most used and
liquids are those that are liquid at room temperature, detailed
structural features are easily accessible by
looking at those that crystallize and that have their crystal
structure determined (indeed, the investigation of structural
properties in the liquid state requires considerable experimental
effort, as for example the X-Ray spectroscopy, and little information
can be gained if heavy metals are missing).
A simple comparison of these data
can yield important information about the structural
features of the same material in the liquid phase.
view in the X-ray studies reported in the last years on the
structure of 1,3-dialkylimidazoilum salts reveals a typical
trend: they form in the solid state an extended network of
cations and anions connected together by hydrogen bonds.
The monomeric unit is always constituted of one
imidazolium cation surrounded by at least three anions
and in turn each anion is surrounded by at least three
imidazolium cations (Figure 2).
Although the number of anions that surround the cation
(and vice-versa) can change depending upon the anion
size and type of the N-alkyl imidazolium substituents, this structural
is a general trend in imidazolium salts
(for examples see Table 2).
bond always involves the most acidic H2 of the
imidazolium cation (pKa=23.0 for the 1,3-dimethyl
imidazolium cation) followed by the other two
hydrogens (H4 and H5) of the imidazolium nucleus and/
or the hydrogens of the N-alkyl radicals (H6, H7 and H8, Table 2).
bonds possess properties of weak to
moderate hydrogen bonds – they are mostly electrostatic
in nature - (H……X bond lengths > 2.2Å; C-H……X bond
angles between 100°-180°).
that even in
the case B(Ar)4 anions the
formed through relatively strong C-H…π hydrogen bonds.
In the case of the octahedral PF6 anion the equatorial F
atoms participate preferentially in hydrogen bonding
network and in the case of the tetrahedral BF4 anion only
three of the F atoms are usually involved in the linkage.
three dimensional arrangement of the imidazolium
ionic liquids are generally formed trough chains of the
imidazolium rings (Figure 3).
In some cases there are
typical π-π stacking interactions among the imidazolium
rings and in the case of 1-alkyl-3-methylimidazolium salts
a relatively weak C-H…π interaction via the methyl group
and the imidazolium ring-π system can also be found. This
molecular arrangement can generate channels in which
the spherical anions are accommodated as chains. This
structural pattern depends on the anion geometry, and the
internal arrangements along the imidazolium columns vary
with the type of the N-alkyl substituents. Of note that other
effects than π-π stacking, such as the entropy effect and
electrostatic interactions may not favorable the formation
of structures of the type shown in Figure 3.
Therefore, it can be proposed that the best
representation for the imidazolium salts in the solid phase is: [(DAI)x(X)x-n]n+
[(DAI)x-n(X)x)]n-, where DAI is the
1,3-dialkylimidazolium cation and X is the anion.
IL salts have
attracted some interest due to their liquid
crystalline (LC) properties. The origin for these can be found in the
formation of domains "Coulombic layers" where the ionic head-groups
interact with the counterions, and "van der Waals" layers built from
(anti)parallel stacking of the alkyl chains.
Hexafluorophosphate salts with cations up to C20MIM have
been investigated by differential thermal analysis and show one or more
LC transitions. Melting to isotropic liquids occur at rather high
temperatures (> 100°C). The phase behaviour of long chain-length
imidazolium and pyridinium chlorides, tetrachlorocobaltates and
tetrachloronickelates showing LC properties has also been reported.