Chemistry Reference
In-Depth Information
this topic to give a lengthy description of polymorphism and in this section we will
deal with a few concepts intimately related to molecular organic crystals. Those
willing to learn more about this intriguing phenomenon are recommended to read
Bernstein, 2002.
Polymorphism is very important for MOMs because of the weak intermolecu-
lar interactions involved. This implies that molecular crystals exhibit low cohesion
energies, as previously discussed, and molecules may adopt different shapes, result-
ing in what is termed conformational polymorphism (Bernstein & Hagler, 1978).
Torsion about single bonds defines the molecular conformation. The energy in-
volved for torsion is
c
. 0.05 eV molec
−
1
, comparable to the energy difference
between different crystalline forms. It thus becomes clear that for molecules which
possess torsional degrees of freedom, various polymorphs are expected. The con-
formational degrees of freedom are thus intrinsic factors. The previously discussed
cases of BEDT-TTF and
p
-NPNN molecules are good examples of conformational
polymorphism because of the presence of mobile ethylenedithio groups and the tor-
sion of the carbon-carbon bond connecting the nitrophenyl and nitronyl nitroxide
groups, respectively. BEDT-TTF salts and
p
-NPNN exhibit several polymorphs,
but for the vast majority of MOMs only one or two crystallographic phases have
been
observed
.
In fact, the number of polymorphs of a given material can be larger than the
number established to date. It mainly depends on the chosen synthesis route. Thus,
many polymorphs are still waiting to be discovered! Extrinsic factors influencing
the observation of polymorphs are more difficult to define and refer to the dimen-
sionality of the parameter hyperspace. Themysterious phenomenon of disappearing
polymorphs should serve as an example (Dunitz & Bernstein, 1995). Polymorphs
of several materials have been
unambiguously
prepared and characterized, and for
some unknown and uncontrollable factors have never been found again, despite
longstanding efforts. Logic tells us that if they have been prepared at some stage
it should always be possible to obtain them again, provided that the right exper-
imental conditions are met again. It is of course a matter of time, dedication and
systematic work, sometimes incompatible with the ever decreasing scientific time
scale, urged by the absurd competition towards fast reporting of results. It is clear
that the larger the dimensionality of the parameter hyperspace (the relevant external
parameters involved), the larger the time needed for successful synthesis. This is
further complicated by the small energy barriers between polymorphs, whichmeans
that small variations in e.g., temperature by few degrees makes the target synthetic
route unviable. On the other hand, a given material, because of its extraordinary
properties, becomes synthesized in a single way after its discovery leading to a
unique crystallographic phase. However, when synthesized differently, new phases
