Strong anion-anion hydrogen bond in the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate

This work discusses the remarkable differences in the mid- and far-infrared regions of the spectrum of the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate, [C(2)C(1)im][HSO4], in comparison to ionic liquids based on other anions. The infrared spectrum unveils the occurrence of anion-anion hydrogen bond in [C(2)C(1)im][HSO4]. The band related to the anion O-H stretching mode,.(OH), covers a large frequency range (2000-3400 cm(-1)) and exhibits the spectral pattern of strongly hydrogen bonded systems. The far-infrared (FIR) spectrumof [C(2)C(1)im][HSO4] also exhibits the signature of the anion-anion interaction, that is, the stretching mode of the hydrogen bond,.(O.O), at similar to 170 cm(-1). Hydrogen bond between [HSO4](-) anions also occurs in ionic liquids containing imidazolium cation with longer alkyl chain ([C(4)C(1)im]+) or tetraalkylammonium cation. The vibrational frequencies of nu(OH) and nu(O center dot center dot center dot O) modes satisfy empirical correlations that characterize the anionanion interaction in [C(2)C(1)Im][HSO4] as a strong hydrogen bond. (C) 2020 Elsevier B.V. All rights reserved.