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Molecules 2014, 19(1), 459-481; doi:10.3390/molecules19010459
Article

NMR Structural Study of the Prototropic Equilibrium in Solution of Schiff Bases as Model Compounds

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1 Laboratorio de Posgrado, Facultad de Ciencias Químicas, Universidad de Colima, Km 9 Carretera Colima-Coquimatlán, Colima 28400, Mexico 2 Departamento de Química, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional, Av. IPN 2508, San Pedro Zacatenco 07360, México D. F., Mexico 3 Departamento de Ciencias Básicas, Unidad Profesional Interdisciplinaria de Biotecnología del Instituto Politécnico Nacional, Av. Acueducto s/n Barrio la Laguna Ticomán, México D. F. 07340, Mexico
* Author to whom correspondence should be addressed.
Received: 28 October 2013 / Revised: 12 December 2013 / Accepted: 13 December 2013 / Published: 31 December 2013

Abstract

An NMR titration method has been used to simultaneously measure the acid dissociation constant (pKa) and the intramolecular NHO prototropic constant ΔKNHO on a set of Schiff bases. The model compounds were synthesized from benzylamine and substituted ortho-hydroxyaldehydes, appropriately substituted with electron-donating and electron-withdrawing groups to modulate the acidity of the intramolecular NHO hydrogen bond. The structure in solution was established by 1H-, 13C- and 15N-NMR spectroscopy. The physicochemical parameters of the intramolecular NHO hydrogen bond (pKa, ΔKNHO and ΔΔG°) were obtained from 1H-NMR titration data and pH measurements. The Henderson–Hasselbalch data analysis indicated that the systems are weakly acidic, and the predominant NHO equilibrium was established using Polster–Lachmann δ-diagram analysis and Perrin model data linearization.
Keywords: Schiff bases; NHO prototropic tautomerism; NMR titration; δ-diagram Schiff bases; NHO prototropic tautomerism; NMR titration; δ-diagram
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).
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Ortegón-Reyna, D.; Garcías-Morales, C.; Padilla-Martínez, I.; García-Báez, E.; Aríza-Castolo, A.; Peraza-Campos, A.; Martínez-Martínez, F. NMR Structural Study of the Prototropic Equilibrium in Solution of Schiff Bases as Model Compounds. Molecules 2014, 19, 459-481.

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