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

• Published in: Journal of molecular liquids 2020-07, Vol.310, p.113178, Article 113178
• Main Author: Ribeiro, Mauro C.C.
• Format: Article
• Language: English
• ISSN: 0167-7322; 1873-3166
• DOI: 10.1016/j.molliq.2020.113178

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, [C2C1im][HSO4], in comparison to ionic liquids based on other anions. The infrared spectrum unveils the occurrence of anion–anion hydrogen bond in [C2C1im][HSO4]. The band related to the anion OH stretching mode, ν(OH), covers a large frequency range (2000–3400 cm−1) and exhibits the spectral pattern of strongly hydrogen bonded systems. The far-infrared (FIR) spectrum of [C2C1im][HSO4] also exhibits the signature of the anion–anion interaction, that is, the stretching mode of the hydrogen bond, ν(O…O), at ~170 cm−1. Hydrogen bond between [HSO4]− anions also occurs in ionic liquids containing imidazolium cation with longer alkyl chain ([C4C1im]+) or tetraalkylammonium cation. The vibrational frequencies of ν(OH) and ν(O…O) modes satisfy empirical correlations that characterize the anion–anion interaction in [C2C1Im][HSO4] as a strong hydrogen bond. •There is anion–anion hydrogen bond in ionic liquids based on the hydrogen sulfate anion.•The stretching of the hydrogen bond is clearly identified in the far-infrared spectrum of [C2C1im][HSO4].•The high- and the low-frequency vibrations characterize the strong nature of the hydrogen bond between [HSO4]− anions.