The role of the C2 position in interionic interactions of imidazolium based ionic liquids: a vibrational and NMR spectroscopic study

Abstract

Methylation of the C2 position of 1,3-dialkylimidazolium based ionic liquids disrupts the predominant hydrogen-bonding interaction between cation and anion leading to unexpected changes of the physicochemical properties. We found the viscosity of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁C₁Im][Tf₂N], for example, to be about three times higher than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁Im][Tf₂N]. In order to explain these macroscopic changes upon methylation we investigated the vibrational as well as the magnetic resonance structure of [Tf₂N]⁻ salts involving the cations 1-ethyl-3-methylimidazolium [C₂C₁Im]⁺, 1-ethyl-2,3-dimethylimidazolium [C₂C₁C₁Im]⁺, 1-butyl-3-methylimidazolium [C₄C₁Im]⁺, and 1-butyl-2,3-dimethylimidazolium [C₄C₁C₁Im]⁺ by means of Fourier-transform infrared (FTIR), Raman and ¹³C NMR as well as ¹H NMR spectroscopy aiming a better microscopic understanding of the cation–anion interaction. To reveal the impact of methylating the C2 position and changing the alkyl side chain length of the imidazolium a detailed assignment of the individual peaks is followed by a comparative discussion of the spectral features also considering already published work. Our spectroscopic findings deduce electron density changes leading to changes in the position and strength of interionic interactions and reduced configurational variations. Both facts are represented on a macroscopic level by the viscosity and melting point. Therefore changes on a macroscopic level clearly express molecular alterations which in turn can be observed using spectroscopic methods as Raman, IR and NMR.