Quantum chemical calculations of formation enthalpies of cations and anions of ionic liquids

Abstract

The development of advanced ionic-liquid-based applications requires accurate thermochemical data for both ionic liquids and their constituent ions. Herein, the formation enthalpies of alkylimidazolium cations ([CnMIm+], n = 2, 4, 6, 8, 10) and various anions (trifluoromethanesulfonate ([TfO−]), ethyl sulfate ([ES−]), dicyanamide ([DCA−]), and bis(trifluoromethane)sulfonylimide ([NTf2−])) in the ideal gas state were obtained by quantum chemical calculations using density functional theory (DFT: B3LYP, B98, M06, M06-2X) and composite (G4) thermochemical models in the framework of the isodesmic reaction approach. The enthalpies of heterolytic dissociation of neutral ion pairs (NIPs) for the [CnMIm+][X−] series (n = 2, 4, 6, 8, 10; X− = [TfO−], [ES−], [NTf2−], [DCA−], [BF4−], [PF6−], [Cl−], [Br−], [I−]) were derived using the obtained data and literature formation enthalpies (ΔfH°) of NIPs. Analysis of the thermochemical property trends revealed that the formation enthalpies of [CnMIm+] decrease linearly as the alkyl chain length increases. In addition, the use of the isodesmic reaction approach improved the reliability of the computed data. Furthermore, owing to its internal self-consistency, the G4 method was found to be the most appropriate for describing the thermodynamic quantities of the cations and anions of ionic liquids.