Evaluating the acidity levels in super-acidic ionic liquids by Raman Spectroscopy

Understanding fundamental levels of acidity accessible in non-aqueous solvents, often expressed as the proton transfer activity coefficient, which is related to the free energy of transfer when the proton is transferred from one solvent to another, experiencing difference in solvation[1], is critical for developing new chemical syntheses and applications in non-aqueous media[2]. For the proton, values for the transfer activity coefficient can only be experimentally estimated from extrathermodynamic models applied to electrode potentials, solubility or spectroscopic measurements[3,4]. Experimental values for the transfer activity coefficient of proton are essential to confront the computed values, as determining free energies of proton solvation, either experimentally or theoretically, in non-aqueous solvents is one of the most discussed questions in physical chemistry[5]. Proton solvation are particularly interesting to study in stable room temperature ionic liquids (RTILs), resulting from the combination of organic cations (often derivatives of N,N′-substituted imidazolium, N-substituted pyridinium, quaternary ammonium or phosphonium) and inorganic or organic anions (e.g. carboxylates, sulphates, halogens), which emerged as an interesting alternative to volatile organic solvents for inorganic and organic syntheses, catalyses, electrolytes and microextraction, especially for their capacity to dissolve sugar-based biopolymers such as lignin[6–10]. The presence of small fractions of molecular impurities in RTILs has major impacts on their physico-chemical properties, especially the acidity levels accessible in these solvents, which need to be addressed to develop more robust applications. Here we report on the determination of the free energy of solvation for proton in stable N,N′-substituted imidazolium ionic liquids using farfield classical Raman spectroscopy and the acidity function proposed by Hammett[11].
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