The vapour of imidazolium-based ionic liquids: a mass spectrometry study

Abstract

Eight common dialkylimidazolium-based ionic liquids have been successfully evaporated in ultra-high vacuum and their vapours analysed by line of sight mass spectrometry using electron ionisation. The ionic liquids investigated were 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]imide, [C_nC₁Im][Tf₂N] (where n = 2, 4, 6, 8), 1-alkyl-3-methylimidazolium tetrafluoroborate, [C_nC₁Im][BF₄] (where n = 4, 8), 1-butyl-3-methylimidazolium octylsulfate, [C₄C₁Im][C₈OSO₃] and 1-butyl-3-methylimidazolium tetrachloroferrate, [C₄C₁Im][FeCl₄]. All ionic liquids studied here evaporated as neutral ion pairs; no evidence of decomposition products in the vapour phase were observed. Key fragment cations of the ionised vapour of the ionic liquids are identified. The appearance energies, E_app, of the parent cation were measured and used to estimate the ionisation energies, E_i, for the vapour phase neutral ion pairs. Measured ionisation energies ranged from 10.5 eV to 13.0 eV. Using both the identity and E_app values, the fragmentation pathways for a number of fragment cations are postulated. It will be shown that the enthalpy of vaporisation, Δ_vapH, can successfully be measured using more than one fragment cation, although caution is required as many fragment cations can also be formed by ionisation of decomposition products.