The Ionicity And Thermodynamics Of Pro Tic Ionic Liquids

Protic ionic liquids(PILs)are a significant subclass of ionic liquids(ILs),and are generated via proton transferring between Br(?)nsted acids and Br(?)nsted bases.As there is an equilibrium,PILs are actually consisted of not only anions and cations,but also neutral molecules,and the proportion could be described by the term of ’degree of proton transfer’.Up to now,lots of contributions have been made to analyze the degree of proton transfer qualitatively,proving its significant influences on the field of application.However,only a limited number of preliminary methods have been developed to quantitatively analyze the degree of proton transfer,and little studies touched on the determination of its thermodynamic parameters.In this work,we improved the method using 1H NMR to measure the degree of proton transfer,which was developed previously by our group,and applied it on different kind of PILs.By using van’t Hoff analysis,the thermodynamic parameters were obtained as well.On this basis,the factors influencing the degree of proton transfer in PILs were discussed,improving the understanding of the equilibrium in PILs.To determine the degree of proton transfer in PILs,the 1H NMR method was introduced,where the chemical shift of exchangeable proton could shift with the proportion of ions in PILs.Therefore,the degree of proton transfer could be obtained by simulating the function between chemical shift and the proportion of ions.Different from the previous method which ignored solvent effects and treated the chemical shift of proton in pure acids as the same value of that in PILs,we treated this value as an unknown parameter and gained together with others.This work determined the degrees of proton transfer of six different PILs using the modified method,and the values spread over a large range.However,we found that the degree of proton transfer doesn’t have a good correlation with aqueous △pKa.Therefore,it is worth investigating the factors influencing the equilibrium of proton transfer and how to predict the degree of proton transfer.To determine the thermodynamic parameters of proton transfer in PILs,we used van’t Hoff analytic method and obtained the standard enthalpy change(△rHmθ)and the standard enthalpy change(△rSmθ)of the reaction.The commonly used method determining these parameters is calorimetry,and the indirect solution and Calvet-drop calorimetry measurement is always applied because the heat of reaction between pure Br(?)nsted acid and base is too large.The values of enthalpy change we gained are comparable to the literature value determined by complicated Calvet-drop microcalorimetry which could not determine the value of entropy change.Correlating the thermodynamic parameters and degree of proton transfer,we found that for most PILs,the enthalpy change is the key factor influencing the degree of proton transfer,while the entropy change mainly distributes at a narrow negative range.However,there are two exceptions,where the entropy change is abnormally high or low,resulting in high or low degree of proton transfer.Thanks to 1H NMR spectra,we concluded that the final value of entropy change derives from the effect of symmetry,and introduced the entropy change of symmetry to explain this phenomenon.Because of big difference of symmetry between the Br(?)nsted base and its conjugate acid,the resulted abnormally entropy change significantly influences the degree of proton transfer.