z Organic & Supramolecular Chemistry Tautomeric Study of Schiff Bases Derived from o- Dihydroxybenzaldehyde by UV-Vis, IR, 1 H NMR, 13 C NMR Spectroscopy and Computational Modeling. Carmen María Atzin-Macedo, [a] Cándida Pastor-Ramírez, [a] Rafael González-Peláez, [a] Francisco Javier Pérez-Flores, [b] Samuel Hernández-Anzaldo, [a] Hugo Vazquez-Lima,* [a] and Yasmi Reyes-Ortega* [a] Tautomeric equilibria at room temperature were studied for three aromatic imine polyol compounds, previously reported 2- ((2-hydroxybenzylidene)amino)-2-(hydroxymethyl)propane-1,3- diol 1 and two new compounds 2-(((1,3-dihydroxy-2- (hydroxymethyl)propan-2-yl)imino)methyl)benzene-1,4-diol 2 and 3-(((1,3-dihydroxy-2-(hydroxymethyl)propan-2 yl)imino) methyl)benzene-1,2-diol 3. The dynamic behavior of these equilibria is described by a combination of IR, UV-Vis and NMR spectroscopic measurements with DFT and TDDFT calculations in condensed phase. Among the different approaches, the combination of UV-Vis experimental spectra with the oscillator strengths values estimated from TDDFT calculations allowed for the assessment of the tautomeric populations in DMSO and methanol. These results were consistent with different func- tionals employed GGA, Hybrid and MetaGGA. Introduction Aromatic Schiff bases of the general formula Ar À CH =N À R can be obtained from a condensation reaction between an aromatic aldehyde or ketone and a primary amine. [1–2] This reaction offers the means to obtain a large variety of molecules due to its simplicity and relevance in the design of new ligands used in coordination chemistry. Many of such coordination compounds exhibit relevant magnetic properties as single molecule magnets (SMMs) or as catalysts. [3–6] Schiff bases containing a hydroxyl group on the ortho position of the aromatic ring show tautomeric equilibria due to an intra- molecular proton transfer between the enol-imine ei/ka keto- amine forms. [7–9] For years the tautomeric equilibrium of Schiff bases has been studied by different spectroscopic techniques in solid and liquid states. [10–13] Tautomeric equilibrium constants and thermodynamic parameters have been determined using NMR and UV-Vis spectroscopies. [14–15] Ground state equilibrium and excited state deactivation have been studied by solvato- chromic measurements and the proton transfer rate in crystalline phase was calculated by fluorescence spectroscopy. [16–17] On the other hand, the prototropic tautomerism nature of Schiff bases has been studied through various computational methods in order to aid in the correlation of experimental information. [18–20] This has been done mainly with either one structure or with structures with different molecular skeletons. Here we take advantage of the accuracy and relative speed of standard DFT functionals to provide insight into the spectro- scopic properties of different structural isomers. The aim of this work is to describe the tautomeric equilibrium of the previously reported compound 2-((2-hydroxybenzylidene)amino)-2- (hydroxymethyl)propane-1,3-diol 1 and two new compounds 2- (((1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl)imino)methyl) benzene-1,4-diol 2 and 3-(((1,3-dihydroxy-2-(hydroxymethyl) propan-2-yl)imino)methyl)benzene-1,2-diol 3 by UV-Vis, IR, 1 H NMR, 13 C NMR spectroscopy and to interpret the results with DFT calculations. The results showed that the ei $ ka exchange rate was not clearly described by NMR, but accurately described by UV-Vis spectroscopy. Results and Discussion Synthesis of 1–3 The synthesis of compounds 1–3 was based on previous articles; nevertheless, variations in the reaction conditions were necessary. [21–24] The ei/ka tautomeric equilibrium of aromatic Schiff bases can be observed via UV-Vis, IR and NMR spectroscopy. UV-Vis spectra of 1–3 showed the tautomers’ transitions that were assigned with the help of TDDFT simulations. IR experimental spectra showed the bands located in the range 1500– [a] C. M. Atzin-Macedo, C. Pastor-Ramírez, R. González-Peláez, Dr. S. Hernández-Anzaldo, Dr. H. Vazquez-Lima, Y. Reyes-Ortega Departamento de Química Inorgánica, Instituto de Ciencias, Centro de Química Benemérita Universidad Autónoma de Puebla Edif. IC09, C. U., Jardines de San Manuel, Puebla, Pue., C.P. 72540, México E-mail: hugo.vazquezlima@correo.buap.mx yasmi.reyes@correo.buap.mx [b] Dr. F. J. Pérez-Flores Laboratorio de Espectrometría de Masas, Instituto de Química Universidad Nacional Autónoma de México Circuito Exterior, C. U., Coyoacán, Ciudad de México, C.P. 04510, México Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.202002398 ChemistrySelect Full Papers doi.org/10.1002/slct.202002398 11120 ChemistrySelect 2020, 5, 11120–11126 © 2020 Wiley-VCH GmbH