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Analysis of tautomeric equilibrium in (E)-4,6-dibromo-2-[(4-fluorophenylimino)methyl]-3-methoxyphenol compound
(Pergamon-Elsevier Science Ltd, 2015) Kastas, Cigdem Albayrak; Kastas, Gokhan; Gur, Mahmut; Muglu, Halit; Buyukgungor, Orhan
In this study, the tautomeric equilibrium between the phenol-imine and keto-amine structural forms of (E)-4,6-dibromo-2-[(4-fluorophenylimino)methyl]-3-methoxyphenol compound has been investigated with experimental (XRD, UV-vis and NMR) and theoretical (DFT and TD-DFT) methods. The results clearly show that structural preference of the compound is definitely depended on its state. Namely, the compound exists in phenol-imine form in the solid state while one or both of these forms can be seen in solvent media. For example, the compound prefers phenol-imine form in benzene while both forms exist in EtOH and DMSO solvents. Coexistence of two forms has been quantified with NMR studies, giving a ratio of 11:9 for phenol and keto structures of the compound in acetone-d(6) solvent. (C) 2015 Elsevier B.V. All rights reserved.
Coordination Dimer of Pyromellitic Acid Based on Supramolecular Architecture
(Int Union Crystallography, 2013) Pasaoglu, Humeyra; Aycan, Tugba; Kastas, Gokhan
[No abstract available]
DEPENDENCE OF TAUTOMERISM ON SUBSTITUENT TYPE IN o-HYDROXY SCHIFF BASES
(Soc Chemists Technologists Madeconia, 2019) Kastas, Cigdem Albayrak; Kastas, Gokhan
Quantum computational methods were used to elucidate the structures of the o-hydroxy Schiff bases with different substituents. It is possible for a Schiff base to have different tautomeric structures depending on intramolecular proton transfer from the phenolic oxygen atom to the nitrogen atom. Proton transfer results in two tautomeric structures known as the phenol-imine and keto-amine forms. To explain the substituent effect on the proton transfer process in five o-hydroxy-Schiff bases, possible geometric structures in gas phase were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. To describe tautomerism including intramolecular proton transfer, potential energy surface (PES) scans were performed starting from the optimized geometry of the phenol-imine form. HOMA indices were calculated in order to estimate pi-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.
Existence of a resonance hybrid structure as a result of proton tautomerism in (±)-(E)-4-Bromo-2-[(2,3-dihydroxypropylimino)methyl]phenol racemate
(Pergamon-Elsevier Science Ltd, 2014) Albayrak, Cigdem; Kastas, Gokhan; Odabasoglu, Mustafa; Buyukgungor, Orhan
o-Hydroxy Schiff bases have two tautomers known as phenol-imine and keto-amine forms. In the present work, the tautomerism in (E)-4-Bromo-2-[(2,3-dihydroxypropylimino)methyl]phenol compound has been investigated by experimental (XRD, FT-IR and UV-vis) and computational (DFT and TD-DFT) methods. The X-ray diffraction (XRD) study reveals that the title compound favors a resonance hybrid structure of phenol-imine and keto-amine forms in the solid state rather than having these forms separately or jointly. Experimental UV-vis study of proton transfer process in solvent media (Benzene, DMSO and EtOH) shows the preference of phenol-imine form in benzene while both phenol-imine and keto-amine characteristics are present in EtOH and DMSO. (C) 2013 Elsevier B.V. All rights reserved.
Investigation of molecular structure and solvent/temperature effect on tautomerism in (E)-4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl] phenol, a new thermochromic Schiff base, by using XRD, FT-IR, UV-vis, NMR and DFT methods
(Pergamon-Elsevier Science Ltd, 2019) Kastas, Gokhan; Kastas, Cigdem Albayrak; Tabak, Ahmet
The molecular structure and the solvent temperature effect on the tautomerism in a new Schiff base, (E)-4,6-clibromo-3-methoxy-2-[(p-tolylimino)methyllphenol, were investigated using spectroscopic (NMR, UV-vis, FT-1R), crystallographic (XRD), computational (DFT and TD-DET) methods and harmonic oscillator model of aromaticity (HOMA). The XRD, DFT and MIR results show that the compound exists in the phenol-imine form in the solid state. HOMA indices support the aromatic structure of the compound. DFT calculations were performed to understand proton transfer process and relatively close values were obtained for the energies of tautomers. UV-vis studies prove the solvent dependence of the tautomerism in the compound by revealing the existence of both phenol-imine and keto-amine forms in polar solvents and only the phenol-imine form in apolar solvent. The TD-DFT results for the electronic transitions lead to the same conclusion as the absorption spectra. H-1 NMR and C-13 NMR studies at room and low (-60 degrees C) temperatures indicate that the tautomeric equilibrium occurs rapidly in the compound. Therefore, it is difficult to observe two tautomers. However, the presence of tautomeric structures is clearly seen in acetone d(6), alternatively underlying the solvent and temperature dependence of tautomerism in the title compound. (C) 2019 Elsevier B.V. All lights reserved.
Probing the compound (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol mainly from the point of tautomerism in solvent media and the solid state by experimental and computational methods
(Elsevier, 2011) Albayrak, Cigdem; Kastas, Gokhan; Odabasoglu, Mustafa; Buyukgungor, Orhan
In this study, (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol compound was investigated by mainly focusing on stacking interactions assembling the supramolecular network of the compound and on tautomerism in solvent media and in the solid state. In doing so. the molecular structure and spectroscopic properties of (E)-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol were experimentally characterized by X-ray diffraction, FT-IR. NMR and UV/Vis spectroscopic techniques and computationally by DFT method. The X-ray diffraction and FT-IR analyses of the title compound reveal the existence of enol form in the solid state. The non-covalent C-H center dot center dot center dot pi and inter-molecular hydrogen bonding interactions assemble the supramolecular structure of the title compound by forming 4-connected (4,4)-net in Wells nomenclature. The dependence of tautomerism on solvent types was studied on the basis of UV/Vis spectra recorded in different organic solvents. The results showed that the title compound exists in enol form in all solvents. Computational investigation of enol-keto tautomerism was carried out at B3LYP (6-311G(d,p)) level for both enol and keto forms. The results obtained for enol form are more compatible to the experimental results than those of keto form. TD-DFT calculations carried out in both gas and solution phases indicate that the title compound adopt only enol form in solution. The enol-keto tautomerism was also investigated by evaluating the changes in thermodynamic properties (heat capacity, entropy, enthalpy and Gibbs free energy) with varying temperatures, showing that the formation of tautomerism in the title compound is non-spontaneous between 100 and 500 K and that the title compound must exist in enol form. (C) 2011 Elsevier B.V. All rights reserved.
Probing the compound (E)-5-(diethylamino)-2-[(4-methylphenylimino)methyl]phenol mainly from the point of tautomerism in solvent media and the solid state by experimental and computational methods
(Pergamon-Elsevier Science Ltd, 2011) Albayrak, Cigdem; Kastas, Gokhan; Odabasoglu, Mustafa; Frank, Rene
In this study, the molecular structure and spectroscopic properties of (E)-5-(diethylamino)-2-[(4-methylphenylimino)methyl]phenol were characterized experimentally by X-ray diffraction, FT-IR and UV-vis spectroscopic techniques and computationally by DFT method. It is concluded on the basis of X-ray diffraction and FT-IR analyses that the title compound exists in enol form in the solid state. UV-vis spectra of the title compound were recorded in different organic solvents to investigate the dependence of tautomerism on solvent types. The tautomerism-solvent relation was also studied by computational methods to have more insight on structural properties. The geometry optimization of the title compound in gas phase was performed by using DFT (B3LYP) method with 6-311G(d,p) basis set. The geometry optimizations in solvent media were carried out at the same theory level by the polarizable continuum model (PCM). In the calculation of excitation energies, TD-DFT calculations were carried out in both gas and solution phases. The computational investigation of non-linear optical properties indicates that the title compound has a good second order nonlinear optical property. The thermodynamic properties were obtained in the range of 100-500 K. (C) 2011 Elsevier B.V. All rights reserved.
Scrutinizing the two new o-hydroxy Schiff bases from the point of tautomeric behavior and non-covalent interactions (H-bond, Br•••Br, π•••π and C-H•••π) in their supramolecular architectures
(Elsevier, 2019) Kastas, Gokhan; Kastas, Cigdem Albayrak
Two new Schiff bases, (E)-4,6-dibromo-2-[(5-chloro-2-methylphenylimino)methyl]-3-methoxyphenol (1) and (E)-4-bromo-2-[(4-bromophenylimino)methyl]-5-methoxyphenol (2), have been investigated by focusing on the prototropy-related-changes in the geometric parameters, the molecular planarity and the way of crystal packing. X-ray diffraction (XRD), density functional theory (DFT), Hartree-Fock theory (HF) and Moller-Plesset perturbation theory (MP2) and harmonic oscillator model of aromaticity (HOMA) studies show the preference of phenol-imine form by the compounds. The crystal packings of the compounds have been studied in detail by noting the importance of the substituent type and position in regulating the non-covalent interactions, thus, the formation of supramolecular networks. The results underline the fact that the halogen atoms of high polarizability and their presence in the appropriate position dominate the construction of supramolecular structure in Schiff bases. (C) 2019 Elsevier B.V. All rights reserved.
Single stranded helical chains of C-H•••π interactions further connected by halogen-halogen interactions of type I to construct supramolecular structure of (E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol compound
(Pergamon-Elsevier Science Ltd, 2012) Kastas, Gokhan; Albayrak, Cigdem; Odabasoglu, Mustafa; Frank, Rene
In this study, (E)-5-(diethylamino)-2-[(4-iodophenylimino)methyl]phenol compound was investigated from the point of stacking interactions assembling the supramolecular network, conformational isomerism and tautomerism. For this purpose, X-ray diffraction, FT-IR and UV/Vis spectroscopic techniques were used, giving the following structural and spectroscopic properties of the compound: The title compound has two conformers (anti and eclipsed) in the crystal structure resulting from rotation about C-N single bond of ethyl group. Both conformers prefer enol form in the solid state, adopting E configuration about the C=N double bond. The supramolecular architecture of the compound is constructed by two non-covalent interactions as C-H center dot center dot center dot pi and halogen-halogen interactions. The repetition of C-H center dot center dot center dot pi interactions is resulted in a single-stranded helical structure. The helical structures are further connected by C-I center dot center dot center dot I-C interactions of Type I to construct the two dimensional supramolecular network defined as (6,3)-net in Wells nomenclature. The title compound adopts both enol and keto forms in EtOH (a polar and protic solvent) while enol form is preferred in the solid state. (C) 2012 Elsevier B.V. All rights reserved.
Spectroscopic, XRD, Hirshfeld surface and DFT approach (chemical activity, ECT, NBO, FFA, NLO, MEP, NPA& MPA) of (E)-4-bromo-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol
(Elsevier, 2019) Demircioglu, Zeynep; Kastas, Gokhan; Kastas, Cigdem Albayrak; Frank, Rene
In the present work, the compound (E)-4-bromo-2-[(4-bromophenylimino)methyl]-6-ethoxyphenol was synthesized and characterized by spectroscopic (FT-IR and UV-Vis) and single crystal X-ray diffraction techniques. The intermolecular contacts in the compound was examined by Hirshfeld surfaces (HS) and fingerprint plots (FP) using the X-ray diffraction data. For other properties of interest, the geometry of the compound has been optimized by using the density functional theory (DFT) at B3LYP/6-311G (d,p) level. Molecular electrostatic potential (MEP), Fukui function (FF), nonlinear optical properties (NLO) and natural bond orbital (NBO) analyses have been performed and the global reactivity parameters have been obtained. The interactions between the title molecule and DNA bases (such as adenine, cytosine, guanine, and thymine) were investigated by using the ECT (electrophilicity-based charge transfer) method and Delta N (charge transfer). (C) 2019 Published by Elsevier B.V.
Structural, catalytic, antimicrobial and antioxidant properties of Cu(II)-complexes prepared by o-hydroxy Schiff bases with fluorine and ethoxy substituents
(Taylor & Francis Ltd, 2025) Kastas, Gokhan; Kastas, Cigdem Albayrak; Avsar, Cumhur
Cu(II) complexes of three Schiff bases were synthesized and characterized via X-ray diffraction, Fourier transform infrared, and UV-vis spectroscopies. The Schiff bases act as bidentate ligands, chelating Cu(II) ions in seesaw coordination geometry. Changing the substituent types and positions creates various non-covalent interactions within the crystal packings, yielding notable topological differences. GC-MS analysis reveals that Cu(II) complexes induce 1.38%-11.43% efficacy in Suzuki-Miyaura cross-coupling reactions. The biological activity of complexes correlates with substituent type and position in the aromatic ring. The one with ethoxy exhibits antimicrobial effects solely against Candida parapsilosis, while those with fluorine enhance the activity, particularly in the meta position.
Supramolecular Structure Constructed by Coordination Dimer of Pyromellitate Ligand
(Int Union Crystallography, 2013) Aycan, Tugba; Pasaoglu, Humeyra; Kastas, Gokhan
[No abstract available]
Survey of conformational isomerism in (E)-2-[(4-bromophenolimino)methyl]-5-(diethylamino)phenol compound from structural and thermochemical points of view
(Pergamon-Elsevier Science Ltd, 2012) Albayrak, Cigdem; Kastas, Gokhan; Odabasoglu, Mustafa; Frank, Rene
In this study, (E)-2[(4-bromophenylimino)methyl]-5-(diethylamino)phenol compound was investigated by mainly focusing on conformational isomerism. For this purpose, molecular structure and spectroscopic properties of the compound were experimentally characterized by X-ray diffraction, FT-IR and UV-Vis spectroscopic techniques, and computationally by DFT method. The X-ray diffraction analysis of the compound shows the formation of two conformers (anti and eclipsed) related to the ethyl groups of the compound. The two conformers are connected to each other by non-covalent C-H center dot center dot center dot Br and C-H center dot center dot center dot pi interactions. The combination of these interactions is resulted in fused R-2(2)(10) and R-2(4)(20) synthons which are responsible for the tape structure of crystal packing arrangement. The X-ray diffraction and FT-IR analyses also reveal the existence of enol form in the solid state. From thermochemical point of view, the computational investigation of isomerism includes three studies: the calculation of (a) the rate constants for transmission from anti or eclipsed conformations to transition state by using Eyring equation, (b) the activation energy needed for isomerism by using Arrhenius equation, (c) the equilibrium constant from anti conformer to eclipsed conformer by using the equation including the change in Gibbs free energy. The dependence of tautomerism on solvent types was studied on the basis of UV-Vis spectra recorded in different organic solvents. The results showed that the compound exists in enol form in all solvents except ethyl alcohol. (C) 2012 Elsevier B.V. All rights reserved.
Synthesis, crystal structure, computational chemistry studies and Hirshfeld surface analysis of two Schiff bases, (E)-2-[(4-bromo-2-methylphenylimino)methyl]-4-methylphenol and (E)-2-[(4-bromo-2-methylphenylimino)methyl]-6-methylphenol
(Taylor & Francis Ltd, 2021) Demircioglu, Zeynep; Kastas, Cigdem Albayrak; Kastas, Gokhan; Ersanli, Cem Cuneyt
In this study, two Schiff bases, (E)-2-[(4-bromo-2-methylphenylimino)methyl]-4-methylphenol (I) and (E)-2-[(4-bromo-2-methylphenylimino)methyl]-6-methylphenol (II) are synthesized and characterized by XRD and UV-Vis spectroscopy. The presence of various intermolecular interactions and 2 D-fingerprint regions are well supported by the Hirshfeld surface analysis. Also experimental optical energy band and gap studies are discussed. All theoretical computations (NBO, FMOs, MEP, NP, MP, FF) were calculated by Density Functional Theory (DFT) at B3LYP level by using 6-311 G(d,p) basis set. The interactions between the title molecules and DNA bases were investigated by using the ECT (electrophilicity-based charge transfer) method.
The effect of the change in substituents' positions on the formation of supramolecular networks and the solvent type/substituent dependence of prototropic behavior in three new o-hydroxy Schiff bases
(Elsevier, 2020) Kastas, Gokhan; Kastas, Cigdem Albayrak; Kirca, Basak Kosar; Ersanli, Cem Cuneyt
The effect of substituent position on the formation of supramolecular topology, intra-molecular geometry and tautomeric behavior, and the dependence of prototropy on the solvent type have been investigated applying the XRD, UV-Vis, NMR, DFT, PES and HOMA methods on three new o-hydroxy Schiff bases. Based on key geometric parameters, total molecular energy and aromaticity calculations, the XRD, DFT and HOMA studies propose the phenol-imine structure as the most consistent tautomeric form for the compounds. PES and HOMA studies give results that relate the preference of the phenol-imine form to its high p-electron delocalization. For the solvent type dependence of tautomerism, the investigation of UV-Vis and NMR spectra and the PES studies for different solvent media reveals that the co-existence of two tautomeric structures becomes easier by increasing solvent polarity. As to the effect of substituent position on the supramolecular topology, XRD studies show that different substituent positions cause different types of non-covalent inter-molecular interactions in the crystal packings of the compounds. This is resulted in a significant change in the formation and topology of the supramolecular architecture, emphasizing the importance of a substituent position as a regulator in crystal engineering. (C) 2019 Elsevier B.V. All rights reserved.
The prototropic tautomerism and substituent effect through strong electron-withdrawing group in (E)-5-(diethylamino)-2-[(3-nitrophenylimino)methyl]phenol
(Pergamon-Elsevier Science Ltd, 2013) Albayrak, Cigdem; Kastas, Gokhan; Odabasoglu, Mustafa; Frank, Rene
The prototropic tautomerism in o-Hydroxy Schiff bases results in two forms called phenol-imine and keto-amine. The preference of a particular form by the compound changes in the solid and solvent media. The choice can also be regulated by a substituent with a different electron-donating or withdrawing group. In the present study, the above-mentioned factors are considered in the investigation of (E)-5-(diethylamino)-2-[(3-nitrophenylimino)methyl]phenol compound (an o-Hydroxy Schiff basis) by experimental (XRD, FT-IR and UV-vis) and computational (DFT and TD-DFT) methods. The results show that the title compound adopts only phenol-imine form in the solid and solvent media. This was attributed to the substituent effect through strong electron-withdrawing nitro group. (C) 2013 Elsevier B.V. All rights reserved.

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