Enol-imino–Keto-enamine Tautomerism in a Diazepine Derivative: How Decisive Are the Intermolecular Interactions in the Equilibrium?

Abstract

The strange tautomeric equilibrium behaviour exhibited by a new o-hydroxyphenyl diazepine derivative, when the compound is analysed both in solution and solid state, open the structural study of the enol-imino-keto-enamine forms and the influence of the intermolecular interactions in their equilibrium. The expected enol-imino form, in which the enol is part of a phenyl system and a strong O-H ... N intramolecular hydrogen bond is established, results the most stable tautomer in gas phase (theoretical calculations) and was detected by NMR spectroscopy when the compound was dissolved in aprotic solvents. Nevertheless, the keto-enamine form in which the keto group integrate a cyclohexadienone moiety and the aromaticity of the phenol is loss, was the only tautomer in the crystal lattice according to single-crystal X-ray diffraction, vibrational spectroscopy and diffuse reflectance results. The last form was also found as the main tautomer in UV-Vis and NMR spectroscopy, when a protic solvent was employed.

Electrochemical and spectroscopic characterization of biologically important Schiff bases

“…Value of entropy became less negative with increasing temperature and it indicates that electrode reaction becomes less ordered with increased temperature. Dependence of ΔG # and ΔS # on temperature implies that processes are both enthalpy and entropy driven [38][39][40]. This also shows that these redox processes are nonspontaneous, and entropy opposed but become facile to some degree as temperature increases.…”

“…ΔG # , ΔH # and ΔS # at different temperatures were evaluated by using following simple relations [39]. On using Arrhenius type equation and by plotting ln k s vs. 1/T, value of activation energy can be found which is useful in determining the values of ΔH # at corresponding temperatures by utilizing following relation [40] While knowing the values of ΔG # and ΔH # at different temperatures, value of ΔS # at corresponding temperatures can be evaluated by using following well known relation Temperature elevation intensified the peak current which was attributed to the fact that as temperature rise, solvation sphere between the electroactive moiety of the analyte and electrode surface shrinks, subsequently increasing the sensitivity of electrode and making the approach of analyte more viable towards electrode.…”

Tautomeric Study of Schiff Bases Derived from o‐Dihydroxybenzaldehyde by UV‐Vis, IR, 1H NMR, 13C NMR Spectroscopy and Computational Modeling.

“…Based on TDDFT simulations and scientific publications, the bands that appear between 280 and 500 nm are assigned to the π!π* transitions of the corresponding ka/ei tautomers, vide infra. [25][26][27] From the electronic spectra, two factors which affected the position and intensity of the bands were assigned: i) the OH substituents and ii) the solvent polarity. Compound 1 in methanol presented an absorption band at 320 nm (ε = 2040 M À 1 cm À 1 ) assigned to a π!π* transitions of the ei tautomer and another at 400 nm (ε = 2870 M À 1 cm À 1 ) assigned to π!π* transitions, in this case from the ka tautomer ( Figure 1a).…”

Synthesis, Structure, and Biological Assays of Novel Trifluoromethyldiazepine–Metal Complexes

“…The ligand HDZP was synthesised as previously reported [32,33] by mixing ethanolic solutions of 2-trifluoromethylchromone (4 mmol) and ethylendiamine (…”

“…Recently, this compound was fully characterised and its tautomerism was studied in solid, solution, and gas phase. [32] The synthesised compounds were characterised by FT-IR, UV-vis, and diffuse reflectance spectroscopic studies. The crystal structure of the Cu II complex has been solved by X-ray diffraction methods and a detailed analysis of the main intermolecular interactions has been evaluated by Hirshfeld surface (HS) analysis.…”