Chemistry Reference
In-Depth Information
which the (intramolecular) distance between non-bonded atoms is smaller than the
sum of the van der Waals radii of the involved atoms [
9
,
10
]. The fjord regions in
BAEs resemble the unique rim motif in [5]helicene (dibenzo[
c
,
g
]phenanthrene)
[
31
]. [n]Helicenes are non-planar for
n
4. Accordingly, cove and fjord regions
introduce non-planarity in polycyclic aromatic hydrocarbons (PAHs) [
32
].
In BAEs, the intramolecular overcrowding requires out-of-plane deformations in
order to accommodate the sterically demanding bistricyclic moieties without pro-
hibitively close contacts of non-bonded atoms in the fjord regions on both sides of
the central double bond (C
9
¼
C
9
0
). A hypothetical coplanar bistricyclic aromatic
ene based on conventional bond lengths [
33
] and bond angles would maintain very
short non-bonded carbon-carbon, carbon-hydrogen, and hydrogen-hydrogen dis-
tances at positions 1, 1
0
, 8, and 8
0
, leading to a considerable overlap of the van der
Waals radii. The associated repulsive interactions may be relieved by deviations
from coplanarity and by various bond angle and bond length distortions.
The non-planarity of an ethylene group (a double bond and the four attached
atoms) may be described by three parameters [
34
-
36
]. In this work, the pure twist
ω
and the two pyramidalization angles of the double bond carbons C
9
and C
9
0
will be
used. The pure ethylenic twist
is defined as the average of the two torsion angles
that are smaller than 90
[
1
,
34
-
36
]:
ω
C
9a
-C
9
-C
9
0
-C
9a
0
C
8a
-C
9
-C
9
0
-C
8a
0
ω
¼
1
=
2
τ
þ τ
ð
1
Þ
For the pyramidalization, several parameters have been introduced. Their advan-
tages and disadvantages have been reviewed [
37
,
38
]. In the present study, the
angles
(C
9
) and
(C
9
0
) will be used [
1
,
34
-
36
,
38
]:
ˇ
ˇ
χ
(C
9
) = (
τ
(C
9a
-C
9
-C
9
-C
8a
) MOD 360°) −180°
9
c(C
9
)
9
ð
2
Þ
9a
8a
χ
(C
9
) = (
τ
(C
9a
-C
9
-C
9
-C
8a
) MOD 360°) −180°
is sensitive to small pyramidalization effects and not dependent on local
symmetry. The values are readily accessible from computational as well as crys-
tallographic data. In
syn
-pyramidalized double bonds, the pyramidalization angles
χ
χ
(C
9
0
) have identical signs, whereas in
anti
-pyramidalized double bonds
they have opposite signs [
1
,
34
,
35
]. Pyramidalization of
sp
2
-carbons leads to a
rehybridization, adding some
s
character to the
(C
9
) and
χ
-orbital (
s
m
p
-hybrid rather than
pure
p
-orbital). The rehybridization and the direction of the
π
π
-orbital relative to the
˃
-bonds and to neighboring
π
-orbitals (Fig.
3
) have been extensively studied using
π
the
-orbital axis vector (POAV) theory [
37
,
39
,
40
].
The non-planarity of the tricyclic moieties may be described by the torsion
angles defining the conformations of the central rings. Alternatively, the folding
dihedral A-B (and C-D), defined as the dihedral of the least-squares-planes of the
