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 (ΔrHmθ) is from -23.30 to -7.80 kJ mol-1 and the standard entropy change of the reaction (ΔrSmθ) is from -42.70 to -8.07 J mol-1 K-1. The correlation between the degree of proton transfer and aqueous ΔpKa or ΔrHmθ was investigated as well. Furthermore, in some special cases, ΔrSmθ, especially the entropy change of symmetry, also plays an important role in affecting the degree of proton transfer.