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.     

Basicity of azoles

According to the data of photoelectron spectroscopy, a linear dependence exists between the energies of nonbonding electrons in azoles and azines, on the one hand, and their benzoannelated derivatives, on the other hand. Substituents do not disturb this relationship; those in position 2 of N-heterocycles exert effects of the same type on the ionization potentials of the n-level of 1,3-diazole and pyridine.For part 2, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1600–1603, September, 1994.     

Dipole moment and tautomeric form of 3(5)-nitropyrazole in dioxane solution

The dipole moments of 3(5)-nitropyrazole, its methyl-substituted derivatives, and H-complexes with dioxane were measured experimentally and estimated byab initio calculations (6-31G^* basis set). Comparison of the experimental and calculated dipole moments suggests a shift of the tautomeric equilibrium toward the 3-nitroisomer. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2202–2203, November, 1999.     

Molecular Conformation of 2,2'-Dinitrodiphenylamine According to Dielectric Studies and Ab Initio Quantum Chemical Calculations

According to the results of ab initio calculations employing the HF/6-31G* approach, the isolated 2,2'-dinitrodiphenylamine molecule exists as an sc, sc conformer stabilized by a symmetric intramolecular bifurcated (three-center) hydrogen bond. In protophilic solvents (1,4-dioxane), the conformational equilibrium is also shifted in the direction of this rotational isomer.     

Defect formation in tellurium in various gravity conditions

The effect of gravity (from micro-to a rather high gravity, 5g ₀) acting during tellurium crystallization on the concentration of neutral (N _D ) and electrically active (N _AD ) acceptor-type structural defects in samples grown both under complete remelting of the starting ingot and under directed seed recrystallization of an ingot was studied. The concentrations N _AD and N _D and their distribution over the sample length were derived from the electrical characteristics (conductivity and the Hall effect) measured along the ingots in the temperature range 1.6–300 K. The contributions of various mechanisms to hole scattering were found from an analysis of the temperature behavior of the mobility. The results obtained were compared with the characteristics of samples grown following a similar program under normal conditions. The presence of N _AD defects is characteristic of the initial crystallization stage of all samples. N _AD is substantially lower (N _AD ～ 10¹⁵ cm^?3) than N _D ～ 10¹⁸ cm^?3 and decreases exponentially in the course of sample crystallization. Complete remelting under microgravity revealed indications of strong supercooling and spontaneous crystallization, as well as spatial oscillations of the electrical resistivity over the sample length caused by N _D modulation. These observations are related to the specific features of the melting and crystallization in zero gravity, namely, the melt breaking away from the wall of the ampoule and the increasing role of Marangoni convection.     

Molecular structure of complexes with bifurcated hydrogen bond: IV. Solvate H-complexes of <Emphasis Type="Italic">N</Emphasis>-methyltrifluoromethanesulfonamide in aprotic protophilic media

Analysis of the IR spectra of N-methyltrifluoromethanesulfonamide, as well as quantum-chemical calculations of solvate complexes of its monomer and open-chain dimer have demonstrated inadequacy of the model assuming the presence in protophilic solvents of a 1:1 monomer H-complex only. A model is suggested, which is based on an equilibrium between the 1:1 monomer H-complexes and open-chain dimer complexes of variable composition. The proposed model is much better consistent with experimental data.     

Electronic absorption spectra of cation radicals as a source of information regarding their conformations

On the basis of differences in the energy gaps between the molecular orbitals of the type of biphenyl, 2,2-dithienyl, 2-phenyl(thienyl)pyrrole, 2-phenylselenophene (photoelectronic spectroscopy), and their cation radicals (nanosecond laser spectroscopy) it was shown that ionization causes transition to a more planar molecular conformation.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1213–1216, September, 1989.     

Solvatochromism of Heteroaromatic Compounds: XVIII. Reversible Effect of the Medium on the UV spectra of 2-(2-Benzoyl-1-phenylethenyl)-5-phenylpyrrole

Theoretical (B3LYP/6-31G^*) and experimental (NMR, UV, and IR spectroscopy) data on the electronic and steric structure of 2-(2-benzoyl-1-phenylethenyl)-5-phenylpyrrole have been analyzed in the context of the hypothesis of the zwitter ionic nature of its ground state. The molecule involves a strong NH···O bond, and its ground state can be described as a hybrid of the neutral and zwitter ionic canonical forms; this conclusion was confirmed by the observation of a reversible medium effect on the electronic absorption spectrum of the substrate.     

Solvatochromism of Heteroaromatic Compounds: XXV. Effect of Bifurcate Hydrogen Bond with a Weak Intramolecular Component on the IR,NMR, and UV Spectra of 2,6-Dibromo-4-nitrophenol

Dissolution of 2,6-dibromo-4-nitrophenol in aprotic protophilic solvents forms a three-center solvation H-complex with a sharply weakened intramolecular hydrogen bond. This weakening is associated with a deformation of the bond angle of the hydroxy group. It is only on interaction with weak π-bases that the intramolecular component of the bifurcate hydrogen bond appreciably affects the spectral position of the OH stretching vibration band; this results in a nonlinear dependence between OH and the Kamlet-Taft sol-vatochromic parameter.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 1, 2005, pp. 111–122.Original Russian Text Copyright © 2005 by Fedorov, Krivoruchka, Shulunova, Sherstyannikova, Trofimova, Vokin, Turchaninov.