On the Colossal and Highly Anisotropic Thermal Expansion Exhibited by Imidazolium Salts

The imidazolium salts 1-ethyl-2,3-dimethylimidazolium
chloride,
Edimim­[Cl], and bromide, Edimim­[Br], exhibit negative and positive
thermal expansions, as determined by variable-temperature synchrotron
powder X-ray diffraction experiments. Both compounds crystallize in
the same monoclinic centrosymmetric space group, showing an anisotropic
H-bonding network and imidazolium–imidazolium π+–π+ interactions, which have been corroborated
by density functional theory studies. The chloride derivative displays
a highly anisotropic thermal expansion with a colossal positive coefficient
along one direction. Replacement of Cl– by Br– in the same crystal structure produces an increase
of the colossal coefficient and induces a biaxial negative thermal
expansion. By studying the molecular vibration factors and the H-bonding
framework in their crystals as a function of temperature, it was possible
to rationalize at the molecular level the mechanism for the observed
anomalies in thermal expansion.