Tautomeric transformations of arylidenebarbiturates in solutions

IR and UV spectroscopy and quantum chemistry were employed to investigate the structure of arylidenebarbiturates able to undergo tautomeric transformations. The composition of tautomeric mixtures depends on the properties of the solvent and on the concentrations of the solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 769–775, April, 1997.     

Quantum-chemical study of the electronic structure and reactivity of 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid methyl ester

We have used the MNDO approximation to carry out a quantum-chemical study showing that the selectivity of acylation of 2-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid methyl ester under Friedel—Crafts reaction conditions in the presence of AlCl₃ depends more on the electron density distribution in the complexes than on the structure parameters. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 291–298, February, 2006.     

Tautomeric equilibria of 2‐ and 4‐thiouracil in gas phase and in solvent: A density functional study

The relative stabilities of the five favored tautomers of 2‐ and 4‐thiouracil in gas phase and in water solution were determined by density functional theory employing the Becke, Lee, Yang, and Parr (B3LYP) exchange–correlation potential and the three 6‐31G(d,p), 6‐311++G(d,p), and triple‐zeta valence (TZVP) basis sets. Zero‐point vibrational corrections were also computed. Bulk solvent effects were studied in the framework of the self‐consistent reaction field approach by the polarizable continuum model. All calculations indicate that the most stable tautomer for both species, in the gas phase as well as in solution, has the oxo‐thione form, in full agreement with the previous ab initio and experimental studies. The tautomeric stability orders obtained in the aqueous solution are sensibly different from that in the gas phase. At B3LYP/6‐311++G(d,p) level in the gas phase, the following orders of stability for 2‐ and 4‐thiouracil tautomers were observed, respectively: S2U1>S2U2>S2U4>S2U5>S2U3 and S4U1>S4U2>S4U3>S4U4>S4U5. The corresponding trends in the aqueous phase are S2U1>S2U3>S2U2>S2U5>S2U4 and S4U1>S4U2>S4U3>S4U5>S4U4. On the basis of the computed energy differences we can hypothesize that only the oxo‐thione forms of 2‐ and 4‐thiouracil should exist in the gas phase and in water solution. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 44–52, 2001     

Experimental and quantum-chemical studies of the 3-pyridine-7-hydroxy-2-iminocoumarin and 3-(2-methylthiazole)-7-hydroxy-2-iminocoumarin molecules

The luminescence spectral properties of 3-pyridyl-7-hydroxy-2-iminocoumarin and 3-(2-methylthiazole)-7-hydroxy-2-iminocoumarin were studied in ethanol solutions at different values of pH and also in other solvents differing in polarity. In their ethanol solutions, the compounds can exist as neutral molecules, anions, cations, and zwitterions. The spectral properties and geometry were studied for all possible forms of these compounds by the INDO/S quantum-chemical method.     

Dynamic Effects Dictate the Mechanism and Selectivity of Dehydration–Rearrangement Reactions of Protonated Alcohols [Me2(R)CCH(OH2)Me]+ (R=Me,Et, iPr) in the Gas Phase

The gas‐phase dehydration–rearrangement (DR) reactions of protonated alcohols [Me₂(R)CCH(OH₂)Me]⁺ [R=Me ( ME ), Et ( ET ), and iPr ( I‐PR )] were studied by using static approaches (intrinsic reaction coordinate (IRC), Rice–Ramsperger–Kassel–Marcus theory) and dynamics (quasiclassical trajectory) simulations at the B3LYP/6‐31G(d) level of theory. The concerted mechanism involves simultaneous water dissociation and alkyl migration, whereas in the stepwise reaction pathway the dehydration step leads to a secondary carbocation intermediate followed by alkyl migration. Internal rotation (IR) can change the relative position of the migrating alkyl group and the leaving group (water), so distinct products may be obtained: [Me(R)CCH(Me)Me ??? OH₂]⁺ and [Me(Me)CCH(R)Me ??? OH₂]⁺. The static approach predicts that these reactions are concerted, with the selectivity towards these different products determined by the proportion of the conformers of the initial protonated alcohols. These selectivities are explained by the DR processes being much faster than IR. These results are in direct contradiction with the dynamics simulations, which indicate a predominantly stepwise mechanism and selectivities that depend on the alkyl groups and dynamics effects. Indeed, despite the lifetimes of the secondary carbocations being short (<0.5 ps), IR can take place and thus provide a rich selectivity. These different selectivities, particularly for ET and I‐PR , are amenable to experimental observation and provide evidence for the minor role played by potential‐energy surface and the relevance of the dynamics effects (non‐IRC pathways, IR) in determining the reaction mechanisms and product distribution (selectivity).     

Nature of π-Electron Transitions in the Spectra of 2,3-Polymethylene-3,4-dihydro-4-quinazolones and Their Analogs

We have studied the nature of electronic transitions in the UV spectra of 2,3-polymethylene-3,4-dihydro-4-quinazolones and their analogs. The results of LCAO MO quantum chemical calculations in the ZINDO/S approximation, taking into account configuration interaction, correlate satisfactorily with the positions of the bands in the experimental spectra of the studied compounds.     

Quantum-chemical investigation of the mechanism of cyclocondensation of 4-hydroxy-4-methylpentan-2-one with cyanoacetamide using the AM1 method

The mechanism of formation of 4,6,6-trimethyl-2-oxo-1,2,5,6-tetrahydropyridine-3-carbonitrile from 4-hydroxy-4-methylpentan-2-one and cyanoacetamide in the presence of ammonium acetate has been studied by the AM1 method. It was found that, under the reaction conditions, the amide is readily converted to an iminol tautomeric form which takes part in subsequent reaction. It was shown that the reaction is a cascade process forming two intermediates. The final product 4,6,6-trimethyl-2-oxo-1,2,5,6-tetrahydropyridine-3-carbonitrile is formed from the product of a Knoevenagel condensation via an intramolecular nucleophilic substitution mechanism. On the basis of the activation energies obtained it can be deduced that the limiting stage is the deprotonation process of the cyanoacetiminol. Dedicated to Professor L. I. Belen’kii with gratitude and deep respect. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 209–216, February, 2006.     

Experimental and theoretical investigation of the electronic absorption spectra of 2-H(methyl, methylthio, phenyl)-3,4-dihydro-4-quinazolinylidenecyanoacetic ester

The effect of substituents (Me, SMe, Ph) in position 2 has been studied on the nature of the absorption bands of 2-H-3,4-dihydro-4-quinazolinylidenecyanoacetic esters and a very high sensitivity on the substituent has been detected for the short wave maxima. On the basis of data calculated by the ZINDO/S method the featureless long-wave absorption band was assigned to an electronic transition from HOMO to LUMO caused by charge transfer from fragments of the benzene ring and the N=C—N bond to the >C=C—C=O fragment. Dedicated to Professor L. I. Belen’kii on his 75th birthday. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 227–232, February, 2006.     

Stabilization of the Triplet Biradical Intermediate of 5-Methylcytosine Enhances Cyclobutane Pyrimidine Dimer (CPD) Formation in DNA

Cyclobutane pyrimidine dimer (CPD) is a photoproduct formed by two stacked pyrimidine bases through a cycloaddition reaction upon irradiation. Owing to its close association with skin cancer, the mechanism of CPD formation has been studied thoroughly. Among many aspects of CPD, its formation involving 5-methylcytosine (5mC) has been of special interest because the CPD yield is known to increase with C5-methylation of cytosine. In this work, high-level quantum mechanics/molecular mechanics (QM/MM) calculations are used to examine a previously experimentally detected pathway for CPD formation in hetero (thymine-cytosine and thymine-5mC) dipyrimidines, which is facilitated through intersystem crossing in thymine and formation of a triplet biradical intermediate. A DNA duplex model system containing a core sequence TmCG or TCG is used. The stabilization of a radical center in the biradical intermediate by the methyl group of 5mC can lead to increased CPD yield in TmCG compared with its non-methylated counterpart, TCG, thereby suggesting the existence of a new pathway of CPD formation enhanced by 5mC.     

Interpretation of photoelectron spectra in the AM1 semiempirical method. 3. 2-Substituted benzimidazoles

Analysis of photoelectron spectra of 2-substituted benzimidazoles showed that their orbital structure is determined by the form and energy position of the fragment orbital (-symmetry) of the substituent.For previous communication, see [1].Irkutsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk 664033. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 880–884, April, 1992.     

Calculations of thermodynamic stability of CpCoC<Subscript>2</Subscript>B<Subscript>9</Subscript>H<Subscript>11</Subscript> cobaltacarborane isomers

Quantum-chemical calculations of the geometry and energies of nine possible isomers of 12-vertex cobaltacarborane CpCoC₂B₉H₁₁ (1) were carried out by the DFT method (PBEPBE/DGDZVP/DGA1). Thermodynamic stability of the isomers increases with increasing distance between the carbon atoms in the cage and is virtually independent of the position of the CpCo vertex. The relative stabilities of the 1,2,3-(17.57 kcal mol⁻¹), 1,2,4-(3.72 kcal mol⁻¹), and 1,2,9-isomers of 1 (0 kcal mol⁻¹) are similar to the corresponding values for the ortho (17.61 kcal mol⁻¹), meta (3.21 kcal mol⁻¹), and para isomers (0 kcal mol⁻¹) of carborane C₂B₁₀H₁₂. The results of the present study confirm a close similarity of the CpCo and BH fragments in metallacarborane chemistry. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1557–1559, July, 2005.     

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 68–73, January, 2000.     

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 68–73, January, 2000.     

Theoretical studies of the structure of nitrimines. I. Structure of 2-nitroguanidine and its alkyl derivatives

The molecular structure of 2-nitroguanidine and its 1-methyl, 1-ethyl, and 1,1,3,3-tetramethyl derivatives was studied by quantum-chemical methods. The results were compared with X-ray data. Using a basis larger than 6-31G and polarization functions and including electron correlation in calculations did not improve agreement with experiment. The major reason for this is the considerable effect of the crystal field on the geometrical parameters of the molecules.     

Paths for the formation of dioxin-type xenobiotics: Quantum-chemical treatment 1. Formation of polychlorobiphenylenes from polychlorobenzenes

The heats of formation of polychloroarynes (PCA's) from polychlorobenzenes (PCB's) have been calculated by the AMI method as a function of the composition and structure of the PCB's according to a two-step scheme including deprotonation with the formation of carbanions (PCCA's, H₁) and dechlorination to PCA's (H₂). The influence of the solvent on H₁ and H₂ has been evaluated in the framework of the solvaton model. It has been shown that the value of H₁ decreases as the number of C1 atoms in the PCB is increased, while H₂ increases along the same series due to the increase in the relative stability of the PCCA as the number of C1 atoms is increased. Although the dimerization of PCA's with the formation of polychlorobiphenylenes (PCBP's) is forbidden by orbital symmetry rules, calculations of fragments of the potential-energy surface have shown that this reaction takes place with an energy barrier amounting to about 1 kcal/mole.A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 350–355, February, 1992.     

Double Bond Migration Mechanism in Allyl Systems Involving the Hydroxide Ion. 2. Polarizable Continuum Model (PCM)

Isomerization processes of a double bond site in propene and methylthiopropene molecules with the hydroxide ion were studied in the framework of the RHF/6-31+G*, MP2/6-31+G*, and B3LYP/6-31+G* (density functional) ab initio methods. The solvent effect was taken into account using PCM in its IEFPCM and SCIPCM versions. It is shown that to construct a reaction profile for propene rearrangement, it suffices to perform geometry optimization of stationary points within the Born–Onsager model with further refinement of the energy using IEFPCM. The reaction profiles obtained display that the multiple bond migration mechanism involving the hydroxide ion proton is energetically preferable to the two-stage mechanism forming a solvated carbanion for the propene molecule and for the methylthiopropene molecule that forms a much more stable carbanion.     

Quantum-chemical Studies of Pyrimidin-4-ones. 2. Stability in the Gas Phase of Tautomers of Pyrimidin-2,4-dione, 2-Thioxo-, 2-Selenoxo-, 2-Amino-, and 2-Acetylaminopyrimidin-4-ones, and Their 6-Methyl- and 6-Phenyl Derivatives

The relative stability in the gas phase and the most probable reaction centers for electrophilic attack for the tautomers of pyrimidin-2,4-dione, 2-thioxo-, 2-selenoxo-, 2-amino-, and 2-acetylaminopyrimidin-4-ones, and their 6-methyl- and 6-phenyl derivatives have been investigated on the basis of the results of quantum-chemical calculations, using the semi-empirical PM3 method, without taking solvent effects into account.     

Ab initio study on tautomerism of 2-thiouracil in the gas phase and in solution

Ab initio geometry optimizations were carried out at the HF/3-21G and HF/6-31+G^** levels for the six tautomeric forms of 2-thiouracil (2TU, 2TU1, 2TU2, 2TU3, 2TU4, 2TU5) in the gas phase and in solution. To obtain a more definitive estimate of the relative stabilities for 2-thiouracil tautomers in the gas phase, single-point MP2/6-31+G^** calculations were performed on the HF/6-31+G^** optimized geometries. The tautomeric equilibria in 1,4-dioxane (=2.21), acetonitrile (=38), and in water (=78.54) were studied using the self-consistent reaction field (SCRF) theory. The calculated relative free energies indicated that 2TU is the energetically preferred tautomer in the gas phase and in solution. The stability order of 2-thiouracil tautomers depends on the level of theory and the dielectric constant of the solvent. The obtained results are compared with the available experimental data.