Effect of anion and alkyl chain length on the structure and interactions of N-alkyl pyridinium ionic liquids

The structure and interactions of a series of ionic liquids composed of N-alkyl pyridinium cation ([CnPy]+, n = 2, 4, 6, 8, 10, 12, 14) with varied anions like bromide (Br−), tetrafluoroborate ([BF4]−), bis(trifluoromethanolsulfonyl)imide ([NTf2]−) and hexafluorophosphate ([PF6]−) were investigated using ATR - FTIR and Raman spectroscopy. The observed frequencies were compared to the frequencies computed by means of DFT theory at the B3LYP/6-311 ++G(d,p) level. Significant variations were observed in the vibrational spectra of the pyridinium based ionic liquids, with increasing chain length of the alkyl group in the cation. The nature and extent of the chain-length dependent variations in the spectra was also dependent on the anionic species present in the sample. Comparison of the observed spectra to the computed spectra and previously reported data for imidazolium based ionic liquids indicate mesoscopic segregation of polar and nonpolar regions in the pyridinium ionic liquids. The strength of the cation-anion interaction determines the nature and extent of such microscopic heterogeneity, which is reflected in the anion-specific variations in the vibrational spectra. While the mesoscopic reorganization of pyridinium based ionic liquids is analogous to that in imidazolium based ionic liquids, some important differences have also been noted. The observations highlight the importance of all three structural features - cation, anion and alkyl chain length - in determining the microscopic organization and properties of ionic liquids. The results emphasize the need for broadening the scope of ionic liquid research to include more studies on the non-imidazolium based ionic liquids.