Quantum-chemical investigation of some oligomeric heteroaromatic compounds

The characteristics of the electronic structures of a number of model aromatic oxazoles realized in aromatic polyoxazoles were obtained within the framework of the Pariser-Parr-Pople method. The UV spectra of most of them were interpreted. The stereoisomerism of dibenzoxazoles is examined.     

Quantum-chemical investigation of some oligomeric heteroaromatic compounds

Indigo and its dimer, trimer, dehydro, and leuco forms were investigated within the framework of the Pariser-Parr-Pople method. The electronic structures of the compounds in the ground and excited states and the effect of chain lengthening are discussed, and the UV absorption spectra are interpreted. Several characteristics of the chromaticity of the investigated compounds as representatives of vat dyes are examined. The estimated (from calculations) energetic favorability of the indigoid, dehydro, and leuco forms of indigo is used in discussing the observed and experimental ease of redox transformations of indigo.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 794–798, June, 1972.     

Quantum-chemical investigation of some oligomeric heteroaromatic compounds

Those structures of aromatic imidazoles that can be realized in polybenzimidazoles were investigated within the framework of the computational scheme of the Pariser-Parr-Pople method. The features of the electronic structures of the investigated molecules were exposed. A judgment is expressed relative to the strength characteristics of the bonds in the imidazole rings during thermal or thermal oxidative destruction of aromatic polybenzimidazoles. The stereoisomerism of dibenzimidazoles was examined. The UV absorption spectra of the investigated compounds were interpreted.See [1] for communication III.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 10, pp. 1396–1402, October, 1973.     

Quantum-chemical investigation of tautomerism of hydrophosphoryl compounds

The tautomerization energies for the reaction R₂P(O)H R₂P-OH, where R= OH, OCH₃, OC₂H₅, CH₃, and CF₃, have been calculated by the CNDO/2 method with optimization of the exponents of the Slater 3d AO's according to the criterion of a minimum total energy for the molecule. The results are in qualitative agreement with the experimental data. The MNDO and CNDO/2 calculations with the use of a standard sp basis predict greater stability for the structures with a three-coordinate phosphorus atom, in contradiction to experiment.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 4, pp. 486–490, July–August, 1989.In conclusion, we express our thanks to V. L. Foss for showing an interest in this work and for some valuable discussions.     

Quantum-chemical calculations of the spectroscopic parameters of heteroaromatic compounds containing sulfur and selenium atoms

Conclusions A new single set of spectroscopic parameters of the CNDO method for compounds containing sulfur and selenium atoms, which permits the simultaneous calculation of the ionization potentials and energies of the electronic transitions on a semiquantitative level, has been proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2251–2253, October, 1986.     

Electron-impact and thermal fragmentation patterns of some perfluoroalkylene-linked aromatic and heteroaromatic compounds

The pyrolysis of some perfluoroalkylene-linked aromatic and heteroaromatic polymer precursors and a poly(1,3-phenylenehexafluorotrimethylene) has been studied using a combined pyrolysis-gas chromatographic-mass spectrometric method. Whereas the electron-impact induced fragmentations of the polymer precursors are almost exclusively controlled by cleavage β to the aromatic or heteroaromatic ring, thermolysis of these compounds yields products whose formation is consistent with the occurrence of primary cleavage processes that are not confined β to the ring.     

Quantum-chemical investigation of some features of the interconversions of the gossypol molecule

The electron conformations and tautomeric transformations of the gossypol molecule have been investigated by the MO LCAO method in the AMI approximation. On the basis of an analysis of the electronic structures of 24 model molecules, each forming, as it were, a component part of gossypol, the role of dynamic intramolecular processes in its tautomeric interconversions has been elucidated. The exceptional role of intramolecular hydrogen bonds in the formation of the electron patterns in different structural forms has been shown. The correlation of the dependence of the chemical shifts of the¹³C nuclei on the electron density distribution in the structural forms of gossypol has been studied.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 517–529, July–August, 1996. Original article submitted June 23, 1993; revision submitted May 16, 1996.     

Quantum-chemical investigation of the silicon and carbon coordination bond in their isostructural compounds

Ab initio calculations within the minimal basis set of STO -4LGTO have been carried out on molecules of type H₃MX (M = Si, C; X = H, NH₂, OH, F). The influence of the MH₃-group inversion on the electronic structure of these compounds has been investigated and illustrated by MO electron density maps. The ability of the central atom to form an additional bond has been estimated with the help of calculations on the complexes of these molecules with the hydrogen negative ion. The complexes of type [H₄SiX]^? have been found to be more stable than their unbonded components. The [H₄CX]^? complex formation has not proved to be advantageous.     

Quantum-chemical investigation of monosubstituted cyclopropanes

The applicability of the semiempirical MINDO/3 and MNDO methods is demonstrated for the determination of the relative changes in the strain of the three-carbon ring and examination of the contributions from the effects of substituents X to the stabilization energy of monosubstituted cyclopropanes (substituents H, OCH₃, NH₂, F, Cl, CH₃, CF₃, COCH₃, CN, NO₂). The best results, comparable with the data from nonempirical calculations, were obtained by the MINDO/3 method. The method correctly confers the changes in the geometry of substituted cyclopropane rings but systematically underestimates the strain energy by 20–40 kJ/mole. To judge from the AE_strain values, the -accepting effect of the substituents NO₂, COCH₃, and CN plays a major role in determining the electronic structure, suppressing the destabilizing -acceptor effect. In the case of the substituents F, Cl, OCH₃, and NH₂ the stabilizing -donor effect is either completely suppressed or is partly balanced by the -acceptor effect.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 21, No. 4, pp. 399–407, July–August, 1985.     

Quantum-chemical calculations for some model compounds containing a transannular Si ← N bond

M. V. Lomonosov Institute for Fine Chemical Technology, Moscow. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 6, pp. 126–128, November–December, 1988.     

Electrophilic activation of acetyl-substituted heteroaromatic compounds

The chemistry of acetyl-substituted pyridines, thiazoles, quinoline, isoquinolines, and pyrazine (1-9 and 28) has been studied. These heteroarenes (1-8) condense with benzene in good yields (74-96%) in the Bronsted superacid, CF(3)SO(3)H (triflic acid). In these acid-catalyzed hydroxyalkylation reactions, compounds 1-8 are significantly more reactive than acetophenone. It is proposed that compounds 1-8 readily form dicationic electrophiles in triflic acid.     

Solvent induced quadrupole polarization of the molecules of diazo and diazocarbonyl compounds: Quantum-chemical investigation

Equilibrium structural parameters, atomic charges, and quadrupole moments of the molecules of 2,4,6-tris(diazo)cyclohexane-1,3,5-trione, 3,6-bis(diazo)cyclohexane-1,2,4,5-tetraone and its anion, isomeric 3,6-bis(diazonium)cyclohexanediondioles and 4,4′-bis(diazonium)-1,1′-biphenyl in a vacuum and in dichloromethane were calculated by the quantum chemical method PBE0/cc-pVTZ using the polarizable continuum model (PCM).The p-quinoid dication is more favorable than o-quinoid dication by 20 kcal mol-1 in a vacuum and by 13 kcal mol⁻¹ in dichloromethane.The norm of the quadrupole moment of the centrosymmetric molecules in a polarizable medium increases while that of the dications decreases. The quadrupole polarization of molecules and molecular ions by the solvent does not change their symmetry point group.