Structure and hydrogen-bond properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide and ethanol: A combination of FTIR and theoretical studies

Abstract

Pyrrolidinium-based ionic liquids (ILs) demonstrate multiple irreplaceable features. One key to a better understanding of their unique properties is spectroscopic tools combined with density functional theory (DFT) calculations to study their structure features. In this regard, we investigated the structure features of four N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([CnMPyr] [Tf2N], n=3, 4, 6 and 8) ILs and ethanol mixtures using infrared spectroscopy (IR) and DFT calculations. Excess and two-dimensional correlation spectroscopy were employed to progressively enhance the infrared resolution and extract the structural information of the binary liquid systems from the O−D stretching vibrational region (v(O−D)) of the CD3CD2OD hydroxyl group. The mixing process is non-ideal, and different interaction complexes (the alcohol cyclic trimer/tetramer/larger multimers, anion−ethanol, ion pair−ethanol, and ion cluster−ethanol complexes) were identified. With increasing [CnMPyr] [Tf2N], the cyclic ethanol multimers were gradually broken apart, with the larger cyclic multimers vanishing first. For the hydrogen-bonded complex between IL and ethanol, the anion−ethanol complex appeared first, followed by the ion pair−ethanol complex. The ion cluster−ethanol appeared lastly in the mixture. The hydrogen-bonds with weak strength, closed shells and electrostatically dominant characteristics were found in different complexes.