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 experiment. Both compounds crystallize in the same monoclinic centrosymmetric space group, showing an anisotropic H-bonding network and imidazolium-imidazolium pi(+)-pi(-) 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.