Structural behaviour of ionic liquids

Andrew Dolan, Tom Welton
The ring flipping behaviour of some pyrrolidinium based ionic liquids has been examined using variable temperature NMR spectroscopy, which allows the calculation of the Gibb's free energy of activation of the ring flip. As it was impossible to examine the neat ionic liquids, the work was carried out in solution. The role of the solvent was investigated and its effect on the ring flip activation energy of N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide was found to be accurately
more » ... accurately described by the Kamlet-Taft parameters of the solvent. A range of ionic liquids were examined in dimethylsulfoxide solutions. For a range of N-methylpyrrolidinium ionic liquids there was no correlation found between any property of the system and the ring flip activation energy. For a range of N-alkylpyrrolidinium hydrogensulfate ionic liquids there was a positive correlation between the ring flip activation energy and the free volume of the solution. While the free volume term correlates well, it is believed to be indicative of some underlying entropic effect. A range of 1-alkyl-3-methylimidazolium and 1,3-dialkylimidazolium ionic liquids were examined using small angle X-Ray diffractometry. It was found that 1-alkyl-3- methylimidazolium thiocyanates have a unique pre-peak in their diffraction patterns, the structural origin of this peak is a repeating structure that is about twice the length of the cation. Ionic liquids with other anions reported to date, and those studied here, all have a pre-peak originating from a repeating structure that is about the length of the cation. This strongly suggests 1-alkyl-3-methylimidazolium thiocyanates have a different liquid structure than other ionic liquids. A pre-peak was found in 1-butyl-3-methylimidazolium thiocyanate, which is the first report of a pre-peak in an imidazolium based IL with such a short alkyl chain. It was also found that 1,3-dihexylimidazolium ionic liquids have two peaks in the very small angle regime. It was not possible to precisely determined the structural origin o [...]
doi:10.25560/14462 fatcat:mv6b6q6oj5c6xdhr6dbzggsupu