Determination of the relative pKa's of mixtures of organic acids using NMR titration experiments based on aliased 1H–13C HSQC spectra

Abstract

NMR titrations can be used to simultaneously determine the pK_a's of mixtures of organic acids and bases provided chemical shifts are sensitive to their change of protonation states. Carbons are in principle better probes than protons because their chemical shifts are typically more sensitive to the changes in the electronic densities of the functional groups they are bound to. They are also less prone to overlap, do not suffer from second-order effects and are less sensitive to changes in solvent conditions. Recording HSQC spectra satisfactorily reduces the lack of sensitivity of carbon-detected experiments and the signal dispersion in two dimensions allows to automatically and finely track the drifts of a large number of signals simultaneously. The application of spectral aliasing to the carbon dimensions allows to quickly reach the high resolution required to precisely measure variations in carbon chemical shifts. The paper extends the methodology to the treatment of the data stemming from the simultaneous analysis of mixtures of compounds. It also introduces methods to improve the normalization process and deal with missing points. The application is not limited to the determination of pK_a and can be readily extended to the determination of other chemical constants in experiments where the components of the mixture dynamically compete for substrates other than protons.