Ionization and conformational equilibria of citric acid: delocalized proton binding in solution

Abstract

The micro-speciation of citric acid is studied by analyzing NMR titration data. When the site binding (SB) model, which assumes fully local- ized proton binding to the carboxylic groups, is used to obtain microscopic energy parameters (dissociation constants, pair and triplet interaction energies between charged carboxylate groups), contradictory results are obtained. The resulting macroscopic constants are in very good agreement with the values reported in the literature using potentiometry. However, the found pair interaction energy between the terminal carboxylates and the triplet interaction energy are physically meaningless. In order to solve this apparent contradiction we consider the possibility of delocalized pro- ton binding, so that the proton can be exchanged at high velocity in the NMR time scale through short, strong, low-barrier (SSLB) hydrogen bonds. With this aim, ab initio MP2 calculations using the SMD polarizable continuum model for the solvent were performed and the fully roto-microspeciation elucidated. Firstly, fully localized proton binding was assumed, and the resulting micro-state probabilities are in reason- able agreement with those reported in previous works which use selective blocking of the carboxylic groups. They are, however, in clear disagreement with the microstate probabilities derived from the NMR titration data, which predict, within a very narrow confidence interval, a unique micro-species for the symmetric di-ionized form. Moreover, counterintuitively, the interaction between terminal charged groups is much larger than that between central and terminal groups. As a consequence, we have explored the possibility of delocalized proton binding by calculating the energy of intermediate proton positions between two carbolxylic groups. The results reveal that the exchange of the proton through the hydrogen bonds is in some cases produced without energetic barrier. This efect is specially relevant in the di-ionized form, with all the most stable conformations forming a SSLB, which together would constitute the only micro-state detected by NMR. An alternative reaction scheme for the ionization process, based on proton delocalization, is proposed.