Relative Stabilities Of Tautomers Research Articles

External control over tautomeric distribution and inter-conversion: new insights into the realm of catalyzed tautomerization

Relative stabilities of tautomers and tautomeric barriers are extensively modulated by use of carboxylic acids and amine bases.

Structure and relative stability of neutral, protonated and deprotonated thiazolidine-2-one-4-thione tautomers: DFT calculations

Relative stabilities of tautomers and rotamers of neutral, protonated and deprotonated thiazolidine-2-one-4-thione (TZD) were studied theoretically using the B3LYP/6-311++G(2df,2p)//6-311+G(d,p) method of the Density Functional Theory (DFT) in the gas phase and solution. Gas phase calculated relative energies showed good correlation with those obtained in solution. Proton affinities ( PA ), molecular basicities ( GB ) and deprotonation enthalpies (D H dep) of the title compound were calculated at the same level of theory. Moreover, the isomerization processes leading from the most stable adduct to other stable conformers were investigated. The results reveal that the thione–oxo tautomer is the most stable tautomer in all cases and TZD behaves as sulfur base. Consideration of the solvent causes some reordering of the relative stability of the tautomers and a drastic change in the values of PA , GB and D H dep.

Таутомерія 4-гідразинохіназолінів, коливальні спектри та комп'ютерне моделювання

The tautomerism of 4-hydrazinoquinazoline and its derivatives was investigated. Geometry and thermodynamic parameters were computed theoretically using Gaussian 03 software. All calculations were performed at the MP2 level of theory using the standard 6-31G(d) basis. Energetics and relative stabilities of tautomers were compared and analyzed in a gas phase. The effect of solvents (1,4-dioxane, acetic acid, ethanol and water) on the tautomeric equlibria was evaluated using PCM. It was determined that solvents induced slight changes in the relative stability. In all cases 4-hydrazinoquinazoline exists predominantly as the amino form. The variation of dipole moments was studied. The anharmonic vibrational wavenumbers for unsubstituted 4-hydrazinoquinazoline were calculated at MP2/6-31G(d) level and compared with experimental data. The modes of IR spectra were assigned. The calculated herein wavenumbers and intensities of amino form are in good agreement with those observed experimentally.

DFT STUDY OF KINET IC AND THERMODYNAMIC PARAMETERS OF TAUTOMERISM IN 4?ACYL PYRAZOLONE

In the present work, DFT calculations are employed to obtain the optimized structures of 4- acyl pyrazolone tautomers (19 tautomers) using B3LYP/6-311++G** calculations. In addition, molecular parameters, IR frequencies and relative energies are extracted for all tautomers. The existence of aromatic ring, keto tautomer (versus enol tautomer), N-H bond (versus C-H bond) and C=N double bond (versus N=N double bond) are stabilizing factors in relative stabilities of tautomers. Calculation of vibrational frequencies showed that, in accordance with reported values, intramolecular hydrogen bond (existed in some tautomers) decreased the value of OH frequency. The solvent effects on relative stabilities of tautomers are calculated. The relative stabilities of all the tautomers in acetone, tetrahydrofurane and chloroform (in all solvents, except water) were relatively the same as those in the gas phase. In addition, a nearly good relationship is found between dipole moments of tautomers and their 7Gsolv in chloroform. This relation shows that by increasing the dipole moment, the absolute amount of 7Gsolv in chloroform increases.

Identification, classification and relative stability of tautomers in the cambridge structural database

Families of tautomers in the Cambridge Structural Database (CSD) have been identified, analysed and classified. We identified 108 molecules that crystallise in two different tautomeric forms. Most commonly, pairs of tautomers crystallise together in the same crystal structure. Tautomeric polymorphs—pairs of tautomers crystallising in different crystal structures with no other or identical components—are very rare. The calculation of the relative stabilities of the different pairs of tautomers mirrors the relative frequencies with which tautomeric forms are observed in the CSD. Our improved understanding of the factors influencing tautomeric preferences in crystal structures may allow the prediction and design of crystal structures containing tautomeric forms that are, as yet, unobserved in the solid state.

Kinetic and thermodynamic study of inter- and intramolecular proton transfer in N ′-acetyl formohydrazide tautomers

Nine tautomers and eleven possible tautomeric interconversions of N′-acetyl formohydrazide have been studied at B3LYP/6-311++G** level of theory. From these calculations, optimized geometries, molecular parameters, IR frequencies, NMR chemical shifts, and energetic results are obtained. In all tautomers except tautomers 4, E isomer is more stable than Z isomer. Energetic data were used to calculate the energy barriers of tautomeric interconversions and very high energy barriers were obtained for all tautomeric interconversions. Moreover, study of solvent effects on relative stabilities of tautomers and transition states showed that they are similar to those in the gas phase. In addition, intermolecular proton transfer with the assistance of one to three water molecules has been studied and the results showed that activation barriers in water-assisted tautomerism are in general lower than those in the gas phase. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Tautomerism of representative aromatic α-hydroxy carbaldehyde anils as studied by spectroscopic methods and AM1 calculations. Synthesis of 10-hydroxyphenanthrene-9-carbaldehyde

The synthesis of 10-hydroxyphenanthrene-9-carbaldehyde and its anil are described. The structure of the latter compound has been thoroughly studied by 1H and 13C NMR, UV-visible absorption, fluorescence and IR spectroscopies. All the experimental results support the existence of this anil mainly in the keto-enamine tautomeric form. A comparison is presented with previously studied anils derived from salicylaldehyde and 2-hydroxynaphthalene-1-carbaldehyde. Semiempirical calculations (AM1) concerning the relative stability of tautomers as well as the optimized molecular geometries are in good agreement with the experimental findings

13C NMR spectroscopic and AM1 study of the intramolecular proton transfer in anils of salicylaldehyde and 2-hydroxynaphthalene-1-carbaldehyde

Tautomeric equilibria in intramolecularly hydrogen bonded Schiff bases is studied on the basis of solution 13C NMR chemical shifts. Variable temperature NMR spectroscopic data and comparison with appropriate models show that while anils of salicaldehyde exist as phenolic tautomers, those of 2-hydroxynaphthalene-1-carbaldehyde exist as equilibrium mixtures containing appreciable amounts of both enol and keto tautomers, the latter being slightly predominant. AM1 results concerning the relative stability of tautomers are in good agreement with the NMR spectroscopic information.