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

•ΔfH° of imidazolium cations CnMIm+ (n = 2, 4, 6, 8, 10) were determined by quantum chemical calculations.•ΔfH° of trifluoromethanesulfonate, ethyl sulfate, dicyanamide, and bis(trifluoromethane)sulfonylimide anions were also found.•The enthalpies of heterolytic dissociation of neutral ion pairs were derived.•G4 method was found to be most appropriate for describing the thermodynamic quantities of the ions of ionic liquids.