Equilibrium in Protic Ionic Liquids: The Degree of Proton Transfer and Thermodynamic Properties.

The degree of proton transfer and thermodynamic parameters of six acetate-based protic ionic liquids (PILs) were measured using nuclear magnetic resonance spectroscopy and calculated by van't Hoff analytical method, respectively. The degree of proton transfer of these PILs at 298 K spread over a large range, which is from 39.6% (1-methylpyrrolidinium acetate, [MpyrH][AcO]) to 94.4% (1-butylimidazolium acetate, [BuimH][AcO]). The calculated standard enthalpy change of the reaction (Δr Hm θ ) is from -23.30 to -7.80 kJ mol-1 and the standard entropy change of the reaction (Δr Sm θ ) is from -42.70 to -8.07 J mol-1 K-1 . The correlation between the degree of proton transfer and aqueous ΔpKa or Δr Hm θ was investigated as well. Furthermore, in some special cases, Δr Sm θ , especially the entropy change of symmetry, also plays an important role in affecting the degree of proton transfer.