Measurement of the p K a Values of Organic Molecules in Aqueous-Organic Solvent Mixtures by 1 H NMR without External Calibrants.

Aqueous-organic solvent mixtures are commonly used for reactions or analyses, where the components of a system are insoluble in pure water. The acid dissociation constant is an important property to measure in these media as it determines the charge state, solubility, and reactivity of a molecule. While NMR spectroscopy is an established tool for the measurement of p K a in water, its use in aqueous-organic solvents is greatly hindered by the requirement for external calibrants on which a working pH scale is set. Such calibrants include buffer solutions, "anchor" molecules with known p K a values, and pH electrodes that have undergone lengthy calibration procedures in the solvent mixture of interest. However, such calibrations are often inconvenient to perform, while literature p K a data covering the required range may not be available at the solvent composition or the temperature of interest. Here, we present a method to determine p K a in aqueous-organic solvents directly by NMR. We first determine p K a of an organic acid such as 2,6-dihydroxybenzoic acid (2,6-DHB) by measuring its 1 H chemical shift as a function of concentration along a concentration gradient using chemical shift imaging (CSI). Using 2,6-DHB as a reference, we then determine p K a of less acidic molecules in single CSI experiments via the variation of their 1 H chemical shifts along pH gradients. As proof of concept, we determine the p K a values of organic acids and bases up to p K a 10 in 50% (v/v) 1-propanol/water, 50% (v/v) dimethyl sulfoxide/water, and 30% (v/v) acetonitrile/water and obtain good agreement with the literature values.