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dc.contributor.authorMadurga, Sergio
dc.contributor.authorNedyalkova, Miroslava
dc.contributor.authorMas, Francesc
dc.contributor.authorGarcés, Josep Lluís
dc.date.accessioned2020-11-04T10:58:50Z
dc.date.available2020-11-04T10:58:50Z
dc.date.issued2017-07-13
dc.identifier.issn1520-5215
dc.identifier.urihttp://hdl.handle.net/10459.1/69771
dc.description.abstractThe 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.ca_ES
dc.description.sponsorshipWe acknowledge the financial support from Generalitat de Catalunya (Grants 2014SGR1017, 2014SGR1132, and XrQTC). J.L.G. acknowledge the Spanish Ministry of Science and Innovation (project CTM2016-78798-C2-1-P). S.M., M.N., and F.M. acknowledge the funding of the 8SEWP-HORIZON-2020 grant “Materials Networking” (692146).ca_ES
dc.language.isoengca_ES
dc.publisherAmerican Chemical Societyca_ES
dc.relationMINECO/PN2013-2016/CTM2016-78798-C2-1-Pca_ES
dc.relation.isformatofVersió postprint del document publicat a: https://doi.org/10.1021/acs.jpca.7b05089ca_ES
dc.relation.ispartofThe Journal of Physical Chemistry A, 2017, vol. 121, núm. 31, p. 5894-5906ca_ES
dc.rights(c) American Chemical Society, 2017ca_ES
dc.subjectReaction mechanismsca_ES
dc.subjectMoleculesca_ES
dc.subjectConformationca_ES
dc.subjectTitrationca_ES
dc.titleIonization and Conformational Equilibria of Citric Acid: Delocalized Proton Binding in Solutionca_ES
dc.typeinfo:eu-repo/semantics/articleca_ES
dc.identifier.idgrec026469
dc.type.versioninfo:eu-repo/semantics/acceptedVersionca_ES
dc.rights.accessRightsinfo:eu-repo/semantics/openAccessca_ES
dc.identifier.doihttps://doi.org/10.1021/acs.jpca.7b05089
dc.relation.projectIDinfo:eu-repo/grantAgreement/EC/H2020/692146ca_ES


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