Study Of Tautomerism Research Articles

Tautomeric, spectroscopic, DFT calculations and X-ray studies on O 2N-4-C 6H 4–NHN [dbnd]C(COCH 3) 2

This paper deals with the structural and tautomeric studies on O 2N-4-phenyl hydrazone derivative of acetylacetone, of formula O 2N-4-C 6H 4NHN C(COCH 3) 2, I, using IR, 1H NMR and 13C NMR spectroscopies, X-ray diffraction analysis and quantum mechanical calculations. The crystallographic data show that this compound has a β-diketohydrazone structure, containing an intramolecular H-bond assisted by resonance (RAHB), with N⋯O distance of 2.6025(16) Å. These results are in agreement with the spectroscopic studies and with theoretical calculations. The collected information confirms that I shows the same structure in CDCl 3 solution, solid state and gaseous phase.

Enol-keto tautomerism of aromatic photochromic Schiff base N,N′-bis(salicylidene)-p-phenylenediamine: Ground state equilibrium and excited state deactivation studied by solvatochromic measurements on ultrafast time scale

A photochromic symmetric Schiff base, N,N'-bis(salicylidene)-p-phenylenediamine, is proposed as a probe for the study of solvent dependent enol-keto tautomerism in the ground and excited states. The ground state equilibrium between the enol-keto tautomers is found to depend mainly not on polarity but on the proton donating ability of the solvent. Upon selective excitation of each of these tautomers, the same excited state of a keto tautomer is created: in enol, after the ultrafast excited state intramolecular proton transfer (ESIPT), reaction, and in keto tautomer, directly. Then some part (<30%) of excited molecules are transferred to the photochromic form in its ground state. The evidence of another ultrafast deactivation channel in the excited enol tautomer competing with ESIPT has been found. The solvent does not influence the ESIPT dynamics nor the efficiency of the creation of the photochrome.

1H, 13C, N [sbnd]H, H [sbnd]H, C [sbnd]H COSY, H [sbnd]H NOESY NMR and UV–vis studies of Solophenyl red 3BL dye azo-hydrazone tautomerism in various solvents

Azo-hydrazone tautomeric behavior of polyazo Solophenyl red 3BL (C.I. Direct 80) dye in different solvents (water, methanol and DMSO) was investigated using 1H, 13C, N H, H H, C H COSY, H H NOESY NMR techniques and UV–vis spectroscopy. Two-dimensional NMR experiments were used to assign 1H, 13C and 15N NMR lines unambiguously. Results showed that the hydrazone-form proton NMR signal appeared in the weakest field with respect to tetramethylsilane, in comparison with the amide and phenolic proton NMR signals. UV–vis absorption spectroscopic evidences showed that azo-hydrazone mixture exists in water and DMSO solvents, but in methanol, only azo tautomer was dominant, which was in a good agreement with NMR spectroscopic results.

Ring−Chain Tautomerism of the Novel 2-Ferrocenyl-2,4-dihydro-1H-3,1-benzoxazine

The synthesis and the study of the spectroscopic and electrochemical properties as well as the solution behavior of the novel 2-ferrocenyl-2,4-dihydro-1H-3,1-benzoxazine (1a) are described. NMR studies reveal the existence of a tautomeric equilibria between the cyclic (1a) and the open-chain form (2a). Electrochemical studies based on cyclic voltametry and 57Fe Mossbauer spectroscopy as well as a comparative study of the ring-chain tautomerism of 1a and that of 2-phenyl-2,4-dihydro-1H-3,1-benzoxazine (3a) are also reported.

Spectroscopic studies of Solophenyl red 3BL polyazo dye tautomerism in different solvents using UV–visible, 1H NMR and steady-state fluorescence techniques

Azo–hydrazone tautomerism of Solophenyl red 3BL (C.I. Direct 80) polyazo dye in dimethylsulfoxide, methanol, propionic acid, formamide, dimethylformamide and water was investigated using UV–visible, 1H NMR and steady-state fluorescence techniques. The results showed that 3BL dye molecules exist as an equilibrium mixture of azo and hydrazone tautomer in all solvents, but azo form was dominant. In the UV–visible absorption spectra a broad intense band at 570 nm appeared which is related to azo (at 520 nm) and hydrazone (at 560 nm) tautomer bands and UV–visible band contour analyses further confirmed this statement. UV–visible absorption spectra of the two tautomric forms were completely distinguished in DMSO and DMF solvents while in water and propionic acid they were slightly distinguished. Furthermore, in methanol only azo tautomer was dominant. By increasing solvent polarity from water (1.82 D) to DMSO (3.96 D), the splitting of azo and hydrazone tautomers was increased. 1H NMR studies also confirmed the azo and hydrazone tautomerization in D 2O and DMSO– d 6 solvents. Finally, steady-state fluorescence spectroscopy results showed that 3BL dye molecules did not exhibit any appreciable light emission in acidic, neutral and basic aqueous solutions or in other solvents.

Quantum mechanical study of tautomerism of methimazole and the stability of methimazole–I 2 complexes

DFT and ab initio theoretical methods were used to calculate the relative stability of tautomers in the methimazole (MMI). The calculations show that the thione form of MMI 1 is more stable than the thiol tautomer in good agreement with the experimental results. The DFT and ab initio calculations were also used to determine the stability of MMI–I 2 complexes. All methods suggest that the methimazole in the MMI–I 2 complex exists almost exclusively as the thione tautomer. The Gibbs free energy difference between planar and perpendicular forms of thione tautomer of MMI–I 2 complex indicates that the planar form is the predominant complex. The counterpoise corrected Gibbs free energy also shows that the MMI–I 2(plan.) complex is more stable than the MMI–I 2(perp.) complex. These predictions are in good agreement with the experimental results. By using the natural bond orbital (NBO) approach, the effects of charge transfer interactions on the stability of MMI–I 2 complexes were investigated. The LP 3(S)→σ*(I–I) and LP 3(I)→σ*(N–H) charge transfer interactions may be very important in the stability of the planar form. The results show that the LP 3(S)→σ*(I–I) charge transfer interaction causes a greater increase in the σ*(I–I) antibond occupation number, and concomitantly, a greater increase in the corresponding I–I bond length in the planar complex with respect to the perpendicular complex. The LP 3(S)→σ*(I–I) charge transfer interaction is assisted by NH⋯I intermolecular hydrogen bonding. The atom in molecule (AIM) analysis shows that the charge density and its Laplacian at the S⋯I bond critical point of the planar complex is greater than the perpendicular complex.

A study of peculiar tautomerism of pyrido62,3-c961,2,69thiadiazine 2,2-dioxide system

A systematic study of the tautomerism of 4-amino-1 H -pyrido[2,3- c ][1,2,6]thiadiazine 2,2-dioxide has been carried out. Thus, the relativity stability of the possible tautomers has been studied in the gas phase by ab initio and functional density theory (DFT) methods. The theoretical study in solution has been carried out using several continuum solvation models as self consistent reaction field (SCRF) and the Poisson-Boltzmann method. In addition, complexes with one or two molecules of water have been calculated in order to consider explicit solute–solvent interactions. These results have been compared with UV spectrophotometry data, giving good agreement between theoretical and experimental results in aqueous solution. Finally, the gauge invariant atomic orbital method (GIAO) for calculating magnetic absolute shielding with both the ab initio method (RHF/6-31G*) and density functional theory (B3LYP/6-31G*) have been applied to explore this method to study the tautomeric equilibrium in the pyrido[2,3- c ][1,2,6]thiadiazine 2,2-dioxide system.

A PM3 study of tautomerism in some 3,3$prime;-methylenebis64-hydroxycoumarin9 systems

This Parametric Method 3 (PM3) study represents one of the first attempts to investigate the relative stabilities of the tautomers of dicoumarol and its derivatives. The calculations have been carried out both in the gas-phase and in a continuum model of water, on isolated tautomers of 10 systems obtained by single proton transfer and their heats of formation (Δ H f ), relative heats of formation ( δ Δ H f ) and dipole moments have been estimated. In most cases, the PM3 estimates of δ Δ H f indicate the predominance of α,γ′-benzopyran tautomeric structure in gas-phase. Macroscopic solvent effects on the tautomers have been modeled within the framework of continuum solvation model using Conductor like Screening Model (COSMO) method. In aqueous solution, the calculations imply significant qualitative differences in the δ Δ H f of the tautomers and predict the predominance of tetra-keto structure. However, in most of the cases regardless of the surrounding media, calculations indicate α,α′-benzopyran structure as the one having the largest dipole moment. The PM3 method has been demonstrated to be useful in studying such large systems which are otherwise difficult to be handled by ab initio methods for reasons of computational cost and time.

A PM3 study of tautomerism in some 3,3′-methylenebis[4-hydroxycoumarin] systems

This Parametric Method 3 (PM3) study represents one of the first attempts to investigate the relative stabilities of the tautomers of dicoumarol and its derivatives. The calculations have been carried out both in the gas-phase and in a continuum model of water, on isolated tautomers of 10 systems obtained by single proton transfer and their heats of formation (Δ H f), relative heats of formation ( δΔ H f) and dipole moments have been estimated. In most cases, the PM3 estimates of δΔ H f indicate the predominance of α,γ′-benzopyran tautomeric structure in gas-phase. Macroscopic solvent effects on the tautomers have been modeled within the framework of continuum solvation model using Conductor like Screening Model (COSMO) method. In aqueous solution, the calculations imply significant qualitative differences in the δΔ H f of the tautomers and predict the predominance of tetra-keto structure. However, in most of the cases regardless of the surrounding media, calculations indicate α,α′-benzopyran structure as the one having the largest dipole moment. The PM3 method has been demonstrated to be useful in studying such large systems which are otherwise difficult to be handled by ab initio methods for reasons of computational cost and time.

IR-spectral study of photoinduced tautomerization in 1,3-diphenyl-pyrazol-5-one

AbstractThe UV-photoinduced prototropic isomerization of 1,3-diphenyl pyrazol-5-one (DPhP) embedded in polyvinyl chloride (PVC) film is studied by means of difference in IR spectral analysis. The initially existing CH-form in non-irradiated compound is transformed to NH-form, and a hydrochloride salt is stabilized because of the photodegradation of the PVC. An additional comparative IR spectral study of DPhP in different solutions as well as of its hydrochloride salts in solid state is also carried out.

Ab initio study of tautomerism and of basicity center preference in histamine, from gas phase to solution—comparison with experimental data (gas phase, solution, solid state)

AbstractTautomeric and basicity center preferences for isolated neutral and monoprotonated histamine were studied by means of ab initio calculations (HF, MP2 and DFT). The polarizable continuum model (PCM) was applied to the study of the variations of the tautomeric and basicity center preferences in histamine on going from the gas phase to aqueous solution. Twelve solvents of different polarities (from n‐heptane to water) were chosen and calculations were performed for geometries optimized at the HF/6–31G* level. In low‐polarity solvents and in the gas phase the protonation site is identical. A change of the preferred site of protonation takes place in solvents containing heteroatoms (except tetrachloromethane). Under the same conditions, a variation of the tautomeric preference in the monocation occurs. The ring N2‐protonated form (ImH+)—favored in gas phase—is also preferred in non‐polar solvents (n‐heptane, benzene, tetrachloromethane). The ImH+ form becomes less important in more polar solvents. In such a case, the chain N3‐protonated form (AmH+‐T1) predominates. For the neutral histamine, solvation has a relatively small influence on the relative energies (variations are less than 1 kcal mol−1), and does not change the tautomeric preference (HA‐T2). Calculated basicity parameters were compared with those obtained experimentally in the gas phase and in aqueous solution. In the gas phase, the experimental (‘macroscopic’) basicity parameter (PA) is close to the ‘microscopic’ PA calculated for the gauche conformation. In aqueous solution, the microscopic pKa order is similar to that of the Eprot calculated for the trans conformation. In the solid state, both forms of histamine (neutral and monoprotonated) prefer the trans conformation. Some exceptions occur for complexes with metals. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Ab Initio Study of the Prototropic Tautomerism of Cytosine and Guanine and Their Contribution to Spontaneous Point Mutations

High-level quantum-chemical and quantum-dynamics calculations are reported on the tautomerization equilibria and rate constants of isolated and monohydrated cytosine and guanine molecules. The results are used to estimate the fraction of the bases present in the cell during DNA synthesis as the unwanted tautomers that forms irregular base pairs, thus giving rise to a spontaneous GC → AT point mutation. A comparison of the estimated mutation frequencies with the observed frequency in E. coli is used to analyze two proposed mechanisms, differing in the degree of equilibration reached in the tautomerization reaction. It was found that the fraction of the rare tautomer in monohydrated complex of cytosine as well as guanine significantly exceed the amount responsible for the observed values of the GC → AT mutations. In the absence of water the equilibrium concentration of tautomeric forms is relatively large, but the barrier to their formation is high. It is possible that the mechanism in which a high tautomerization barrier keeps the tautomeric transformation far from a state of equilibrium is more likely than a mechanism in which water and/or polymerases produce a low equilibrium concentration of the tautomeric forms.

Multinuclear magnetic resonance and theoretical calculations in the study of structure and tautomerism of some 2-amino- N′-(aryl)-benzamidines

1H, 13C and 15N NMR spectra of eight 2-amino- N′-(aryl)-benzamidines and of the parent compound were recorded, and unequivocal chemical shift assignments through the use of COSY, 1H-J resolved, HETCOR and COLOC sequences were performed. 1H and 13C chemical shifts for the nuclei of the benzamidine aromatic ring were not affected by the substituents present at N′-phenyl group, while the substituent effects in the chemical shifts of the same nuclei of N′-phenyl ring were very similar to the ones reported for the corresponding monosubstituted benzenes, indicating that there is no interaction between the two aromatic rings. 15N NMR spectra (DEPT sequence) show just two hydrogenated nitrogen atoms, which confirm that the amino form is the most stable tautomer, but the observation of a sharp signal and two broad signals ( 15N decoupled spectra), and the corresponding broad signal for the CNH 2 protons (in the 1H spectra), indicates the occurrence of tautomerism between the amino and imino forms, observable for some of the studied benzamidines. Theoretical calculations lead to the conclusion that these compounds occur mostly as the amino tautomer with Z configuration, which is stabilized by hydrogen bonding.

Preparation, Characterization and Thiol—Thione Tautomeric Studies of 2‐Thiono‐4‐methyl‐5‐ (2,2,2‐trifluoro‐1‐trifluoromethylethyl)‐1,3‐thiazoline.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Tautomeric studies on 2-mercapto pyrimidines and their significance in corrosion process

Thermodynamic parameters of some 2-mercapto pyrimidine derivatives were calculated by semi-empirical quantum chemical calculation methods. Full geometry optimization of the molecules in aqueous phase were made and any possible parallelism with the reported experimental data were searched The observed confidence levels of correlations for both tautomerism and corrosion process were satisfactory.

Theoretical study of molecular structure, tautomerism, and geometrical isomerism of N-methyl- and N-phenyl-substituted cyclic imidazolines, oxazolines, and thiazolines

The geometries of various tautomers and isomers of 2-methylamino-2-imidazoline, 2-methylamino-2-oxazoline, 2-methylamino-2-thiazoline, 2-phenylamino-2-imidazoline, 2-phenylamino-2-oxazoline, and 2-phenylamino-2-thiazoline have been studied using the Becke3LYP/6–31+G(d,p) DFT, ONIOM(Becke3LYP/6–31+G(d,p):HF/3–21G*) and ONIOM(Becke3LYP/6–31+G(d,p):AM1) methods. The optimized geometries indicate that these molecules show a distinctly nonplanar configuration of the cyclic moieties. In the gas phase, the amino tautomers (with exception of 2-phenylamino-2-imidazoline) are computed to be more stable than the imino tautomers. Of the two possible (E and Z) isomers of methyl and phenyl derivatives of imino-oxazolidine and imino-thiazolidine species, the (Z) isomers have the lowest energy. The iminozation free energies in the gas phase were found to be 5 – 15 kJ/mol. Absolute values of KT depend strongly on the accuracy of the method used for calculation of free energy. Solvation (using the MD simulations) causes, in most cases, a shift in tautomeric preference toward the imino species.

Oxidative Cyclization of Dithiobiuret under Basic Conditions and Theoretical Tautomeric Studies of 5-Amino-2,3-dihydro-1,2,4-thiadiazole-3-thione

Absract- The oxidative cyclization of dithiobiuret under basic conditions (NaOH-H 2 O 2 or CH 3 CO 3 H) afforded bis(5-amino-1,2,4-thiadiazolyl) 3,3'-disulfide (5), oxidative dimer form of 5-amino-3-mercapto-1,2,4-thiadiazole (3b). The theoretical tautomeric study (ab initio calculations) shows that the most stable tautomeric form of 3 is 5-amino-3-mercapto-1,2,4-thiadiazole (3b) among the four possible tautomers. Thus, 3b is speculated as an intermediate of the formation of the compound (5).

A theoretical study of tautomerism in hexa-aza macrocycles containing 2,2′-bipyridine and 1,10-phenanthroline and their ability to form lithium complexes

Calculations using ab initio restricted Hartree–Fock molecular orbital (RHF MO) theory and the density functional theory (DFT) are reported for several structural isomers of hexa-aza macrocycles 1– 3 containing 2,2′-bipyridine and 1,10-phenanthroline, as well as their lithium complexes. Based on the computational results, we describe the geometries, imine/amine tautomerism, and lithium complexation of these hexa-aza macrocycles. Metal-free macrocycles have the most stable imine form with planar structures, whereas their lithium complexes tend to be in the amine form. We discuss protonation/deprotonation processes with proton transfer in a cavity of macrocycles as a means of interpreting imine/amine tautomerism. Deprotonation and the subsequent geometric changes in tautomeric isomers lead to structural changes, i.e. macrocycles 2 and 3 having 2,2′-bipyridines become non-planar; this theoretical result is consistent with observed experimental behaviors. The present results indicate that unlike macrocycle 1, macrocycles 2 and 3 with 2,2′-bipyridine have the capacity for amine/imine tautomerism, as well as the ability to form complexes with lithium.

Preparation, characterization and thiol–thione tautomeric studies of 2-thiono-4-methyl-5-(2,2,2-trifluoro-1-trifluoromethylethyl)-1,3-thiazoline

The synthesis of a new trifluoromethyl 2-mercapto-1,3-thiadiazole derivative from 5,5,5-trifluoro-4-(trifluoromethyl)-3-penten-2-one, its structural study in the solid state and in the liquid phase and the thiol–thione tautomeric equilibrium study of the title compound are reported. The crystalline structure shows short intermolecular FF contacts. Theoretical calculations using HF and DFT methods were performed. Vibrational spectra were calculated for both tautomers and compared with experimental data. The experimental and theoretical results can be interpreted in terms of the existence of a thione tautomer in the solid and in solution.

Vinylphosphonium salt mediated simple synthesis of 7-oxo-7 H-pyrido[1,2,3- cd]perimidine derivatives. Dynamic NMR spectroscopic study of prototropic tautomerism in ethyl 1 H-perimidine-2-carboxylate

The reactive 1:1 intermediate produced in the reaction between dimethyl acetylenedicarboxylate and triphenylphosphine was trapped by ethyl 1 H-perimidine-2-carboxylate to yield 5-ethyl 9-methyl 7-oxo-7 H-pyrido[1,2,3- cd]perimidine-5,9-dicarboxylate and 5-ethyl 9-methyl 1-[3-methoxy-1-(methoxycarbonyl)-3-oxo-1-propenyl]-7-oxo-7 H-pyrido[1,2,3- cd]perimidine-5,9-dicarboxylate in nearly 7:1 ratio and overall good yields. The first of these compounds is quantitatively converted to ethyl 10-[(2-ethoxy-2-oxoacetyl)amino]-2-hydroxybenzo[ h]quinoline-4-carboxylate by refluxing in water-saturated chloroform, while the later compound remained unchanged under the same reaction conditions. The free-energy barrier (Δ G ≠) for prototropic tautomerism in ethyl 1 H-perimidine-2-carboxylate is determined by dynamic 1H NMR studies to be 50.6 kJ mol −1. The methylene protons of 5-ethyl 9-methyl 7-oxo-7 H-pyrido[1,2,3- cd]perimidine-5,9-dicarboxylate are diastereotopic as a result of peri interaction of carboethoxy group with the adjacent carbonyl bond. The free-energy barrier for conformational racemization of this compond is 56.3 kJ mol −1.