Influence of Alkyl Chain Length on Microscopic Configurations of the Anion in the Crystalline Phases of PYR1A-TFSI

The infrared spectra and their temperature dependence are measured for a series of pyrrolidinium based ionic liquids (ILs) sharing the bis(trifluoromethanesulfonyl)imide (TFSI) anion and having alkyl chains of different length. While in the liquid or glassy state, both conformers of TFSI are retained for all compounds, in the solid state a strong predominance of trans-TFSI occurs in ionic liquids with alkyl chains shorter than five C-H groups; on the contrary, for alkyl chain longer than six C-H groups crystalline phases display only cis-TFSI, which is a rare configuration in solids. Moreover, a mixed system composed of a short chain liquid (PYR14-TFSI) with one having a longer chain (PYR18-TFSI) in a mass ratio of 1:1 is studied. The competition between the two conformers of TFSI hinders the crystallization and gives rise to a glass transition around 183 K.