Spectroscopic and DFT Studies of Cation-Anion Interaction in Imidazolium and Piperidinium Based Ionic Liquids

Abstract

Interaction in various imidazolium and piperidinium Ionic Liquids (ILs) as well as their strength has been discussed using NMR and IR spectroscopic techniques with strapping support from DFT calculation. Strong hydrogen bonding interaction in imidazolium/ piperidinium halide based ILs as compared to PF6, BF4 and NTf2 containing ILs has been explained well with the help of spectroscopy. Higher viscosity of bmimPF6 compared to bmimBF4 and bmimNTf2 has been elucidated due to strong interaction of C2-H with anion, observed in NMR and IR spectroscopy and has been sturdily supported by DFT calculated IR frequencies showing blue shift in C2-H stretching when moving from bmimPF6 to bmimNTf2. Further, a molecular level comparison of PIP14Br and bmimBr (PIP14Br=N-butyl-N-methylpiperidinium bromide, melting point (mp): 241°C and bmimBr = 1-butyl-3-methylimidazolium bromide, mp: 79°C) have pointed out that more number of strong classical hydrogen bonding interactions in the former is primarily responsible for much higher melting point. Hence significance variation in number and strength of H-bonding in ILs predominantly controls the physical property of the salt/ionic liquids.

Keywords: DFT calculation, Hydrogen bonding, Ionic liquids, Spectroscopy.

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