(253b) Monitoring Inter- and Intra-Molecular Interactions in Imidazolium Protic Ionic Liquids HCnImNTf2 (n=0-12)

In this work we synthesized and studied the effect of the alkyl chain size in the structure of the imidazolium based Protic Ionic Liquids (PILs) 1-H-3-alkyl-imidazolium bis(trifluoromethanesulfonyl)imide HC_nImNTf₂ (n=0-12) , using vibrational spectroscopy (FTIR and Raman). The main characteristic of PILs is the presence of a proton on site 1 of the imidazolium ring. As a result PILs can form an extensive network of hydrogen bonds (HB).

The observed N-H vibrational modes in the high frequency spectral region at 3270-3150cm^-1 indicate that the liquid phase of ionic liquids consists of ion pairs. The observed redshifts, of the N-H and C_(i)-H bands frequencies (in the same region) with increasing alkyl chain length, suggest the strengthening of the HB between the anion and the cation. However, the C_(i)-H vibrational modes are primarily affected when n is varied between 0 and 3 (short alkyl chains), whereas N-H modes are affected for when n is varied from 3 to 12.Thus, the shift of the C_(i)-H mode can be attributed to the induction +I effect due to the presence of the alkyl chain in the ring, whereas the shift of the N-H modes is attributed to the intercationic interactions between the imidazolium rings that can be attributed to the parallel packing in the liquid phase structure.

The parallel stacking of the imidazole rings was also suggested from FT-Raman high frequency region. It was shown that when n>3, alkyl groups tend to develop structures similar to those of n-alkane melts, consisting of parallel imidazolium rings with oriented chains, defining polar and non polar regions. Moreover, the conformation of the anion NTf₂^- was studied as a function of temperature and the alkyl chain length. It was shown that the elevation of the temperature favors the cis conformation of the anion, whereas trans conformation is dominant for bigger cations with long alkyl chains.

Finally, the enthalpy ΔΗ_eq of the conformational equilibrium of the anion NTf₂^- for the PILs HC_nImNTf₂ (n=0-12) was experimentally calculated from spectroscopic data. The calculated enthalpy values reduce as the alkyl chain increases. The change in the slope of the calculated values from –ethyl to –propyl may reflect the change from intramolecular to intermolecular interactions.