The structure and hydrogen-bond properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide and DMSO mixtures?

Physical Chemistry Chemical Physics Pub Date: 2020-11-19 DOI: 10.1039/D0CP03640D

Abstract

Mixing ionic liquids (ILs) with molecular solvents can extend the practical applications of ILs and overcome the drawbacks of neat ILs. Knowledge on the structure and hydrogen-bond interaction properties of IL-molecular solvent mixtures is essential for chemical applications. In this work, the structure and hydrogen-bond features of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([CnMPyr][Tf2N], n = 3, 4, 6 and 8) and DMSO mixtures were studied using Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations. Excess infrared absorption spectroscopy and two-dimensional correlation spectroscopy (2D-COS) were employed to extract structural information on the mixtures from the C–D systematic stretching vibrational (νs(C–D)) region of the methyl groups in DMSO-d6. It was found that the mixing process of [CnMPyr][Tf2N] and DMSO is non-ideal and interaction complexes form between [CnMPyr][Tf2N] and DMSO-d6. They are ion cluster-DMSO-d6 complexes and ion pair-DMSO-d6 complexes. In the mixing processes, the species present in pure DMSO gradually decrease from DMSO dimer to DMSO monomer with an increase in ILs. Besides, the ion cluster-DMSO complexes gradually increase, while the ion pair-DMSO complexes decrease due to the strong electrostatic interaction between the cation and anion. In the ion cluster-DMSO complexes and ion pair-DMSO complexes, the ring hydrogen atoms of the methylene group directly attached to the nitrogen atom are the preferred interaction sites of the [CnMPyr]+ cations. All the hydrogen bonds in the identified complexes are closed-shell, electrostatically dominant and weak.

Graphical abstract: The structure and hydrogen-bond properties of N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide and DMSO mixtures
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