Conformational evolution of TFSI- in protic and aprotic ionic liquids

We here report on the conformational evolution of the bis(trifluoromethanesulfonyl) imide anion (TFSI-) in protic and aprotic TFSI--based ionic liquids as a function of temperature. The investigation is performed by Raman spectroscopy in the spectral ranges 240-380 cm(-1) and 715-765 cm(-1), where the interference from bands due to the cations is negligible. The contribution from each TFSI- conformation, i.e. the cisoid (C-1) and the transoid (C-2), is quantified in order to estimate the enthalpy of conformational change, Delta H, which is found to be in the range 3.4-7.3 kJ/mol in the liquid state. Conformational information is for the first time determined from the 740 cm(-1) band, which previously mainly has been used as an indicator of ion-ion interactions. The similarity in Delta H values obtained from the two spectral ranges demonstrates the validity of using also the 740 cm(-1) band for the quantification of the TFSI conformational evolution. Copyright (C) 2010 John Wiley & Sons, Ltd.