Importance of weak interactions and conformational equilibrium in N-butyl-N-methylpiperidinium bis(trifluromethanesulfonyl) imide room temperature ionic liquids: Vibrational and theoretical studies

• Combined vibrational spectroscopic and DFT studies of piperidinium cation based ionic liquids is presented.
• Variation in number and strength of H-bondings found to affect and control the physical properties of ionic liquids.
• Raman studies shows existence of cisoid–transoid conformational equilibrium in NTf2− anion of PIP14NTf2 ionic liquid.
• Calculated interaction energies for PIP14NTf2 and PIP14Br show a linear relationship with melting points.

Piperidinium cation-based room temperature ionic liquids (RTILs) constitute an important class of ILs because of their unique electrochemical properties as well as non-aromatic nature of the cation. However, detailed structural studies are yet to be done. In this paper, we discuss the molecular structure and vibrational spectra of N-butyl-N-methylpiperidinium bis(trifluromethanesulfonyl) imide, (PIP14NTf2; where, PIP14 is N-butyl-N-methylpiperidinium and NTf2 is bis(trifluromethanesulfonyl) imide), obtained with a combined approach of infrared (IR) and Raman spectroscopies in the liquid state and density functional theory (DFT) and Hartree–Fock (H–F) based theoretical calculations. DFT calculations, which are found to produce the most stable geometry compared to other two methods (MP2 and H–F), reproduce the experimental IR and Raman spectra reasonably well. Our findings reveal structural properties that profoundly influence intermolecular interactions and melting point. There exists a large variation in the melting point of the ILs studied. While the bromide salt of the piperidinium derivative (PIP14Br) is solid with very high melting point (241 °C), the corresponding NTf2 salt is low viscous liquid at room temperature (mp: −25 °C). bmimBr (bmim = 1-butyl-1-methylimidazolium) exhibits a substantially lower melting point of 79 °C than PIP14Br, suggesting that more number of strong classical hydrogen bonding interactions in the latter is primarily responsible for the much higher melting point. In addition, involvement of the alkyl group in PIP14 in H-bonding interaction provides additional rigidity in n-butyl chain which is otherwise absent in bmimBr. Interaction energy for PIP14Br is found to be higher than PIP14NTf2, showing a positive correlation between interaction energy and melting point. A blue shift in CH stretching wavenumber as evident from IR and Raman spectra of PIP14Br IL is a clear indication of the stronger hydrogen bonding as compared to PIP14NTf2 IL. Furthermore, we experimentally observe the existence of cisoid–transoid conformational equilibrium of NTf2− anion in the Raman spectrum of PIP14NTf2 for the first time and determined that transoid NTf2− anion to be more stable than the corresponding cisoid conformer by 1.04 kcal/mol using DFT. Examination of various conformational possibilities of the cation shows that the butyl group preferentially exists in gauche conformation.