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

Abstract

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 pK_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 pK_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 pK_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 pK_a in aqueous-organic solvents directly by NMR. We first determine pK_a of an organic acid such as 2,6-dihydroxybenzoic acid (2,6-DHB) by measuring its ¹H 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 pK_a of less acidic molecules in single CSI experiments via the variation of their ¹H chemical shifts along pH gradients. As proof of concept, we determine the pK_a values of organic acids and bases up to pK_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.

Scheme 1. Method to Determine pK_a of Organic Molecules in Aqueous–Organic Mixtures Using CSI and Concentration Gradients of 2,6-DHB

The ¹H chemical shift of 2,6-DHB is measured as a function of concentration in the absence of base (a) and the presence of 40 mM 1,2,4-triazole (b) to determine the pK_a of both compounds. The pK_a values of other indicator molecules are determined relative to triazole and 2,6-DHB (c) allowing the determination of pH from their ¹H chemical shifts and pK_a of other organic molecules using pH gradients and CSI (d).

Figure 1

Plot of ¹H chemical shift of 2,6-DHB (solid symbols) vs C in the absence (a) and presence (b) of 40 mM 1,2,4-triazole in 50% 1-propanol/H₂O (black diamond), 50% DMSO/H₂O (red triangle), and 30% CD₃CN/H₂O (blue square). Solid lines are fits to eqs 1–4. ¹H chemical shift of 1,2,4-triazole (open symbols). Fits to eq 7 (vertical cross) and eqs 2 and 8 (diagonal cross).

Figure 2

Plot of ¹H chemical shifts of indicators vs pH in 50% DMSO/H₂O. Solid lines are fits to eq 13.

Figure 3

Plot of ¹H chemical shifts of monoprotic analytes vs pH in 50% 1-propanol/H₂O (a), 50% DMSO/H₂O (b), and 30% CD₃CN/H₂O (c) and fits to eq 13 (solid lines).

Figure 4

Plot of the ¹H chemical shifts of diprotic analytes. Solid lines are fits to eq 14. (a) Picolinic acid, 50% 1-propanol/H₂O. (b) Pipecolic acid, 50% 1-propanol/H₂O. (c) d-valine, 50% DMSO/H₂O. (d) Phthalic acid, 30% CD₃CN/H₂O. (e) Quinine, 30% CD₃CN/H₂O.