Efficient pKa Determination in a Non-Aqueous Solvent using Chemical Shift Imaging

pKa is an important property of a molecule which impacts on many fields such as drug design, catalysis, reactivity and environmen-tal toxicity. It is often necessary to measure pKa in non-aqueous media due to the poor solubility of an analyte in water, for exam-ple many compounds of pharmaceutical interest. Although NMR methods to measure pKa in water are well established, determining pKa in organic solvents is laborious and problematic. We present an efficient one-shot method to determine the pKa of an analyte in organic solvent in a single measurement. Diffusion of an acid into a basic solution of the analyte and a set of pH indicators establishes a pH gradient in the NMR tube. The chemical shift of a pH sensitive resonance of the analyte and the pH of the solution are then determined simultaneously as a function of position along the pH gradient by recording a chemical shift image of the NMR tube. The pKa of the analyte is then determined using the Henderson-Hasselbalch equation. The method can be implemented in any laboratory with a gradient equipped NMR high-field spectrometer and is demonstrated for a range of pharmaceutical compounds and inorganic phosphazene bases