Chemistry Reference
In-Depth Information
according to their degree of symmetry and topology, which ultimately determine
the physical and chemical properties of the solids they build.
Electronic configuration
From the electronic point of view the most remarkable property of MOMs is that
metals, superconductors and magnets can be built when only
s
- and
p
-electrons
are involved. For metals this is not surprising since alkali metals are based
on
s
-electrons, and for graphite delocalized
p
z
-electrons are responsible for its
semimetallic character. However, the existence of magnetic order without
d
-
electrons is more intriguing, especially when thinking of classical magnets, known
for centuries, which are all based on transition metals. In fact W. Heisenberg con-
cluded, back in 1928, that ferromagnetism could not exist in compounds consisting
only of light elements (Heisenberg, 1928). Fortunately he was wrong, otherwise
Section 1.5 would make little sense.
As will be discussed later (Section 1.5), molecules containing no metallic
elements are able to combine and form materials exhibiting metallic character,
e.g., HMTSF-TCNQ, TTF-TCNQ, etc., or even lose any electrical resistance be-
low a given temperature and thus become superconductors, e.g., (TMTSF)
2
ClO
4
.
Metal-free molecules can also, in the solid state, show magnetic order, such as
p
-NPNN and
p
-NC
CNSSN, where in the absence of
d
-electrons the mag-
netic properties are related to unpaired
p
-electrons.
·
C
6
F
4
·
Symmetry
Symmetry is one of the most important parameters when characterizing molecules
and solids because it permits the rationalization of their physical properties and helps
in simplifying the resolution of the Schrodinger equation (see Section 1.7). Follow-
ing the common practice, Schonflies and short Hermann-Mauguin symbols will be
used for point and space groups, respectively. Readers interested in learning more
about group theory applied to chemistry are encouraged to read e.g., Cotton, 1971.
Let us start with the neutral molecule TTF, the fundamental building block
of an immense pool of CTSs exhibiting magnetic ground states, semiconducting,
metallic or superconducting properties that will be extensively discussed in the
topic. Figure 1.3 shows a TTF molecule oriented in a Cartesian coordinate system,
where the long molecular axis is arbitrarily set along the
x
-axis. We will keep this
convention throughout the topic.
If we assume that TTF is perfectly flat (contained in the
xy
-plane), TTF remains
invariant under the following symmetry operations:
E
,
C
2
(
x
)
,
C
2
(
y
)
,
C
2
(
z
)
,σ
xy
,σ
xz
,σ
yz
,
i
,
