jp7b05089_si_001.pdf (1.22 MB)
Ionization and Conformational Equilibria of Citric Acid: Delocalized Proton Binding in Solution
journal contribution
posted on 2017-07-13, 00:00 authored by Sergio Madurga, Miroslava Nedyalkova, Francesc Mas, Josep Lluís GarcésThe
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.
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triplet interaction energiesterminalpair interaction energySMD polarizable continuum modeldi-ionized formproton bindingcarboxylic groupsSSLBtriplet interaction energyalternative reaction schemeSBNMR titration dataNMR time scalehydrogen bondsab initio MP 2 calculationsDelocalized Proton Bindingdelocalized proton bindingmicrostate probabilities
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