such effects when moving perpendicularly to a bond path and in opposite directions relative to a bcp.
In the next section, a more complex case will be explored, where homoconjugation coexists with aromaticity in a nonbenzenoid 10-membered ring (10MR).
5.3.3 1,6-Methano[10]Annulene
1,6-methano[10]annulene (IIa, C 11 H 10 , Scheme 5.3) was the first discovered stable,
aromatic cyclodecapentaene. In this respect, it represents a true cornerstone in the
history of nonbenzenoid aromatic compounds [75]. Even though the methylene
bridge at the C11 carbon (Scheme 5.4) distorts the hydrocarbon backbone from
being planar, the 10MR is aromatic, as it can be inferred from the NMR spectrum.
Indeed, peripheral H nuclei are deshielded, while the bridging ones are strongly
shielded, due to the significant intra-annular diamagnetic ring current induced by
the magnetic probe [76]. Moreover, the main ring is prone to electrophilic substitution reactions [77, 78]. Interestingly, (IIa) can exist in equilibrium with its norcaradiene valence tautomer (IIb, Scheme 5.3), that undergoes spontaneous valence
tautomerization to the (slightly) more stable (IIa) compound [75, 79]. The equilibrium between these two forms is rather subtle, and it is not surprising that
different substituents R, R′ at the bridging methylene atom can reverse the stability
order, as demonstrated by single-crystal X-ray diffraction experiments [80–83].
In general, bridged methano[10]annulene derivatives are a very attractive class
of compounds for investigating non-trivial aromatic systems. Being quite easy to be
synthesized and showing a somewhat ‘tunable’ conjugation pattern [84] due to the
1
Actually, the pseudo-7MR system is not rigorously planar. Here we intend the mean least-squares
plane passing through the sp
2 atoms.
112
C. Gatti et al.
1 On the other hand, for the C2–C3 bond,
the SF contributions are clearly different whether they are evaluated above or below
the plane. More in detail, SF nn % and SF others % are greater when the rp is chosen in
trans- with respect to the C2 atom, that is by moving the rp towards the interior
rather than to the exterior of the ring and thus to a more symmetric position relative
to the atoms of the ring. Thanks to the degree of freedom ensured by the choice of
the rp, the SF descriptor may be a very efficient sensor of electron delocalization
magnitude asymmetries.
In conclusion, even for a homoconjugated system, like the homotropylium
cation (I), the SF picture nicely fits with that provided by more sophisticated
instruments like the delocalization indices. On top of this, due to its mixed
local/integral nature (choice of the rp and summation of local sources within a
region of space), the SF descriptor may reveal relevant asymmetries in the electron
delocalization processes. These may be related to different magnitudes of the
electron delocalization effects when travelling in opposite directions along a
sequence of bonds, or to different magnitudes of such effects when moving perpendicularly to a bond path and in opposite directions relative to a bcp.
In the next section, a more complex case will be explored, where homoconjugation coexists with aromaticity in a nonbenzenoid 10-membered ring (10MR).
5.3.3 1,6-Methano[10]Annulene
1,6-methano[10]annulene (IIa, C 11 H 10 , Scheme 5.3) was the first discovered stable,
aromatic cyclodecapentaene. In this respect, it represents a true cornerstone in the
history of nonbenzenoid aromatic compounds [75]. Even though the methylene
bridge at the C11 carbon (Scheme 5.4) distorts the hydrocarbon backbone from
being planar, the 10MR is aromatic, as it can be inferred from the NMR spectrum.
Indeed, peripheral H nuclei are deshielded, while the bridging ones are strongly
shielded, due to the significant intra-annular diamagnetic ring current induced by
the magnetic probe [76]. Moreover, the main ring is prone to electrophilic substitution reactions [77, 78]. Interestingly, (IIa) can exist in equilibrium with its norcaradiene valence tautomer (IIb, Scheme 5.3), that undergoes spontaneous valence
tautomerization to the (slightly) more stable (IIa) compound [75, 79]. The equilibrium between these two forms is rather subtle, and it is not surprising that
different substituents R, R′ at the bridging methylene atom can reverse the stability
order, as demonstrated by single-crystal X-ray diffraction experiments [80–83].
In general, bridged methano[10]annulene derivatives are a very attractive class
of compounds for investigating non-trivial aromatic systems. Being quite easy to be
synthesized and showing a somewhat ‘tunable’ conjugation pattern [84] due to the
1
Actually, the pseudo-7MR system is not rigorously planar. Here we intend the mean least-squares
plane passing through the sp
2 atoms.
112
C. Gatti et al.