cg5b00633_si_004.cif (12.76 kB)
On the Colossal and Highly Anisotropic Thermal Expansion Exhibited by Imidazolium Salts
dataset
posted on 2015-11-04, 00:00 authored by I. de Pedro, A. García-Saiz, J. Dupont, P. Migowski, O. Vallcorba, J. Junquera, J. Rius, J. Rodríguez
FernándezThe 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.