Chemistry Reference
In-Depth Information
KEKUL STRUCTURE
A
B
C
BIRADICAL STRUCTURE
Fig. 7. (Left) Bond lengths of
1
(at 200 K), anthracene (94 K),
20a
and phenanthrene
(295 K).
20b
(Right) Resonance structure of bisanthene
3-3
.
estimated to be only 21% according to the Pauling-Brockway
relationship,
19
which represents C-C bond character as a function of an
experimental bond length. Thus the X-ray data agree with the
consideration of the Kekulé structure as having more importance in the
resonance hybrid of
1
.
3.2.
Physical properties
Physical properties of
1
also support the geometrical consideration on the
ground state of
1
. In general, a small HOMO-LUMO energy gap is an
essential factor for open-shell character of a molecule, because a
HOMO-LUMO gap is closely related to the promotion of electrons from
HOMO to LUMO. A single electron HOMO-LUMO excitation along
with spin inversion leads to the triplet biradical state, whereas admixing
a doubly excited configuration
1
Φ
H,H→L,L
into a ground state leads to
singlet biradical character of the system. Electronic absorption spectrum
of
1
in a toluene solution gave an intense low-energy band at 662 nm
(
Ε
= 32,500), from which the optical HOMO-LUMO energy gap of
1
