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 (ΔH) is from -23.30 to -7.80 kJ mol and the standard entropy change of the reaction (ΔS) is from -42.70 to -8.07 J mol K. The correlation between the degree of proton transfer and aqueous ΔpK or ΔH was investigated as well. Furthermore, in some special cases, ΔS, especially the entropy change of symmetry, also plays an important role in affecting the degree of proton transfer.