Abstract :
[en] Ionic liquids (ILs) possess outstanding properties, such as specific solvation abilities and a super-acidic feature, explaining their use as solvents for acid-catalysed reactions, for instance.[1] In that context, determining the levels of acidity accessible in ILs is essential to better apprehend their use as super-acidic media for novel chemical processes. The fundamental level of acidity achievable in ILs is related to the solvation properties of the solvent on the proton. However, data on solvation energies for protons in non-aqueous solvents, and ILs especially, are still missing, since the solvation energy of protons in such solvents can only be determined experimentally through extra-thermodynamic hypotheses. [2] One of the methods relying on such hypotheses is the Hammett acidity function, based on the protonation equilibrium of nitroanilines, monitored by UV-visible spectroscopy (UVS).[3] However, the transfer of the Hammett method from molecular solvents, as originally proposed, to ILs might not be straightforward due to the possible interactions between the nitroanilines and the ions of the ILs (π-staking) as well as the suggested non-dissociating behaviour of ILs. In this work, we implemented, for the first time, the determination of the Hammett acidity function using Raman spectroscopy (RS). Using RS instead of UVS would enable the use of new, non-coloured, more spherical and non-aromatic pH-reporters, better meeting the fundamental hypothesis of Hammett. In this work, the level of acidity of three 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ILs (alkyl = ethyl, butyl, octyl) was determined, to study the influence of the alkyl chain length on the acidity of the solvents, either by UVS and RS to confirm the appropriate implementation of the Hammett acidity function in RS. Since RS has a lower sensitivity than UVS, the influence of the pH-reporter concentration on the observed acidity level (hence the solvation energy) was also assessed. Finally, other constituting cations were selected to study the influence of specific interactions between the IL and the pH-probe: the 1-Butyl-1-methylpyrrolidinium and 1-tert-butyl-3-methylimidazolium cations were chosen to study the consequence of aromaticity and steric hindrance on the acidity, respectively.
