Journal articles: 'Tautomeric constant'

1

KOSAR, BASAK. "THERMODYNAMIC AND SECOND-ORDER NON-LINEAR OPTICAL PROPERTIES OF TAUTOMERIC FORMS OF (E)-4-METHOXY-2-[(4-NITROPHENYL)IMINOMETHYL]PHENOL: A DENSITY FUNCTIONAL STUDY." Journal of Theoretical and Computational Chemistry 10, no. 03 (June 2011): 279–95. http://dx.doi.org/10.1142/s021963361100644x.

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This work presents a computational study on the tautomeric forms of (E)-4-Methoxy-2-[(4-nitrophenyl)iminomethyl]phenol, an ortho-hydroxy Schiff base compound. The electronic structure of title compound has been characterized at the B3LYP/6-311G(d,p) level of density functional theory. The first hyperpolarizability values have been obtained from the molecular polarizabilities for both tautomers. The second-order non-linear optical properties have been investigated based on their relationships with the natural bond orbitals and frontier molecular orbitals. The changes of thermodynamic properties with temperature going from 100 K to 300 K have been investigated for the reactants and the reaction products tautomers. Tautomeric equilibrium constant derived from the difference between the Gibbs free energies of tautomers has been obtained at different temperatures. The relationship between formation enthalpy and entropy changes has been investigated with the enthalpy-entropy compensation.

2

Scrypynets, Yu V., G. O. Fedosenko, G. V. Maltsev, I. I. Chebotars'ka, D. I. Aleksandrova, S. N. Kashutskуy, and A. V. Yegorova. "DETERMINATION OF THE IONIZATION CONSTANT OF FAVIPIRAVIR API." Odesa National University Herald. Chemistry 28, no. 2(85) (September 12, 2023): 52–61. http://dx.doi.org/10.18524/2304-0947.2023.2(85).286603.

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One of the aspects of studying the physical and chemical properties of compounds is the establishment of their ionization constants. These indicators are extremely important from both a theoretical and a practical point of view. Theoretical analysis of proteolytic equilibria and potentiometric titration of an aqueous solution of an oral antiviral drug approved for the treatment of influenza in Japan, AFI favipiravir, were carried out in the paper. The presence of a hydroxy group in the third position of the pyrazine cycle in the molecule determines the ability of favipiravir to exhibit acid-base properties. According to acid-base titration, the ionization constant of Favipiravir API was determined: pKa = 5.05 ± 0.02. On the basis of the found pK value, the degree of formation of protolytic forms was calculated depending on the pH of the solution. It was established that favipiravir API in solutions exists in two protonated (non-ionized), tautomeric forms A1H and A2H and one deprotonated (ionized) form A-. Based on the calculations, the following conclusions can be drawn: - at a pH value of the solution from 0 to 3.5, approximately 100% of the substance will be in two protonated (non-ionized), tautomeric forms A1H and A2H; - at pH = 5, approximately 50% of the substance will be in two protonated (non-ionized), tautomeric forms, A1H and A2H; - at a pH value > 6.5, approximately 100% of the substance will be in one deprotonated (ionized) form of A-. This is confirmed by the nature of the electronic absorption spectra of aqueous solutions of favipiravir. In the spectrum of favipiravir in a 0.1 M hydrochloric acid solution, there are two bands at the wavelengths of 322 nm and 365 nm, which correspond to the two tautomeric forms A1H and A2H. In the spectrum of favipiravir in 0.1 M sodium hydroxide solution, there is one band at a wavelength of 364 nm, which corresponds to one ionized form of A-. At the stage of pharmaceutical development, these data on the acid-base properties of the Favipiravir API can be used in the development of production technology, the creation of quality control methods and their validation, as well as in the study of the solubility of the API and the dissolution profiles of the finished medicinal product.

3

Babu, Numbury Surendra, and Didugu Jayaprakash. "Computational Study of the Stability of Tautomers and equilibrium constants of Cyanuric acid (CA) in Different solvents." JOURNAL OF ADVANCES IN CHEMISTRY 11, no. 2 (January 22, 2015): 3485–97. http://dx.doi.org/10.24297/jac.v11i2.6691.

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In the present investigation, the tautomeric and equilibrium of Cyanuric acid has been studied using Hartifock (HF) method in the gas phase and different solvents using the PCM model. The relative energies of these tautomers have been calculated at the HF level of theory using 6-311++ G (d,p) basis set. Energetics and relative stabilities of the tautomers were compared and analyzed in both the gaseous and different solvents. The results indicate that the keto tautomer (CA1) is the most stable form in the gas phase and other solvents. The order of stability of isomers was found to be CA1 > CA3> CA11> CA2> CA6> CA10> CA7> CA4> CA5> CA9>CA8. Having the largest dipole moment the CA8 tautomer is expected to have the strongest interaction with polar solvents. The HF method calculated tautomeric equilibrium constants with respect to the most stable tautomer CA1 of Cyanuric acid both in the gas and in different solvents. The of the equilibrium constants calculated starting from the general outline of interconversion.

4

Hallé, Jean-Claude, Jacques Lelievre, and François Terrier. "Solvent effect on preferred protonation sites in nicotinate and isonicotinate anions." Canadian Journal of Chemistry 74, no. 4 (April 1, 1996): 613–20. http://dx.doi.org/10.1139/v96-065.

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Potentiometric determinations of the two successive acidities of nicotinic and isonicotinic acids (AH2+) have been carried out in aqueous dimethyl sulfoxide mixtures containing up to 95% Me2SO by volume. In both systems, the results reveal that the addition of Me2SO induces a proton transfer from the pyridinium ring to the carboxylate group, the tautomeric equilibrium between the neutral forms of the two acids being displaced toward the zwitterionic form (AH±) in aqueous solution, but toward the molecular form (AH°) in Me2SO. An analysis of the data by means of Hammett relationships previously established for benzoic acids over the whole range of H2O/Me2SO mixtures allowed the four microscopic acidity constants as well as the tautomeric equilibrium constant KT pertaining to the complete ionization scheme of the two acids to be determined. At 20 °C, there are equal populations of the tautomeric AH° and AH± species in the mixtures containing 38 and 47% Me2SO for the nicotinic and isonicotinic systems, respectively. Hammett relationships describing the ionization behaviour of a number of substituted pyridinium cations in H2O/Me2SO mixtures are also discussed. Possible reasons accounting for the peculiar effects exerted by the NH2, CONH2, and COOH substituents on the process are suggested. Key words: nicotinic and isonicotinic acids, substituted pyridines, acidities, tautomeric equilibrium, protonation sites, water – dimethyl sulfoxide mixtures.

5

Mchedlov-Petrossyan, Nikolay O., and Natalya A. Vodolazkaya. "Protolytic Equilibria in Organized Solutions: Ionization and Tautomerism of Fluorescein Dyes and Related Indicators in Cetyltrimethylammonium Chloride Micellar Solutions at High Ionic Strength of the Bulk Phase." Liquids 1, no. 1 (February 20, 2021): 1–24. http://dx.doi.org/10.3390/liquids1010001.

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Ionic equilibrium of 22 hydroxyxanthenes, including halogen and nitro derivatives of fluorescein, and their thio- and aza analogues, were studied spectrophotometrically in micellar solutions of cetyltrimethylammonium chloride at ionic strength of the bulk phase 4.0 M KCl. This micellar pseudophase is characterized by the electrostatic surface potential of +(15–16) mV and the ETN value of 0.623. In the case of dyes bearing the COOH group, colorless lactone is the predominant tautomer of the molecular form H2R. A new classification of fluoresceins is developed. The dyes were divided into four groups based on the nature of tautomerism of the anions. In the case of the fluorescein type, the monoanions HR− exist predominantly as “carboxylate” tautomers, with ionized carboxylic and non–ionized hydroxylic group. For the dyes of the eosin type, the situation is opposite, while for the intervening type of compounds, the concentrations of the two tautomers are comparable. Dyes capable of forming lactone anions HR− were classified as the fourth type. For some of them, even the dianion R2− exists as a lactone. The relationship between the stepwise ionization constants, Ka1/Ka2, varies from 1.3 to 1.07 × 105 and is determined by the state of tautomeric equilibrium of molecules and ions.

6

Nishimura, N., K. Danjo, Y. Sueishi, and S. Yamamoto. "Solvent and Pressure Effects on the Tautomeric Equilibrium of 4-Phenylazo-1-naphthol." Australian Journal of Chemistry 41, no. 6 (1988): 863. http://dx.doi.org/10.1071/ch9880863.

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The azo-hydrazone tautomerism of 4-phenylazo-1-naphthol has been studied in ethanol-water and acetone-water systems and at high pressures. The hydrazone form is favoured by adding water to organic cosolvents and by applying pressure. The effects of solvent polarity and pressure on the equilibrium constant (KT) between the tautomers were examined by using Kirkwood-type equations applied to chemical equilibrium. Remarkable deviations were observed in the Kirkwood plots for both types of mixtures. Solvent-induced polarization, especially of the hydrazone form in a high polarity region, was suggested. In ethanol-water mixtures, correlation is fair between ln KT and ET, while in acetone-water system the ln KT against ET plot was curved. The above findings, together with the difference in the volume data for the two systems, suggest an existence of special interactions between acetone molecules and hydrophobic parts of 4-phenylazo-1-naphthol.

7

Lopez, Concepcion, Rosa Ma Claramunt, Swiatoslaw Trofimenko, and José Elguero. "A 13C NMR spectroscopy study of the structure of N-H pyrazoles and indazoles." Canadian Journal of Chemistry 71, no. 5 (May 1, 1993): 678–84. http://dx.doi.org/10.1139/v93-092.

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The 13C chemical shifts and some 1H–13C coupling constants of 23 pyrazoles and indazoles are reported and discussed. The spectra were recorded in CDCl3 and in DMSO-d6 solutions and, in several cases, also in the solid state (CP/MAS technique). These data allow us to determine the tautomerism of these compounds in the solid state (usually only one tautomer) and in solution. The position of the tautomeric equilibrium is related to the Hammett σm value of the 3(5)-substituent.

8

Němcová, Irena, Oldřich Valcl, and Jiří Makovička. "The interactions of tensides with calconalide I." Collection of Czechoslovak Chemical Communications 56, no. 10 (1991): 2100–2106. http://dx.doi.org/10.1135/cccc19912100.

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A study was made of the effect of cationogenic, anionogenic and nonionogenic tensides on the absorption spectra and dissociation constant of calconalide I. It was found that the changes in the absorption spectra are a result of influences on the dissociation of the dye and a shift in the equilibrium between the tautomeric forms. The mechanism of the effect of the tenside on dye dissociation was also discussed.

9

Acree, William E., and Sheryl A. Tucker. "Thermochemical Investigations of Tautomeric Equilibrium. Variation of the Calculated Equilibrium Constant with Binary Solvent Composition." Physics and Chemistry of Liquids 20, no. 2-3 (September 1989): 135–45. http://dx.doi.org/10.1080/00319108908036400.

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10

Moskaeva, Elena, Ahina Mosharenkova, Sergey Shekhovtsov, and Nikolay Mchedlov-Petrossyan. "PROTOLYTIC EQUILIBRIUM OF TETRA- AND PENTANITROFLUORESCEINS IN A BINARY SOLVENT ACETONITRILE – DIMETHYL SULFOXIDE (MASS RATIO 96 : 4)." Ukrainian Chemistry Journal 87, no. 5 (June 25, 2021): 25–37. http://dx.doi.org/10.33609/2708-129x.87.05.2021.25-37.

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In this paper, the acid-base and tautomeric equilibria of four nitrofluorescein dyes, 2,4,5,7-tetranitrofluorescein, 2,4,5,7,4’-pentanitrofluorescein, 2,4,5,7,5’-pentanitrofluorescein, and 2,4,5,7-tetranitrofluorescein methyl ester, were studied. As reaction media, a binary solvent acetonitrile – dimethyl sulfoxide (96 : 4 by mass) was used. The acidity scale in this solvent was established previously. The indices of the dissociation constants of the dyes were determined using the spectrophotometric method. Interpreting the values requires an understanding of the state of tautomeric equilibria. The behavior of these compounds differs significantly from that of other fluorescein dyes, e.g., halogen derivatives. In the case of the first three compounds, i.e., for dyes with a free carboxylic group, the lactonic structure is predominant not only for the neutral form, but even for the double-charged anion. The single-charged anionic form exists as an equilibrium mixture of a colored (and fluorescent) tautomer and an almost colorless lactone. The fourth compound with esterified carboxylic group exhibits extreme stability in its anionic form. Evaluation of the tautomerization constants made it possible to calculate the microscopic equilibrium constants of the stepwise dissociation of dye lactones, k1L and k2L. The consideration of the difference (pk2L – pk1L) allowed estimating the effective relative permittivity of the space between the ionizing groups basing on the Bjerrum – Kirkwood – Westheimer equation. Tautomerism of anions was discussed from the point of view of stabilization of symmetric structures.

11

Harris, Victoria H., Clifford L. Smith, W. Jonathan Cummins, Alan L. Hamilton, Harry Adams, Mark Dickman, David P. Hornby, and David M. Williams. "The Effect of Tautomeric Constant on the Specificity of Nucleotide Incorporation during DNA Replication: Support for the Rare Tautomer Hypothesis of Substitution Mutagenesis." Journal of Molecular Biology 326, no. 5 (March 2003): 1389–401. http://dx.doi.org/10.1016/s0022-2836(03)00051-2.

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12

Raczyńska, Ewa D. "Influence of Substituent on Tautomeric Equilibrium Constant in 5(6)-Substituted Benzimidazoles in the Gas Phase." Journal of Chemical Research, no. 11 (1998): 704–5. http://dx.doi.org/10.1039/a802613k.

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13

CHARIF, I. E., S. M. MEKELLECHE, and D. VILLEMIN. "SOLVENT EFFECTS ON THE KETO-ENOL TAUTOMERIC EQUILIBRIUM OF TETRONIC AND ETHYL ACETOACETATE CARBON ACIDS: A THEORETICAL STUDY." Journal of Theoretical and Computational Chemistry 09, no. 06 (December 2010): 1021–32. http://dx.doi.org/10.1142/s0219633610006171.

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The solvent effects on the keto-enol tautomeric equilibriums of ethyl acetoacetate (EAA) and tetronic acid (TA) are theoretically investigated. The present study shows that the most stable keto tautomer of EAA corresponds to the trans diketo, E, Z form; while the most stable enol tautomer corresponds to the structure in which the enolization takes place at the carbonyl group. Our calculations also put in evidence that the keto tautomer of TA prefers the trans diketo, E, E form, while the most stable enol tautomer corresponds to the structure in which the enolization takes place at the carbonyl group. The calculated free energies indicate that, in polar solvents, the keto-enol equilibrium of EAA is shifted towards the keto tautomer, whereas the keto-enol equilibrium of TA is shifted toward the enol tautomer. The trends of the change of equilibrium constants with respect to the change of solvent polarity are well reproduced by both B3LYP and MP2 calculations. The present study shows that the enthalpic term is predominant in the determination of the calculated equilibrium constants and the entropic effect on the calculated Gibbs free energies is found to be very small and has little influence on the studied keto-enol tautomeric equilibriums.

14

Ramírez-Galicia, G. "A new model for the theoretical tautomeric constant (KT) calculation of 2-, 3- and 4-substituted pyridines." Journal of Molecular Structure: THEOCHEM 542, no. 1-3 (June 15, 2001): 1–6. http://dx.doi.org/10.1016/s0166-1280(00)00649-7.

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15

Obukhova, Olena M., Nikolay O. Mchedlov-Petrossyan, Natalya A. Vodolazkaya, Leonid D. Patsenker, and Andrey O. Doroshenko. "Stability of Rhodamine Lactone Cycle in Solutions: Chain–Ring Tautomerism, Acid–Base Equilibria, Interaction with Lewis Acids, and Fluorescence." Colorants 1, no. 1 (February 24, 2022): 58–90. http://dx.doi.org/10.3390/colorants1010006.

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The equilibrium between different tautomers that can be colored or colorless is an important feature for rhodamine dyes. Presently, this phenomenon is mostly discussed for rhodamine B. Herein, we studied the tautomerism and acid–base dissociation (HR+ ⇄ R + H+) of a set of rhodamines in organic media. Form R is an equilibrium mixture of the colored zwitterion R± and colorless lactone R0. Absorption spectra in 90 mass% aqueous acetone reflects the correlation between the dyes structure and the equilibrium constant, KT = [R0]/[R±]. Increase in the pKa value on transferring from water to organic solvents confirms the highly polar character of the R± tautomer. To reveal the role of the solvent nature, the tautomerism of an asymmetrical rhodamine, 2-(12-(diethyliminio)-2,3,5,6,7,12-hexahydro-1H-chromeno[2,3-f]pyrido[3,2,1-ij]quinolin-9-yl)benzoate, was examined in 14 media. This chain–ring tautomerism is an intramolecular acid–base reaction; the central carbon atom acts as a Lewis acid. The interaction with other Lewis acids, Li+, Ca2+, Mg2+, and La3+, results in rupture of lactone cycle. In polar solvents, lactones undergo photocleavage resulting in formation of highly fluorescent R±, whereas the blue fluorescence and abnormally high Stokes shift in low-polar media may be explained either by another photoreaction or by spiroconjugation and charge transfer in the exited state.

16

Cristiani, Franco, Francesco A. Devillanova, Angelo Diaz, Francesco Isaia, Gaetano Verani, Francesco Demartin, and Giuseppe Saba. "2-S-Methyl-5,5-dimethylimidazolin-4-one: chemical behaviour and crystal structure of its Δ1 tautomer." Canadian Journal of Chemistry 69, no. 3 (March 1, 1991): 383–90. http://dx.doi.org/10.1139/v91-059.

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2-S-Methyl-5,5-dimethylimidazolin-4-one can be crystallized as Δ1 or Δ2 tautomers, depending on the solvent polarity. The structure, solved on crystals obtained from benzene, showed it to be the Δ1 tautomer in the anti configuration. Δ1-2-S-Methyl-5,5-dimethylimidazolin-4-one is monoclinic, space group P21/c, with a = 8.347(1), b = 10.121(1), c = 9.996(3) Å, β = 107.20(2)°, R = 0.037 for 1395 independent reflections with I > lσ(I). The quantum mechanical (MNDO, PM3, and AMI) calculations indicate that among the four conformers the anti-Δ1 tautomer has both the lowest dipole moment and the lowest enthalpy of formation. The ultraviolet spectrum of the title compound has been recorded in solvents with different dielectric constants. The infrared spectra of the Δ1 and Δ2 solid samples are discussed relative to the v(NH) and v(CO) absorptions and in comparison with those recorded in carbon tetrachloride and in chloroform solution. Its acid–base behaviour has been studied by determining the pKa in aqueous solutions and the results are compared with those obtained for some related hydantoins. Key words: 2-S-methyl-5,5-dimethylimidazolin-4-one, tautomerism, hydantoin derivatives, crystal structure.

17

Saraswat, Apoorv, and Rana Krishna Pal Singh. "Utilization of Dissociation Constant (pKa) Value Perspective of -CH Acid in Electrochemical Synthesis of 4H-Chromene and its Derivatives." Asian Journal of Chemistry 32, no. 7 (2020): 1697–702. http://dx.doi.org/10.14233/ajchem.2020.22663.

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Activation of acidic-H in C-H chemistry through imposition of proper electrode potential is a matter of concern among synthetic research community. In this study, our main aim is to get deeper insight into the dissociation constant (pKa) value perspective of -CH acid and its utilization in electro-catalytic transformation. Herein, a platinum cathode was being used to attract a covalently bonded electrons that exist between C and active-H of CH-acid (dimedone) toward itself i.e. cathodic part to form acidic-H and thus, form in-situ tautomeric enol form that acts as electro-generated base. Utilization of this notability has been accomplished by electrochemical synthesis of 4H-chromene and its derivatives by performing cathodic reduction. Due to acidic property of dimedone, cathodic platinum electrode draw the potential switched to lower side that is being recorded through the potential-cum-galvanostat which facilitates the faster transfer of electrons from -CH acid to form enolate ion of dimedone that act as electro-generated base for the reaction media which simultaneously undergoes cascade reaction (Michael addition/cyclization) on activated alkene (α-cyanocinnamonitrile derivatives). This approach offers several advantages such as good yield, mild reaction condition, easy accessible, simple work-up procedure and controlled potential selectivity at cathode. Use of no base is pivotal section of this methodology. Electro-generated base is used instead of chemical toxic bases, so the present protocol is green as electrons that acts as intrinsically sole reagent for the reaction media which is renewable, hence sustainable for the safe future.

18

Chebotarev, Alexandr N., Denys V. Snigur, and Kateryna V. Bevziuk. "STUDY OF PROTOLYTIC EQUILIBRIA IN SOLUTIONS OF HYDROXYANTHRAQUINONE DYES BY CHEMICAL CHROMATICITY METHOD." IZVESTIYA VYSSHIKH UCHEBNYKH ZAVEDENIY KHIMIYA KHIMICHESKAYA TEKHNOLOGIYA 60, no. 3 (April 13, 2017): 22. http://dx.doi.org/10.6060/tcct.2017603.5418.

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In this paper, the acid-base properties of hydroxyanthraquinone dyes-alizarin, alizarin red S and alizarin-complexone- were studied in aqueous solutions by chemical chromaticity and spectrophotometry methods. The advantages of chemical chromaticity method over UV-VIS spectrophotometry method were shown in the protolytic equilibria study in dyes solutions. The chemical chromaticity method, unlike spectrophotometry, can determine the deprotonation constant of sulfo group of alizarin red S. Using the chromaticity functions data it is possible to determine all ionization constant of these hydroxyanthraquinone dyes. The overlay of tautomeric equilibria on alizarin-complexone ionization process does not interfere for pK determination of their functional groups by the chemical chromaticity method. A quantitative color description of objects of different nature by calculating their color coordinates by spectrophotometric data allows distinguishing spectrally similar substances and obtaining new data on them. Using the values of chromaticity functions of ion-molecule forms of the hydroxyanthraquinone dyes as an analytical signal allows to get a complete picture of the existing acid-base equilibria in a wide range of medium acidity. A probable acid-base equilibria schemes were theoretically determined for alizarin, alizarin red S and alizarin-complexone using CNDO calculations. The diagrams of the ion-molecular forms distribution of hydroxyanthraquinone were built.For citation:Chebotarev A.N., Snigur D.V., Bevziuk K.V. Study of protolytic equilibria in solutions of hydroxyanthraquinone dyes by chemical chromaticity method. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 3. P. 22-28.

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Rak, Janusz, Piotr Skurski, Ludwika Jozwiak, and Jerzy Blazejowski. "Theoretical Studies on the Effect of the Medium on Tautomeric Phenomena in Neutral and Protonated Acridin-9-amine. Mechanism of Tautomerization in Neutral Entities." Australian Journal of Chemistry 50, no. 2 (1997): 97. http://dx.doi.org/10.1071/c96184.

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Semiempirical AM1 and PM3 (gaseous phase) and AM1-COSMO, PM3-COSMO and PM3-AQ (liquid phase) methods were used to examine the structure, as well as the thermodynamic and physicochemical features (dipole moments and LUMO and HOMO energies), of tautomeric forms of neutral, mono- and di-protonated acridin-9-amine. The energy gaps between possible tautomers are only insignificantly influenced by the medium and reveal the coexistence of neutral and diprotonated entities in two forms, and the monoprotonated entity in one form. Entropy and thermal energy, which can be evaluated only for gaseous systems on the basis of statistical mechanics, both increase with temperature and affect equilibria between tautomers. The mechanism of bimolecular tautomerization of neutral acridin-9-amine was examinated by the PM3 and PM3-AQ methods. The inclusion of entropy changes accompanying tautomerization in the gaseous phase enabled high-pressure-limit rate constants for well-defined steps of the process at the RRKM and transition state level of theory to be predicted. Thermodynamic and kinetic considerations indicate that neutral acridin-9-amine should coexist in amino and imino tautomeric forms, and that both these entities can convert into each other at moderate temperatures.

20

Anderson, John C., and A. Douglas Broadbent. "The influence of pH in the tautomerism of 9,10-anthracenediols and 1,3-diketones." Canadian Journal of Chemistry 64, no. 1 (January 1, 1986): 127–32. http://dx.doi.org/10.1139/v86-022.

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Equations have been derived to describe how the ratio of the total concentration of all keto forms to that of all enolic forms (K′) varies with changing pH, for a tautomeric system in which the enol and ketone are polyprotic acids. The influence of pH on the tautomerism of 2-amino-9,10-anthracenediol, 2,9,10-anthracenetriol, 2,4-pentanedione, and 1-(3-pyridyl)-1,3-butanedione was examined. The equilibrium constants for all the acid–base equilibria involved were calculated from the dependence of K′ on pH.

21

Sargolzaei, M., M. Afshar, and H. Nikoofard. "Solvent Effect on The Equilibrium and Rate Constant of the Tautomeric Reaction in Nexium, Skelaxin, Aldara and Efavirenz Drugs: A Dft Study." Journal of Structural Chemistry 59, no. 2 (March 2018): 297–305. http://dx.doi.org/10.1134/s0022476618020063.

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22

Ou, Zhongping, Xueyan Chen, Lina Ye, Songlin Xue, Yuanyuan Fang, Xiaoqin Jiang, and Karl M. Kadish. "N-confusedmeso-tetraaryl-substituted free-base porphyrins: determination of protonation and deprotonation constants in nonaqueous media." Journal of Porphyrins and Phthalocyanines 19, no. 01-03 (January 2015): 251–60. http://dx.doi.org/10.1142/s1088424614501132.

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The protonation and deprotonation reactions for a series N-confused meso-tetraaryl-substituted free-base porphyrins containing electron-donating or electron-withdrawing substituents was monitored in CHCl3and DMF by UV-visible spectroscopy during titrations with trifluoroacetic acid or tetra-n-butylammonium hydroxide. The spectroscopic data was also used to calculate equilibrium constants for these reactions. The examined compounds are represented as (XPh)4NCPH2, where "NCP" represents the N-confused porphyrin π-conjugated macrocycle and X is a CH3O , CH3, H or Cl para-substituent on the four meso-phenyl rings (Ph) of the compound. The porphyrins can exist in two tautomeric forms depending upon the solvent and each tautomer undergoes two stepwise protonation reactions leading to formation of the mono- and bis-protonated porphyrins, [(XPh)4NCPH3]+and [(XPh)4NCPH4]2+. A single step deprotonation is observed for the same compounds in DMF and the product is assigned as [(XPh)4NCPH]-. Comparisons are made between UV-visible spectra of the protonated, neutral and deprotonated forms of the porphyrin and the effect of the porphyrin ring substituents and tautomeric form of the neutral porphyrin on the UV-visible spectra and protonation constants is discussed along with data from DFT calculations.

23

Albayrak Kaştaş, Çiğdem, and Gökhan Kaştaş. "Dependence of tautomerism on substituent type in o-hydroxy Schiff bases." Macedonian Journal of Chemistry and Chemical Engineering 38, no. 1 (May 16, 2019): 85. http://dx.doi.org/10.20450/mjcce.2019.1531.

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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 p-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.

24

Almdal, Kristoffer, Hanne Eggert, Ole Hammerich, Lars Skattebøl, Povl Krogsgaard-Larsen, Ragnar Ryhage, and Roland Isaksson. "Determination of the Equilibrium Constant for the Tautomeric 9-Hydroxyanthracene/9-Anthrone System in Aprotic Solvents by a Novel Application of Cyclic Voltammetry." Acta Chemica Scandinavica 40b (1986): 230–32. http://dx.doi.org/10.3891/acta.chem.scand.40b-0230.

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Kunyima, Anaclet B., Joseph C. Kuburhanwa, Heritier M. Kaseya, Benoît M. Mukendi, and Michel B. Kunyima. "Kinetic and Thermodynamic Study of Extraction of Oleoresin Containing Curcuminoids from Turmeric (Curcuma Longa L.)." European Journal of Advanced Chemistry Research 3, no. 2 (July 24, 2022): 15–26. http://dx.doi.org/10.24018/ejchem.2022.3.2.107.

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Numerous studies are nowadays undertaken across the world laboratories on turmeric and show its excellent virtues in various fields such as medicine, pharmacology, cosmetics, and so forth. In this paper, to enlarge the domain of scientific knowledge, a kinetic and thermodynamic behavior of curcuminoid extraction from Curcuma longa L has been investigated in the dilute medium using ethanol as solvent under a magnetic stirrer. Through the KUNYIMA method, by means of the KUNYIMA first law, the global kinetic constant has been calculated at 250 rounds per minute (rpm) and the data have shown its Constance at 500 rounds per minute at the same temperature (27.5 °C). The kinetic constant can serve to build a commercial tank of production of curcuminoids using the sizing factor to endow the Democratic Republic of the Congo with its home technology of curcuminoids extraction as is hereby discussed. Extraction enthalpy and entropy have been calculated in a closed system using the Arrhenius relation. The results of enthalpy indicate that the extraction of curcuminoids is an endothermic phenomenon. This endothermic behavior of the extraction of curcuminoids in ethanol under a magnetic stirrer probably suggests the predominance of the enol form of curcumin in presence of its tautomeric ketone form. Also, the extraction of curcuminoids in ethanol is an endothermic phenomenon and this last phenomenon probably suggests its non-bioavailability. The energy promoting solubilization of curcuminoids arises from the solvent, under a magnetic stirrer, that can meet its alike with the hydroxyl group (like dissolves like). The value of the variation of entropy has evidently suggested the disturbance of the natural order after extraction. Note that KUNYIMA's second law has been hereby announced.

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Demukhamedova, S. "THEORETICAL QUANTUM-CHEMICAL SIMULATION OF THE STRUCTURE AND PROPERTIES OF CARNOSINE DIPEPTIDE BY THE DFT METHOD." Russian Journal of Biological Physics and Chemisrty 7, no. 2 (November 15, 2022): 241–50. http://dx.doi.org/10.29039/rusjbpc.2022.0509.

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The spatial and electronic structure of the natural carnosine dipeptide in the N3H tautomeric form of its imidazole ring in gas and water environment was studied by the quantum-chemical method in the framework of the electron density functional theory (DFT) with the three-parameter B3LYP hybrid functional. The sufficiently reliable and time-saving extended basis set, taking into account the polarization and diffuse functions 6-31+G (d, p) was used for calculations. All calculations were carried our using the Gaussian 09 software package. GaussView 6.0.16 was used to visualize the obtained results. Geometrical parameters, values of electronic energy, energies of HOMO and LUMO orbitals and the energy gap between them, reactivity descriptors, values of dipole moments, partial charges on atoms, and theoretical IR spectra for gaseous and aqueous media are obtained. Calculations show that the proposed model of the molecule forms a stable structure. The resulting structure is stabilized due to non-covalent interactions in the dipeptide. Molecular electrostatic potential maps (MEP) are constructed to identify potential binding sites. To study the charge transfer and intramolecular interactions that determine the stability of the molecule, an analysis of natural bond orbitals (NBOs) was carried out. Structural rearrangements and changes in various parameters depending on the dielectric constant of the medium are analyzed.

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Hush, NS, MK Livett, JB Peel, and GD Willett. "Variable-Temperature Ultraviolet Photoelectron-Spectroscopy of the Keto-Enol Tautomers of Pentane-2,4-Dione." Australian Journal of Chemistry 40, no. 3 (1987): 599. http://dx.doi.org/10.1071/ch9870599.

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The HeI photoelectron spectra of pentane-2,4-dione measured at various temperatures have been subjected to a spectrum-stripping technique to enable isolation of the complete HeI photoelectron spectra of the keto and enol tautomers. The resulting spectrum-stripping coefficients allow evaluation of the enolization equilibrium constant at each temperature and subsequently the enthalpy of enolization. Interpretation of the photoelectron spectra is achieved with the use of ab initio molecular orbital calculations. Photoionization of non-bonding oxygen, and π -type electrons, contribute to the low ionization energy region of the spectrum, with a πCC < no- <πCO ordering for the enol tautpmer , and no- < no+ < πCO+ for the keto tautomer . Because of different electronic relaxation effects, ∆SCF calculations are required to predict correctly the observed πCC/no- spacing for the enol tautomer.

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Sigel, H. "Acid-base properties of purine residues and the effect of metal ions: Quantification of rare nucleobase tautomers." Pure and Applied Chemistry 76, no. 10 (January 1, 2004): 1869–86. http://dx.doi.org/10.1351/pac200476101869.

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The macro acidity constants valid for aqueous solutions of several adenine, guanine,and hypoxanthine derivatives are summarized. It is shown how the application of the corresponding constants, e.g., for 7,9-dimethyladenine, allows a quantification of the intrinsic acidic properties of the (N1)H0/+ and (N7)H+ sites via micro acidity constants, and how to use this information for the calculation of the tautomeric ratios regarding the monoprotonated species, that is, N7-N1*H versus H*N7-N1, meaning that in one isomer H+ is at the N1 site and in the other at N7. It is further shown that different metal ions coordinated to a given site, e.g., N7, lead to a different extent of acidification, e.g., at (N1)H; the effect decreases in the series Cu2+>Ni2+>Pt2+ ~Pd2+. Moreover, the application of micro acidity constants proves that the acidifications are reciprocal and identical. This means, Pt2+ coordinated to (N1)–/0 sites in guanine, hypoxanthine, or adenine residues acidifies the (N7)H+ unit to the same extent as (N7)-coordinated Pt 2+ acidifies the (N1)H0/+ site. In other words, an apparently increased basicity of N7 upon Pt2+ coordination at (N1)–/0 sites disappears if the micro acidity constants of the appropriate isocharged tautomers of the ligand are properly taken into account. There is also evidence that proton–proton interactions are more pronounced than divalent metal ion–proton interactions, and that these in turn are possibly larger than divalent metal ion–metal ion interactions. The indicated quantifications of the acid-base properties are meaningful for nucleic acids including the formation of certain nucleobase tautomers in low concentrations, which could give rise to mutations.

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Ouali, M., C. Laboulais, H. Leh, D. Gill, E. Xhuvani, F. Zouhiri, D. Desmaele, et al. "Tautomers of styrylquinoline derivatives containing a methoxy substituent: computation of their population in aqueous solution and their interaction with RSV integrase catalytic core." Acta Biochimica Polonica 47, no. 1 (March 31, 2000): 11–22. http://dx.doi.org/10.18388/abp.2000_4058.

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8-Hydroxy-2-[2-(3-hydroxy-4-methoxyphenyl)ethenyl]-7-quinoline carboxylic acid and 8-hydroxy-2-[2-(3-methoxy-4-hydroxyphenyl)ethenyl]-7-quinoline carboxylic acid inhibit the processing and strand transfer reactions catalyzed by HIV-1 integrase with an IC50 of 2 microM. Some of their spectral properties are briefly reported. Their fluorescence is so weak that it is of no use in an experimental determination of the binding to the protein and we resorted to computer simulation. Both styrylquinoline derivatives, in their monoanionic form, have several dozens of tautomers and each of these forms has four planar rotamers. In this work computer simulations have been performed to determine which tautomer is the most abundant in aqueous solution and which binds to the Rous sarcoma virus (RSV) integrase catalytic core. As the substituents on the quinoline moiety are the same as on salicylic acid, the energies of hydroxy benzoic acid tautomers were also computed both in vacuo and embedded in a continuous medium which had the dielectric constant of bulk water, using the recent CPCM technique. The CPCM method was then applied to the two integrase inhibitors to estimate the tautomer population in water. The binding site of the compounds on the RSV integrase catalytic core was determined through a docking protocol, consisting of coupling a grid search method with full energy minimization. The designed method is a way leading to identification of potent integrase inhibitors using in silico experiments.

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Zdanovskaia, Maria A., Brian J. Esselman, Samuel M. Kougias, Brent K. Amberger, John F. Stanton, R. Claude Woods, and Robert J. McMahon. "Precise equilibrium structures of 1H- and 2H-1,2,3-triazoles (C₂H₃N₃) by millimeter-wave spectroscopy." Journal of Chemical Physics 157, no. 8 (August 28, 2022): 084305. http://dx.doi.org/10.1063/5.0097750.

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The 1 H- and 2 H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130–375 GHz frequency range, we measured the spectroscopic constants for thirteen 1 H-1,2,3-triazole and sixteen 2 H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium ( r eSE) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration–rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a “best theoretical estimate” (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1 H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2 H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and r eSE values is discussed.

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Buncel, Erwin. "1999 R.U. Lemieux Award Lecture Adventures with azo-, azoxy-, and hydrazoarenes: from the Wallach to the benzidine rearrangement. Molecular electronics." Canadian Journal of Chemistry 78, no. 10 (October 1, 2000): 1251–71. http://dx.doi.org/10.1139/v00-126.

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The author's studies with aromatic azo-, azoxy-, and hydrazo-dye molecules, comprising kinetic and equilibrium investigations, as well as synthesis of novel molecules having photogenic properties, are described under the following highlights: A. Wallach rearrangement and cognate studies with azoxyarenes (1) Elucidation of the mechanism of the Wallach rearrangement of azoxybenzene through the kinetic observation of a two-proton process which, together with a pKa study, was interpreted on the basis of formation of a deoxygenated, dicationic, symmetrical species as a key, short-lived reaction intermediate. (2) The proposal of a general acid-catalyzed pathway in concentrated sulfuric acid (catalysis by H2SO4 and H3SO+4. (3) Elucidation of the consecutive sulfonations of reaction products of azoxybenzene in the 100% H2SO4 region, and the diprotonation equilibria for p-hydroxyazobenzene, thus shedding light on past reaction pathway and product studies. (4) The observation of a novel reaction pathway for 2,4,6,2',4',6'-hexamethy lazoxybenzene. (5) The observation of a dichotomy of reaction pathways for α- and β-2-phenylazoxynaphthalenes: reaction via the dicationic intermediate and via quinoid intermediate species; comprising two isomeric compounds reacting by different pathways to give the same product. (6) Identification and structure proof of α- and β-isomers observed for the first time in the peracid oxidation of phenylazopyridine. (7) Observation of a rate constant ratio of 22 000 in the rearrangement of these α- and β-isomers, and the proposal of differential barriers for transition states leading to a tricationic intermediate. B. Benzidine rearrangement and cognate studies (8) Observation of the acid-catalyzed hydroxylation of phenylazopyridine to p-hydroxyphenylazopyridine and the proposal of an SNAr mechanism with formation of an intermediate hydrazo species in the reaction. (9) First study of benzidine type rearrangement-disproportionation of phenylhydrazopyridine in acid media. (10) Proposal of a A [Formula: see text] B [Formula: see text] C [Formula: see text] D type reaction profile for the consecutive hydroxylation[Formula: see text]disproportionation processes of phenylazopyridine in aq H2SO4. (11) Proposal of 10-π and 14π-electron electrocyclic processes in the benzidine type rearrangement-disproportionation of phenylhydrazopyridine. (12) Identification and structural elucidation of a dimer formed from phenylazopyridine as a minor product and proposal of a reaction mechanism. C. Facile acid-catalyzed demethylation via SNAr/A-SE2 mechanisms and studies of tautomerism (13) Observation of an abnormally facile acid-catalyzed cleavage (demethylation) of 4-methoxyphenylazopyridine via an SNAr mechanism. (14) Observation of two reaction pathways, SNAr and A-SE2, for the consecutive demethylations of 3,4-dimethoxyphenylazopyridine, with rate constant ratio of 7 000:1 favoring the SNAr process. (15) Quantitation of the tautomeric and protonation equilibria of 4-hydroxyphenylazopyridine, produced in (13). D. A new solvent polarity scale, molecular switches, and molecular electronics (16) Establishment of a π*azo solvent polarity scale based on solvatochromism of a series of azomerocyanine molecules ("Buncel's dye"). (17) Some glimpses are presented of current forays into molecular electronics, as emanating from the above studies: (a) spiropyran (SP) <> merocyanine (MC) thermo- and photochromic "molecular switch" systems; (b) synthesis and characterization of azo-functionalized star-burst dendrimers with photoswitchable properties and potential applications in optical data storage systems, holographic gratings, and drug delivery systems as host molecules.Key words: Wallach rearrangement, benzidine disproportionation, azoarenes, azoxyarenes, dendrimers, hydrazoarenes, dendrimers, solvatochromism, photochromism, thermochromism, spiropyran-merocyanine molecular switch.

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Nguyen, Nguyen Tran, Vo Viet Dai, Nguyen Ngoc Tri, Luc Van Meervelt, Nguyen Tien Trung, and Wim Dehaen. "Experimental and theoretical studies on the synthesis of 1,4,5-trisubstituted pyrrolidine-2,3-diones." Beilstein Journal of Organic Chemistry 18 (August 31, 2022): 1140–53. http://dx.doi.org/10.3762/bjoc.18.118.

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Substituted 4-acetyl-3-hydroxy-3-pyrroline-2-ones have been prepared via three-component reactions and the tautomerism of these 3-pyrroline-2-ones is due to the slight difference of energy, and the significantly large rate constant of transformation between two tautomers. 1,4,5-Trisubstituted pyrrolidine-2,3-dione derivatives were prepared from the above mentioned 2-pyrrolidinone derivatives and aliphatic amines, which exist in enamine form and are stabilized by an intramolecular hydrogen bond. A possible reaction mechanism between 3-pyrroline-2-one and aliphatic amine (CH3NH2) was proposed based on computational results and the main product is formed favorably following the PES via the lowest ΔG# pathway in both the gas-phase and an ethanol solvent model. DFT calculations showed that kinetic selectivity is more significant than thermodynamic selectivity for forming main products.

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Mari, Matteo, Debora Carrozza, Gianluca Malavasi, Ettore Venturi, Giulia Avino, Pier Cesare Capponi, Michele Iori, et al. "Curcumin-Based β-Diketo Ligands for Ga3+: Thermodynamic Investigation of Potential Metal-Based Drugs." Pharmaceuticals 15, no. 7 (July 12, 2022): 854. http://dx.doi.org/10.3390/ph15070854.

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Curcumin is known for its therapeutic properties; among these, antioxidant, anti-inflammatory and anti-cancer ones stand out. Besides, curcumin metal complexes have shown widespread application in medicine and can be exploited as lead structures for developing metal-based drugs. Unfortunately, curcumin is poorly bioavailable, mainly due to its instability in physiological conditions; this weakness is tightly connected to the presence of the β-diketo moiety undergoing tautomeric equilibrium. Stability and metal-chelating ability can be tuned by modulating the electronic effects and steric hindrance close to the β-diketo moiety; in addition, formation of a metal complex shifts the tautomeric equilibrium towards the β-keto–enol form and increases stability in biological media. Among the metals used in clinical therapy, gallium nitrate has shown to have significant antitumor activity against non-Hodgkin lymphoma and bladder cancer, thus indicating that gallium-based drugs have potential for further development as antineoplastic agents with improved therapeutic activity. Curcuminoids have demonstrated high affinity for gallium(III), allowing the formation of stable positively charged M:L 1:2 β-diketonate complexes that benefit from the therapeutic activity of both the metal and the ligand. Seven new curcumin derivatives were synthesized and completely characterized. The new derivatives retain the solvent-dependent keto–enol tautomerism, with the prevalence of the diketo form in aqueous solution. Enhanced stability in simulated physiological conditions was observed in comparison to the lead compound curcumin. The presence of Ga3+ anticipates the dissociation of the enolic proton, allowing chelate complex formation, and simultaneously it shifts the tautomeric equilibrium towards the keto–enol form. A complete 1H/13C NMR and UV–Vis study was performed to define the metal-to-ligand stoichiometry ratio and the overall stability constants. In addition, we demonstrated that some of the derivatives have increased antiproliferative activity on colon cancer cells compared to curcumin and antioxidant properties. On the whole, the synthesized curcumin-based molecules may act as new gallium(III) chelators with improved stability with respect to curcumin and could open interesting perspectives for the development of novel therapeutic agents for cancer.

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Herrenknecht, Christine, Elisabeth Guernet-Nivaud, Olivier Lafont, Michel Guernet, and Claire Gueutin. "Chemical electrochemical mechanism. Electrochemical reduction of some pyrazolidine-3,5-diones in DMSO. Relation between voltammetric behavior and 1H nuclear magnetic resonance spectra." Canadian Journal of Chemistry 66, no. 5 (May 1, 1988): 1199–202. http://dx.doi.org/10.1139/v88-196.

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In DMSO (TEAP), phenylbutazone, oxyphenbutazone, and kebuzone, pyrazolidine-3,5-dione species are irreversibly reduced in two steps at the mercury electrode. The total reduction process is monoelectronic with production of the enolate carbanion. A chemical–electrochemical mechanism is proposed, each step is attributed to one of the tautomeric forms, ketoenol (E) and diketone (D). The tautomeric equilibrium constants (K = [E]/[D]) are reached both by linear sweep voltammetry at 30 V s−1 and by FT 1H NMR and the global apparent acidity constants are determined by potentiometry.

35

Wierzchowski, Jacek, Agnieszka Bzowska, Katarzyna Stępniak, and David Shugar. "Interactions of Calf Spleen Purine Nucleoside Phosphorylase with 8-Azaguanine, and a Bisubstrate Analogue Inhibitor: Implications for the Reaction Mechanism." Zeitschrift für Naturforschung C 59, no. 9-10 (October 1, 2004): 713–25. http://dx.doi.org/10.1515/znc-2004-9-1017.

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Abstract Interactions of calf spleen purine nucleoside phosphorylase (PNP) with a non-typical substrate, 8-azaguanine (8-azaG), and a bisubstrate analogue inhibitor, 9-(2-phosphonylmethoxyethyl)- 8-azaguanine (PME-azaG), were investigated by means of steady-state fluorescence spectroscopy. Both 8-azaG and PME-azaG form fluorescent complexes with the enzyme, and dissociation constants are comparable to the appropriate parameters (Km or Ki) obtained from kinetic measurements. PME-azaG inhibits both the phosphorolytic and synthetic pathway of the reaction in a competitive mode. The complex of 8-azaG with PNP is much weaker than the previously reported Gua-PNP complex, and its dissociation constant increases at pH > 7, where 8-azaG exists predominantly as the monoanion (pKa ≈ 6.5). The fluorescence difference spectrum of the PNP/8-azaG complex points to participation of the N(7)H or/and N(8)H tautomers of the neutral substrate, and the 9-(2-phosphonylmethoxyethyl) derivative also exists as a neutral species in the complex with PNP. The latter conclusion is based on spectral characteristics of the PNP/PME-azaG complex, confirmed by fluorimetric determination of dissociation constants, which are virtually pH-independent in the range 6-7. These findings testify to involvement of the neutral purine molecule, and not its monoanion, as the substrate in the reverse, synthetic reaction. It is proposed that, in the reverse reaction pathway, the natural purine substrate is bound to the enzyme as the neutral N(7)H tautomer, which is responsible for the reported strong fluorescence of the guanine-PNP complex.

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Leškovskis, Kristaps, Anatoly Mishnev, Irina Novosjolova, and Māris Turks. "SNAr Reactions of 2,4-Diazidopyrido[3,2-d]pyrimidine and Azide-Tetrazole Equilibrium Studies of the Obtained 5-Substituted Tetrazolo[1,5-a]pyrido[2,3-e]pyrimidines." Molecules 27, no. 22 (November 8, 2022): 7675. http://dx.doi.org/10.3390/molecules27227675.

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A straightforward method for the synthesis of 5-substituted tetrazolo[1,5-a]pyrido[2,3-e]pyrimidines from 2,4-diazidopyrido[3,2-d]pyrimidine in SNAr reactions with N-, O-, and S- nucleophiles has been developed. The various N- and S-substituted products were obtained with yields from 47% to 98%, but the substitution with O-nucleophiles gave lower yields (20–32%). Furthermore, the fused tetrazolo[1,5-a]pyrimidine derivatives can be regarded as 2-azidopyrimidines and functionalized in copper(I)-catalyzed azide-alkyne dipolar cycloaddition (CuAAC) and Staudinger reactions due to the presence of a sufficient concentration of the reactive azide tautomer in solution. In total, seven products were fully characterized by their single crystal X-ray studies, while five of them were representatives of the tetrazolo[1,5-a]pyrido[2,3-e]pyrimidine heterocyclic system. Equilibrium constants and thermodynamic values were determined using variable temperature 1H NMR and are in agreement of favoring the tetrazole tautomeric form (ΔG298 = −3.33 to −7.52 (kJ/mol), ΔH = −19.92 to −48.02 (kJ/mol) and ΔS = −43.74 to −143.27 (J/mol·K)). The key starting material 2,4-diazidopyrido[3,2-d]pyrimidine presents a high degree of tautomerization in different solvents.

37

Bunting, John W., James P. Kanter, Raymond Nelander, and Zhennan Wu. "The acidity and tautomerism of β-diketones in aqueous solution." Canadian Journal of Chemistry 73, no. 8 (August 1, 1995): 1305–11. http://dx.doi.org/10.1139/v95-161.

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The acidity and keto–enol tautomerism of a series of symmetrical β-diketones (RCOCH2COR (1): R = methyl (a), phenyl (b), 3-pyridinyl (c), 4-pyridinyl (d), 3-(N-methyl)pyridinio (e), and 4-(N-methyl)pyridinio (f)) and two series of unsymmetrical β-diketones (RCOCH2COCH3 (7a–7f) and RCOCH2COC6H5 (8a–8f)) have been investigated in aqueous solution at 25 °C and ionic strength 0.1. Values of [Formula: see text] were measured spectrophotometrically, and the acidities of the enols [Formula: see text] were obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolateconjugate bases. These data in turn allowed the calculation of the acidities of the keto tautomers [Formula: see text] and the equilibrium constants for enolization (KE = [Enol]/[Keto]). In general, KE is greater for the symmetrical ketones(1) than for the corresponding R-substituted unsymmetrical ketones (7 and 8). KE is much more sensitive to the nature of the R substituent in these three series of β-diketones than in the corresponding series of β-keto esters and amides. Correlations between [Formula: see text] for 8 and 7 combined with the known acidities of the corresponding β-keto esters and amides provide the first accurate estimates of the acidities of dimethyl malonate (pKa = 13.0)and malonamide (pKa = 12.5) in aqueous solution. Keywords: acidity, tautomerism, β-diketones.

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Blanco, Carlos A., and Michael J. Hynes. "Catalysis of the deprotonation of β-diketones during formation of the 1:1 metal complexes." Canadian Journal of Chemistry 70, no. 8 (August 1, 1992): 2285–89. http://dx.doi.org/10.1139/v92-287.

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Catalytic effects on the deprotonation rate constants of β-diketones were observed during kinetic studies of the formation reactions of the 1:1 complexes of these ligands. When vanadium(IV), chromium(III), iron(III), cobalt(II), nickel(II), copper(II), and uranium(VI) react with pentane-2,4-dione, heptane-3,5-dione, 1,1,1-trifluoropentane-2,4-dione, 4,4,4-trifluoro-1-(2-thienyl)-butane-1,3-dione, 1-phenylbutane-1,3-dione, and 2-acetylcyclohexanone a significant catalytic effect on the rate of ionization of the keto tautomer of the ligand was observed. A considerable enhancement of the rate of ionization of the enol tautomers of these ligands was observed during the complex formation reactions with Cu2+. A mechanism is proposed that accounts for these catalytic effects. In the case of Cu2+ the increased rate of proton removal from the ligands may be rationalized in terms of the relatively large reduction potential of this metal ion.

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Sadutto, Daniele, Paolo Guglielmi, Simone Carradori, Daniela Secci, and Roberto Cirilli. "Kinetic Study on the Base-Catalyzed Imine-Enamine Tautomerism of a Chiral Biologically Active Isoxazoline Derivative by HPLC on Amylose Tris(3,5-dimethylphenylcarbamate) Chiral Stationary Phase." Molecules 28, no. 18 (September 8, 2023): 6518. http://dx.doi.org/10.3390/molecules28186518.

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Isoxazoline is a nitrogen- and oxygen-containing five-membered heterocyclic scaffold with diverse biological profiles such as antimicrobial, fungicidal, anticancer, antiviral, analgesic and anti-inflammatory activity. Accordingly, the use of this peculiar structural framework in drug discovery is a successful strategy for the development of new drug candidates. Here, a chiral saccharin/isoxazoline hybrid was considered to investigate the tendency of the imine moiety of the heterocyclic ring to tautomerize to the enamine form in the presence of a basic catalyst. The pseudo-first-order rate constants for the base-catalyzed tautomerization process were measured in different solvents and at different temperatures by off-column kinetic experiments based on the amylose (3,5-dimethylphenylcarbamate)-type chiral stationary phase. The kinetic results obtained in this study may be a useful aid in the perspective of designing experimental conditions to control the stereointegrity of these types of pharmacologically active compounds and drive their synthesis toward the preferred, imine or enamine, tautomer.

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Al-Omari, Hamzeh S. M. "Effect of Different Substituents on Uracil and its 2-Hydroxy-4-oxo Enol Tautomer – A Theoretical Study." Zeitschrift für Naturforschung A 63, no. 10-11 (November 1, 2008): 693–702. http://dx.doi.org/10.1515/zna-2008-10-1113.

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The uracil/2-hydroxy-4-oxo uracil tautomeric system was studied by employing the MNDO semiempirical method for the calculations. The uracil structure was found to be energetically favourable as indicated by the calculated heat of formation, the Gibbs free energy, the HOMO and LUMO energies, and charges. The substitution by F, OH, NH2, CH3, and BH2 at the carbon-6 position was found to affect the geometrical parameters of the substituted molecules. All of the substituents were found to shift the equilibrium in forward direction compared to the unsubstituted tautomeric pair as indicated by the calculated values for the equilibrium constants.

41

Rojas, Antonio, Alfonso Perez-Encabo, Ignacio Herraiz-Sierra, and Carlos A. Blanco. "Tautomerization tendencies of 2-acetylcycloalkanones, 2-acetyl-1,3-cycloalkanediones, and cyclic β-keto esters of five- and six-membered rings." Canadian Journal of Chemistry 79, no. 4 (April 1, 2001): 448–54. http://dx.doi.org/10.1139/v01-061.

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NMR and kinetics studies of the tautomeric equilibrium of 2-acetylcycloalkanones, 2-acetyl-1,3-cycloalkanediones, and methyl 2-oxocycloalkanecarboxylates of five- and six-membered rings have been carried out. By comparing 1H NMR and 13C NMR spectra recorded in D2ODOCD3 and CDCl3, with those results derived using a bromination kinetic procedure in dilute aqueous solution, information concerning tautomeric interconversion is provided. Some discussions considering ionisation equilibria and rate constants involved have also been considered. Solvent and concentration effects on keto-enol equilibria have also been discussed.Key words: 2-acetylcycloalkanones, 2-acetyl-1,3-cycloalkanediones, methyl 2-oxocycloalkanecarboxylates, equilibrium, tautomerization, ionization.

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Capponi, Marco, Ivo G. Gut, Bruno Hellrung, Gaby Persy, and Jakob Wirz. "Ketonization equilibria of phenol in aqueous solution." Canadian Journal of Chemistry 77, no. 5-6 (June 1, 1999): 605–13. http://dx.doi.org/10.1139/v99-048.

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The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.

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Saad, Gamal R., Magdi M. Naoum, and Hanna A. Rizk. "Dielectric polarization and molecular interaction. Part II. Acetylacetone, benzoylacetone, and dibenzoylmethane – triethyl amine – cyclohexane systems." Canadian Journal of Chemistry 67, no. 2 (February 1, 1989): 284–88. http://dx.doi.org/10.1139/v89-047.

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A modified form of Few and Smith equation applicable to dilute solutions of acetylacetone, benzoylacetone, and dibenzoylmethane in constant-ratio mixtures of triethylamine in cyclohexane is confirmed. The changes in dielectric polarization of diketones with amine concentration in the mixed solvents are used to deduce the equilibrium constants of the association process, and the dipole moments of the complexes formed. Further, an inference is made of the relative polarities of the keto and enol tautomers of these diketones, and of the ketoanilides previously investigated (1). Keywords: dielectric polarization, molecular interaction, β-diketones, triethylamine.

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Khandogin, Jana, and Charles L. Brooks. "Constant pH Molecular Dynamics with Proton Tautomerism." Biophysical Journal 89, no. 1 (July 2005): 141–57. http://dx.doi.org/10.1529/biophysj.105.061341.

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Rzepiela, Kacper, Aneta Buczek, Teobald Kupka, and Małgorzata A. Broda. "Factors Governing the Chemical Stability and NMR Parameters of Uracil Tautomers and Its 5-Halogen Derivatives." Molecules 25, no. 17 (August 28, 2020): 3931. http://dx.doi.org/10.3390/molecules25173931.

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We report on the density functional theory (DFT) modelling of structural, energetic and NMR parameters of uracil and its derivatives (5-halogenouracil (5XU), X = F, Cl, Br and I) in vacuum and in water using the polarizable continuum model (PCM) and the solvent model density (SMD) approach. On the basis of the obtained results, we conclude that the intramolecular electrostatic interactions are the main factors governing the stability of the six tautomeric forms of uracil and 5XU. Two indices of aromaticity, the harmonic oscillator model of aromaticity (HOMA), satisfying the geometric criterion, and the nuclear independent chemical shift (NICS), were applied to evaluate the aromaticity of uracil and its derivatives in the gas phase and water. The values of these parameters showed that the most stable tautomer is the least aromatic. A good performance of newly designed xOPBE density functional in combination with both large aug-cc-pVQZ and small STO(1M)−3G basis sets for predicting chemical shifts of uracil and 5-fluorouracil in vacuum and water was observed. As a practical alternative for calculating the chemical shifts of challenging heterocyclic compounds, we also propose B3LYP calculations with small STO(1M)−3G basis set. The indirect spin–spin coupling constants predicted by B3LYP/aug-cc-pVQZ(mixed) method reproduce the experimental data for uracil and 5-fluorouracil well.

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Baron, Marie-Hélène, Sabine Halut-Desportes, Hai-Fen Ye, Jack Huet, and Edith Favre. "Spectroscopic conformational study of PMS 952, a specific imidazoline ligand. Experimental results." Canadian Journal of Chemistry 77, no. 12 (December 5, 1999): 2015–24. http://dx.doi.org/10.1139/v99-191.

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X-ray analysis of PMS 952 displays two kinds of very strong hydrogen bonds, an intramolecular one stabilizing the conformer A and an intermolecular one that associates conformers A to form two infinite chains. The 1H NMR study of vicinal coupling constants based on dihedral angle values, evidences that PMS 952 takes up an eclipsed conformation, in CCl4 and CDCl3 solutions, closely related to the conformation in solid state. Deuterium exchange in methanol-d4 points out the presence of a tautomeric form. However, as there is spectral evidence neither from NMR nor from FTIR studies for such a tautomer, this specie cannot be taken into account to explain the complexity of the FTIR spectra. NMR data also reveal that the imidazoline ring freely rotates while FTIR analysis allows characterizing two conformers (A and B), with lifetime shorter than NMR time scale. These two conformers are stabilized by two different intramolecular H-bonds. Taking into account that conformer A could exist in two states (monomer A and (A)n-chains), an equilibrium between A, (A)n, and B entities allowed the assignment of all main bands of the IR spectra, assigned to vNH, vOH, vCH, and vCN modes. The strength of hydrogen bonds is solvent dependent. Polar solvents, either proton donor or acceptor stabilized the conformer A, while CCl4 favors the conformer B.Key words: hydrogen bonds, X-ray, NMR, FTIR, imidazoline derivative.

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Pachulia, Z., and J. Kereselidze. "Calculation of tautomeric equilibrium constants in complementary DNA pairs." Journal of Biological Physics and Chemistry 16, no. 4 (December 30, 2016): 177–80. http://dx.doi.org/10.4024/20pa16a.jbpc.16.04.

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Shawali, Ahmad S., Mosselhi A. N. Mosselhi, and Thoraya A. Farghaly. "Synthesis and Tautomeric Structure of 2-arylazo-4H-imidazo[2,1-b][1,3,4]thiadiazines." Journal of Chemical Research 2007, no. 8 (August 2007): 479–83. http://dx.doi.org/10.3184/030823407x237885.

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Two series of the title compounds were prepared via reaction of N-aryl 2-oxohydrazonoyl halides with 1-amino-4-phenylimidazoline-2-thione. Their tautomeric structure was elucidated by spectral analysis, and the correlation of their acid dissociation constants with the Hammett equation, to be as the hydrazone form.

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Costantino, Luca, Giulio Rastelli, Maria C. Rossi, and Albano Albasini. "Quantitative measurement of proton dissociation and tautomeric constants of apigeninidin." Journal of the Chemical Society, Perkin Transactions 2, no. 2 (1995): 227. http://dx.doi.org/10.1039/p29950000227.

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Lyčka, Antonín, Libuše Havlíčková, Alois Koloničný, and Josef Jirman. "15N NMR study of amino-imino tautomerism in derivatives of 1,4-bis(substituted amino)-9,10-anthraquinones and 1,4-bis(substituted amino)-2,3-dihydro-9,10-anthraquinones." Collection of Czechoslovak Chemical Communications 52, no. 3 (1987): 736–41. http://dx.doi.org/10.1135/cccc19870736.

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The 15N chemical shifts and 1J(15N, H) coupling constants of 1,4-bis(substituted amino)-9,10-anthraquinones and 1,4-bis(substituted amino)-2,3-dihydro-9,10-anthraquinones indicate that these derivatives exist as true aminoderivatives except for 1,4-bis(phenylamino)-2,3-dihydro-9,10-anthraquinone which forms a tautomeric mixture of the amino and imino forms in deuteriochloroform and hexadeuteriodimethyl sulphoxide.

Journal articles on the topic 'Tautomeric constant'

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