Vaporization of Protic Ionic Liquids Studied by Matrix-Isolation Fourier Transform Infrared Spectroscopy

Several protic ionic liquids (PILs) with a wide range of pK a differences (ΔpK a) between the parent acid and base molecules were thermally evaporated in vacuum, trapped on a CsI plate by a cryogenic neon matrix-isolation method, and studied by Fourier transform infrared spectroscopy and density functional theory calculations. The parent neutral molecules and proton-transferred cation–anion pair species were identified as chemical components evaporated from the PILs with lower and higher ΔpK a values, respectively. The ΔpK a-dependent vaporization mechanism is discussed in terms of thermodynamic equilibrium between acid–base and anion–cation systems in the liquid phase.