Hydrogen-Bonding Interactions in Pyridinium-Based Ionic Liquids and Dimethyl Sulfoxide Binary Systems: A Combined Experimental and Computational Study.

Abstract

The addition of highly polar and aprotic cosolvents to ionic liquids has proven to considerably decrease the viscosity of the solution and improve mass transfer in many chemical reactions. In this work, the interactions between a representative pyridinium-based ionic liquid, N-butylpyridinium dicyanamide ([Bpy][DCA]), and a cosolvent, dimethylsulfoxide (DMSO), were studied in detail by the combined use of attenuated total reflection Fourier transform infrared spectroscopy, hydrogen nuclear magnetic resonance (1H NMR), and density functional theory calculations. Several species in the [Bpy][DCA]–DMSO mixtures have been identified, that is, ion clusters can translate into ion pairs during the dilution process. DMSO formed hydrogen bonds (H bonds) simultaneously with [Bpy]+ cations and [DCA]− anions but stronger hydrogen-bonding interactions with the [Bpy]+ cations than the [DCA]− anions, and the intrinsic hydrogen-bond networks of IL were difficult to interrupt at low DMSO concentrations. Interestingly, hydrogen-bonding interactions reach the strongest when the molar fraction of DMSO is 0.4–0.5. Hydrogen-bonding interactions are prominent in the chemical shifts of hydrogen atoms in [Bpy]+ cations, and anisotropy is the main reason for the upfield shifts of DMSO in the presence of [Bpy][DCA]. The theoretical calculations offer in-depth studies of the structural evolution and NMR calculation.