A Combined Experimental and Theoretical Approach to Understand the Structure and Properties of N-Methylpyrrolidone-Based Protic Ionic Liquids.

Correlation of the structure and properties of ionic liquids (ILs) is essential for the development of optimized materials in the fields of gas capture and separation, battery electrolytes, and cellulose dissolution processes. In view of this, a detailed vibrational spectroscopic analysis and quantum-chemical calculations were performed to explore the interionic interactions in ILs based on the N-methylpyrrolidone cation and a carboxylate anion. FTIR and Raman spectroscopy were applied to identify the hydrogen-bonding interactions between ion pairs, in which redshifted vibrational modes were observed as a function of the anion chain length. This observation was verified by the bond lengthening and enhanced hydrogen-bonding energies, as manifested in the structure and natural bond orbital (NBO) calculations. Furthermore, conductivity was measured at different temperatures to envisage the effect of the alkyl chain on the mobility of ions in the ILs. Finally, rheological measurements were implemented to explain the flow behavior of these ILs, which revealed a decrease in shear viscosity with an increase in temperature, that is, a Newtonian trend over a range of shear rates. The observed trend in transport properties was supported by the ion-pair binding energy. Stronger interactions between the IL cations and anions led to a decrease in the number of free ions and lowered the conductivity. In these protic ILs, the intermolecular N-H⋅⋅⋅O and C-H⋅⋅⋅O interactions played an important role in governing their physicochemical properties.