Characterization of the acidity and basicity of green solvents by NMR techniques

The acidity and basicity of the solvents can influence the reaction outcome notably, and hence the precise measurement of pH is important for reaction. However, not all the pH values of organic solvents can be determined with a classic pH meter straightly. In this research, the acidity and basicity of environmentally friendly green solvents, such as ZnCl2 molten salt hydrate, ionic liquids (ILs) and deep eutectic solvents (DESs), were characterized by 31P and 1H NMR spectroscopy using trimethylphosphine oxide (TMPO) and pyrrole as probe molecules at 298 K. For the ZnCl2 molten salt hydrate, the acidic strength of the ZnCl2 molten salt hydrate increased with the concentration of ZnCl2. By using the 1H-pyrrole NMR approach, it was found that the base strength of amino acid-based ILs follows the order: [Ch][Lys] > [Ch][His].