Conformational analysis of 4-methyl-2-trimethylsiloxy-1,3,2-dioxaphosphinane

Conformational analysis of 4-methyl-2-trimethylsiloxy-1,3,2-dioxaphosphinane was performed by the dipole moment method and quantum-chemical calculations. The 1,3,2-dioxaphosphinane heteroring was found to adopt a chair conformation with equatorial orientation of the 4-methyl group and axial orientation of the irregular trimethylsiloxy substituent. The conformational equilibrium involves non-eclipsed gauche and trans conformers (the latter prevailing) interconvertible through rotation about the exocyclic P-O bond.     

Conformational analysis of 2-methyl-1,3,2-oxaza- and 2-methyl-1,3,2-oxathiaborinane associates with water molecule

The structure and conformational behavior of 1: 1 molecular assotsiates of 2-methyl-1,3,2-oxazaand 2-methyl-1,3,2-oxathiaborinanes with water were studied ab initio in terms of HF/6-31G(d) and PBE/3z quantum-chemical approximations. The most stable complexes are formed via hydrogen bonding with the heteroatoms (oxygen and nitrogen or sulfur). Their conformational behavior implies equilibrium between sofa and half-chair conformers, and intermolecular hydrogen bond is retained in the course of ring interconversion. No associate with a dative O→B bond is formed.     

Reactions of <Emphasis Type="Italic">N</Emphasis>-alkyl- and <Emphasis Type="Italic">N</Emphasis>-aryltrihalogenoacetamides with phosphorus pentachloride

In the study of reactions of N-alkyl- and N-aryltrihalogenoacetamides with PCl₅, it was demonstrated for the first time that 3-alkyl(aryl)-2,2,2,4 tetrachloro-4-trihalomethyl-1,3,2-λ⁵-oxazaphosphetanes are intermediates in the synthesis of trihaloacetimidoyl chlorides. According to quantum-chemical calculations, acyclic N-tetrachlorophosphoranes, which are the primary products in the reactions of trihaloacetamides with PCl⁵, undergo rapid cyclization into the corresponding phosphorates and subsequent 1,3-chlorotropic migration gives rise to oxazaphosphetanes with the five-coordinate P atom.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 739-744, March, 2005.     

Conformational analysis of 1,3,2-dioxathiane and its oxides

Computer simulation of the routes of conformational isomerization of 1,3,2-dioxathiane and its Soxides by nonempirical quantum-chemical method RHF/6-31G(d) revealed the main and local minima and transition states of this process. It was shown that the barrier of ring inversion is reduced in going from 1,3,2-dioxathiane to its oxides. The established ΔG ⁰ (300 K) value of S=O group in cyclic sulfite (−15.0 kJ mol⁻¹) is in good agreement with the experimental data.     

Evaluation of MNDO approximation in quantum-chemical calculations of anisotropic hyperfine coupling tensors in free radicals

The MNDO approximation was tested for applicability in quantum-chemical calculations of anisotropic hyperfine coupling tensors using the same set of 17 free radicals as that used previously in evaluating a new procedure for quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in the framework of the same approach. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2242–2247, December, 1999.     

Experimental and theoretical study of the pyrolysis of trichlorosilane

Pyrolysis of trichlorosilane (TCS) and copyrolysis of TCS with 1,3-butadiene were studied. The enthalpies and activation energies for the reactions of the products of TCS pyrolysis were found by quantum-chemical calculations. A direct study of the pyrolysis of TCS by mass spectrometry was carried out. Based on the thermochemical parameters found by quantum-chemical calculations and on the results of GLC and mass spectrometry concerning the composition of the pyrolysis products, it was concluded that the pyrolysis of TCS follows a scheme that includes formation of radicals and silylenes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1659–1662, September, 1997.     

Quantum-chemical investigation of conformational characteristics of the complex of 2-methyl-1,3,2-dioxaborinane with dimethyl ether

With the help of the quantum-chemical approximation MP2/6-31G(d)//HF/6-31G(d) conformational characteristics were investigated of donor-acceptor associate of 2-methyl-1,3,2-dioxaborinane with dimethyl ether, 1:1. The character of the conformational isomerization of the adduct differs from that established for the proper cyclic boric ester and involves an interconversion between two conformers nondegenerate by the energy. The stability of the associate under study is comparable with those of the molecular complexes of 2-methyl-1,3,2-dioxaborinane with water and methanol.     

Conformational analysis of 2,4-dialkyl-1,3,2-dioxaborinanes

Computer modeling using the quantum-chemical empirical MM+ method and nonempirical RHF//6-31G(d), MP2//6-31G(d), and, in individual cases, RHF//3-21G methods was employed to study the potential energy surface of 2,4-dialkyl-1,3,2-dioxaborinanes. The optimal geometry and ¹H NMR spectral data gave values for ∆G ⁰ of the methyl and hexyl substituents at C-4 of the heterocyclic ring equal to 0.6 and 1.6 kcal/mol, respectively.     

Structure of the Products of Stimulated Carbonization of Aromatic Hydrocarbons

This paper deals with X-ray photoelectron and electron microscopy studies of the structure of carbon–metal-containing micro- and nanotubulenes. Ab initio quantum-chemical calculations were performed to analyze the possibility and mechanism of tubulene formation.     

Electron Diffraction Study of the Molecular Structure of 6,6'-Bis(1,5-diazabicyclo[3.1.0]hexane)

Gas-phase electron diffraction using quantum-chemical calculations was employed to study the molecular structure for 6,6'-bis(1,5-diazabicyclo[3.1.0]hexane). The conformation of the bicyclic fragment is boat or anti relative to the exocyclic bond.     

Conformational analysis of 1,3,2-polymethyl-substituted dioxaborinanes

Study of conformational isomerization of 4,4,6-trimethyl- and 4,4,6,6-tetramethyl-1,3,2-dioxaborinanes containing hydrogen, as well as methyl and vinyl groups at the boron atom, by means of nonempirical quantum-chemical calculation in HF/6–31G (d) and PBE/3z approximations, as well as by analysis of ¹H NMR spectroscopic data was carried out. The molecules of the analogs with three methyl groups in the ring are in equilibrium between the half-chair and sofa conformers, the equilibrium is shifted almost completely toward the latter form. For the 4,4,6,6-tetramethyl-substituted cyclic boronic esters is characteristic the conformational equilibrium between energetically degenerated forms of either sofa, or half-chair.     

Modern methods for estimation of ionization constants of organic compounds in solution

The review considers main up-to-date approaches to the calculation of ionization constants of organic compounds in solutions. Methods based on quantum-chemical calculations, continuum solvent models, cluster models, and quantitative structure-property relationships are discussed. The results of calculations of pK _a values by different methods, obtained in the past decade, are compared.     

Investigation of the structure of and the properties of the potentially tautomeric 1,2,3,4-tetrahydro-5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazine-1,4-diones in the gaseous and aqueous phases using the AM1 semi-empirical method

Potentially tautomeric 1,2,3,4-tetrahydro-5,7-dimethyl-6H-pyrrolo[3,4-d]pyridazine-1,4-diones and their fixed tautomeric forms have been studied in order to predict their tautomeric equilibrium constants and pK_a values using semi-empirical AM1 quantum-chemical calculations at the SCF level in the gas phase and in aqueous solution. Hydroxy-oxo forms were found to be more stable than dioxo and dihydroxy forms. The results obtained from the tautomeric equilibria and basicity calculations are in good agreement with experimental data.     

Conformational composition of the 2,4,6-trimethyl-1,3,2-dioxaborinane stereoisomers

A study of conformational transformations of 2,4,6-trimethyl-1,3,2-dioxaborinane cis- and trans-isomers in non-empirical quantum-chemical HF/6-31G (d) and PBE/3z approximations showed that the transformation routes include the sofa conformers with different orientations of the substituents at the ring atoms C⁴ and C⁶ through the transition states corresponding to diequatorial, axial-equatorial, and dixial conformations of the 2,5-twist form. The calculation and ¹H NMR data show that the conformational equilibrium of cis-isomer is almost entirely shifted to diequatorial sofa, and the trans-form is characterized by the interconversion between the two axial-equatorial sofa conformers.     

A new scheme for the mechanism of Fischer indolization of arylhydrazones

A new scheme for the mechanism of Fischer indolization of arylhydrazones has been suggested based on quantum-chemical calculations by the MNDO AM1 method.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 961–962, May, 1994.     

Computer modelling of the conformational equilibrium of 2-and 2,5,5-substituted 1,3,2-dioxaborinanes

Empirical, semiempirical, and nonempirical quantum-chemical methods were used to study the conformational equilibrium of 2,5,5-substituted 1,3,2-dioxaborinanes. The sofa invertomers were found to correspond to the local and global minima on the potential energy surface. The position of the equilibrium between these forms is a function of the substituents at C₍₅₎ of the heterocycle. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1860–1865, December, 2007.     

Chemistry of Ruthenium Polypyridine Complexes: IV. Quantum-Chemical Simulation of the Effect of Solvation Shell on the Electronic Structure of Ru(II) Complexes with Nitrogen-containing Ligands

Ab initio quantum-chemical calculations of Ru(II) complexes have been fulfilled with consideration for solvation within the framework of the polarized continuum model. Energy levels of fragments of Ru(II) complexes with organic ligands are shifted relative to each other by electrostatic interactions with the solvation shells.     

Quantum-chemical calculations of the dissociation energy of the C-H bond in hydrocarbons,alcohols, and ethers

The dissociation energy of the C-H bonds in hydrocarbons, alcohols, and ethers were calculated by semiempirical MNDO, AM1, and PM3 methods. The average error of calculations of theD(C-H) values by using various quantum-chemical methods is 1.3 kcal mol^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2858–2861, December, 1996.     

Stereochemistry of the formation of 2-alkyl-4,5-dimethyl-1,3,2-dioxaborinanes

It is demonstrated with the help of¹H and¹³C NMR spectroscopy and GLC that the formation of the stereoisomers of 2-alkyl-4,5-dimethyl-1,3,2-dioxaborinanes from 2-methyl-1,3-butanediol and acyclic boric acid esters occurs stereospecifically. This conclusion was confirmed by calculations of the optimal geometries and relative energies of the molecules of thecis-andtrans-isomers of model 4,5-dimethyl-1,3,2-dioxaborinanes and the proposed intermediate with a tetravalent boron atom using the method of molecular mechanics.A. V. Bogatskii Physicochemical Institute, National Academy of Sciences of Ukraine, Odessa 270080, Ukraine. Institute of Organic Chemistry, Ufa Science Center, Russian Academy of Science, Ufa 450054, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 400–404, March, 1998.     

Electron Diffraction Determination of Equilibrium Structure for Rigid Molecules. Equilibrium Configuration of 1,2-Thiaarsol According to Gas-Phase Electron Diffraction and Quantum-Chemical Data

For rigid polyatomic molecules, a procedure is described for determining their equilibrium geometrical structures by processing gas-phase electron diffraction, spectroscopy, and quantum-chemical data. The efficiency of the procedure is demonstrated by reference to the 1,2-thiaarsol molecule. For this molecule, quantum-chemical calculations of different degrees of complexity have been carried out, and harmonic and anharmonic force fields have been constructed. The force constants were employed for determining the equilibrium geometry from experimental data. Analysis of the results of this study suggests that the calculated vibrational corrections to the internuclear distances are almost independent of the level of the quantum-chemical calculations.Original Russian Text Copyright © 2004 by Yu. I. Tarasov, I. V. Kochikov, D. M. Kovtun, N. Vogt, B. K. Novosadov, and A. S. Saakyan__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 822–829. September–October, 2004.