Chemistry Reference
In-Depth Information
in the systems having three or more Clar sextets in the biradical
structures in comparison with the Kekulé structures. Aromatic
stabilization energy (ASE) of benzene based on homodesmic
stabilization energy (HSE) is ca. 90 kJ/mol,
14
three times of which is
comparable to the C-C Π-bonding energy of 272 kJ/mol.
15
Thus two
unpaired electrons in the biradical structure of
3-3
(the difference of the
Clar sextet is two) prefer electron pairing to form a closed-shell ground
state, whereas
3-5
(the difference of the Clar sextet is three) is subjected
to competition between the pairing of two unpaired electrons and the
aromatization of the six-membered rings.
Table 1. The degrees of open-shell character
Y
0
of
polyacenes and polyperiacenes
A
-
B
. In parentheses, spin
contaminations are indicated.
B
A
2
3
4
1
0.00 (0.00)
0.00 (0.00)
0.01 (0.31)
0.00 (0.00)
0.12 (1.01)
0.60 (1.63)
3
5
0.01 (0.35)
0.59 (1.66)
0.91 (1.90)
7
0.05 (0.83)
0.84 (1.83)
0.98 (2.05)
2.3.
Aromaticity of each ring
The nucleus-independent chemical shift (NICS) calculation
16a,b,c,d
is
informative for an aromatic contribution of each six-membered ring.
We performed the NICS calculation of polyperiacenes
A
-
B
with the
GIAO
16e
-B3LYP/6-31G* method. Figure 5 shows the NICS(1) values of
polyacenes and polyperiacenes. Bisanthene
3-3
has the largest aromatic
contribution in the ring B, which suggests a dominant contribution of the
Kekulé structure in Fig. 4 to the ground state.
17
On the other hand, the
NICS(1) value of the ring B in
3-5
approaches the non-aromatic and a
large aromatic contribution is found in the ring A and D. This finding
clearly indicates decrease and increase of Kekulé and biradical
contributions to the ground state of
3-5
, respectively. Larger homolog
3-7
shows more non-aromatic NICS(1) values in the ring B and E,
supporting its more pronounced biradical character.
