Proton Transfer and Ionicity: An ¹⁵N NMR Study of Pyridine Base Protonation

Protic ionic liquids (PILs) are made by proton transfer from a Brønsted acid to a base and are of interest for their solvent and electrolyte properties such as high ionic conductivity. Unfortunately, many PILs have been misnamed, because their ionic content is minimal due to an insufficient driving force for the proton transfer. Here we review this problem and introduce a new method, using ¹⁵N NMR spectroscopy, of characterizing the relation between the extent of proton transfer to a given base and the strength of the proton-donating acid. The experimental data reveal a sigmoid "titration type" curve that indicates clearly the acid strength, at which molecule bases, of substituted pyridine type, are fully protonated. We compare results for two bases of similar shape but different basicity, protonated by equimolar amounts of the different acids. The extent of protonation is also reflected in the ionic conductivity, and we show that the important part of the protonation sigmoid is quantitatively reproduced by data for conductivity and viscosity displayed in the form of a Walden plot (log equivalent conductivity vs log fluidity). The acid strength, for this study, is based on gas phase proton affinities, but we note that a similar sigmoid is obtained if we use the condensed phase Hammett acidity functions instead. Our findings allow us to rank the AlCl₄^- anion as the weakest proton acceptor in use in IL studies, consistent with its role in the most conductive ILs.