Chemistry Reference
In-Depth Information
2. ELECTRONIC STRUCTURE OF EDGE STATE AND STM/STS OBSERVATIONS
Let us start with the relationship between aromaticity in small
hydrocarbon molecules and the issue on edge state in
nanographene/graphene edges. Benzene, which is a typical Kekul
molecule and the primary building block of graphene, consists of three
bonding Π- and three antibonding Π*-orbitals split mutually with a large
HOMO-LUMO gap, which corresponds to energy stabilization of
aromaticity. When benzene rings are fused to each other to form
condensed polycyclic hydrocarbon molecules, the Kekulé structure with
a large HOMO-LUMO gap is usually conserved, as evidenced in
naphthalene, anthracene, etc. However, there are exceptional molecules
called non-Kekulé molecules, in which the energy stabilization is
failed.
16-20
A typical example is phenalenyl radical consisting of three
benzene rings fused in a triangle shape as shown in Fig. 1(b). In this
molecule, an additional Π-electron state is present at the Fermi level in
the energy gap as a nonbonding Π-electron state, which works to
destabilize the molecule. A similar situation in the electronic structure is
present in triangle molecules having larger sizes, as shown in Figs. 1(c)
and (d), where the triangle molecules consisting of 6 and 10 benzene
rings have 2 and 3 nonbonding Π-electron states, respectively. Here,
Lieb's theorem
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can tell us how large number of nonbonding states is
present in a molecule concerned. According to the Lieb's theorem, the
(A)
S = 0
(B)
(C)
(D)
(E)
S = 3/2
S = 1/2
S = 1
Fig. 1. (a) Kekulé molecules, (b), (c), (d) triangle-shaped non-Kekulé molecules consisting
of 3, 6 and 10 benzene rings, (e) spatial distribution of the nonbonding Π-electron state.
