Ionization and Conformational Equilibria of Citric Acid: Delocalized Proton Binding in Solution

Abstract

The microspeciation of citric acid is studied by analyzing NMR titration data. When the site binding (SB) model, which assumes fully localized 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. To solve this apparent contradiction, we consider the possibility of delocalized proton 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. First, fully localized proton binding was assumed, and the resulting microstate probabilities are in reasonable agreement with those reported in previous works that 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 microspecies 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 effect is specially relevant in the di-ionized form, with all the most stable conformations forming a SSLB, which together would constitute the only microstate detected by NMR. An alternative reaction scheme for the ionization process, based on proton delocalization, is proposed.

Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations

“…Spectroscopy and X-ray crystallography data, reported for some salts of protonated solvents with O atom donors, generally support the notion of disolvation of the proton–i.e., [ether-H (+) -ether]­[anion] is a valid molecular representation in general. ,,− For the observed product of a reaction of H 2 with ether solutions of BCF or its analogs, the notion of two ether molecules coordinated to the proton is consistent with reported observations. ,, In part, persistent interest in knowing more about the manner of hydrogen bonding in an oxonium cation and specifics of the cation–anion interaction stems from notable effects in vibrational (IR) spectra. , The vibrational OHO-contribution can shift from its typical position at ca. 3000 cm –1 for normal hydrogen bonding (O–H···O) to as low as 1000 cm –1 for SSLB hydrogen bonding (O–H–O) , e.g., the IR spectra of salts including oxonium cations. − , Better understanding of SSLB-type O–H (+) –O bonding, which is quite different from typical N–H···O or O–H···O bonding with asymmetric distances between the proton and hetero atoms, is highly desirable, especially with account of molecular motion of cation perturbed by docking to a structurally complex anion. Differently from previously known salts composed of an oxonium cation and borate anion, e.g., Jutzi’s acid [(Et 2 O) 2 H (+) ]­[(C 6 F 5 ) 4 B (−) ] or other salts with borate anions [(EWG) 4 B (−) ] having four substituents on boron, an anion of the type [(EWG) 3 B–H (−) ] has only three boron-bound groups and a certain direction defined by B → H “vector” that is perpendicular to the “equatorial” plane encompassing the trio of the boron-bound C atoms.…”

Structural Diversity of Protonated Citric Acid-Ammonia Clusters and Its Atmospheric Implication

“…For these three isomers 4CA + NH 4 + _1­(2, 3), the total number of intracluster hydrogen bonds is up to 16, including two N–H•••O–H, two N–H•••OC, eight O–H•••OC and four O–H•••O–H types of H-bonds. In addition, all these clusters feature NH 4 + -embedded structures and a shape like a “lantern”, where citric acid units all adopt cis-conformation …”

“…Furthermore, theoretical calculations were employed to perform a global search for the structures of the [(CA) 2 + NH 4 ] 1+ clusters. Taking into account the various protonated sites 47 and flexible conformation of citric acid units, over 50,000 structures were generated, optimized, and selected at different levels. At last, the eight most stable isomers were determined at the M06-2X/6-311 + g(2d, p) level, which were also confirmed by the method of B3YLP (Table S1).…”

“…In addition, all these clusters feature NH 4 +embedded structures and a shape like a "lantern", where citric acid units all adopt cis-conformation. 47 Like the neutral dimer, the neutral structures of 4CA+NH 3 were also considered by removing one proton from the most stable cationic cluster 4CA + NH 4 + _1(_2, _3 and _4) at different sites. Then, the top eight structures optimized at the level of M06-2X/6-31 + G(d) are shown in Figure S7.…”

Equilibria and Dynamics of Sodium Citrate Aqueous Solutions: The Hydration of Citrate and Formation of the Na3Cit0 Ion Aggregate

“…The citrate anion (Scheme ) is not centrosymmetric and highly flexible at the same time. , Accordingly, its center of charge and its center of hydrodynamic stress (the pivot for ion rotation in a viscous medium) do not coincide, and the resulting charge arm yields an effective dipole moment . Thus, the rotation of free anions certainly contributes to mode 1.…”