Quantum-chemical investigation of bond orders in symmetric polymethine dyes

The behavior of the bond orders in symmetric model polymethine dyes has been investigated by the HMO, PPP, and CNDO/2 methods. It has been established that the greatest equalization of the bonds is achieved in the case of dyes with moderately basic terminal groups. In other cases, alternation of the bond orders, which diminishes with increasing length of the polymethine chain and attenuates with increasing distance from the end, has been discovered at the ends of the polymethine chain. The amplitude of the alternation of the bond orders in a polymethine chain is greater, the more the basicity of the terminal groups, as characterized by the electron-donor parameter _o (0 _o 90°), deviates in either direction from the mean (_o = 45°). It is considerably smaller in the first excited state than in the ground state. The -electronic approximation is a correct method for describing the electronic structure of a polymethine chain. The neglect of the electrons can distort the bond orders of the terminal groups.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 138–146, March–April, 1985.     

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 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 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

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

The results of calculations by the Pariser-Parr-Pople method and the expanded Hückel method of oligomeric compounds and fragments from which aromatic polyimide macromolecules are constructed are presented and discussed. In connection with the controversial character of the problem of the structures of the intermediates in the destruction of polyimides and the use, in this case, of the concept of imide-isoimide rearrangement, the relative advantageousness of the tautomeric forms of imides is evaluated, the participation of the imide ring in one or another form in conjugation is discussed, the spectral differences in the forms are established, etc. The strength characteristics of the bonds in polyimides were estimated in connection with the possibility of homolytic and hydrolytic cleavage of polyimides at these bonds.See [1] for communication II.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No.8, pp. 1048–1056, August, 1972.     

Quantum-chemical investigation of hydrogen bond in complexes of ice-forming reagents with water

The MNDO method has been used in a quantum-chemical calculation of the electronic structure of complexes IFR...H₂O and IFR...2H₂O (IFR is an ice-forming reagent — 1,3,5-trihydroxybenzene, 1,5-dihyrdoxynapthalene, or 2,6-dihydroxynaphthalene), with complete optimization of geometry. The calculated results are compared with data on the ice-forming activity and structural features of the crystals of these IFRs.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 41–47, January–February, 1992.     

Quantum-chemical study of compounds containing intramolecular O-H…O=C bond

New results of the study of stable intermediates containing an intramolecular hydrogen bond O-H…O=C in the gas phase and solvents, carbon tetrachloride and dioxane, were analyzed. The structural and energy characteristics of the stable conformers of these compounds were determined by a MR2/6-311++G(d, p) method. The most stable is the hydrogen-bonded conformer of 1-hydroxy-1-chloroethyl acetate molecule. The approaches to accounting for the effects of the O-H…O=C intramolecular hydrogen bonding on the molecules reactivity were discussed.     

Quantum-chemical investigation of the Si←N bond in adducts of silane derivatives with ammonta

M. V. Lomonosov Moscow Institute of Fine Chemical Technology. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 1, pp. 161–164, January–February, 1990.     

Theoretical investigation of the carbon nitrogen double bond

Ab initio Gaussian type calculations are reported for the ground and excited states of the hypothetical molecule formaldimine (H₂CNH). The C=N group is compared with the C=O group.

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Zusammenfassung Die Ergebnisse einer ab initio Rechnung mit Gau\-Orbitalen für das hypothetische Formaldimin-Molekül (H₂CNH) werden mitgeteilt. Es werden sowohl Grundzustand als auch einige angeregte ZustÄnde untersucht und die beiden Gruppen C=O und N=O verglichen.

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Résumé On présente les résultats de calculs ab initio utilisant une base Gaussienne pour la molécule hypothétique de formaldimine (H₂CNH). L'étude porte sur l'état fondamental et quelques états excités. Une comparaison est faite entre le groupement C=N et le groupement C=O.

     

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.     

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.     

Quantum-chemical modeling of metal coordination compounds with photoswitchable magnetic properties controlled by ligand rearrangements

The complexation of chromenes with transition metal ions was studied using the B3LYP*/6-311++G(d,p) method. We are the first to show that transformation of the spin state of a transition metal complex through an LD LISC mechanism can be achieved by means of configurational isomerization of the metal-chelate coordination site induced by ligand rearrangement.     

The coordination number of silicon in silicon-oxygen compounds: the special case of 6-fold coordination in thaumasite

In most silicon-oxygen compounds, the silicon atom is 4-fold coordinated. Exceptionally, silicon is found in 6-fold coordination, as in the mineral thaumasite, which also may be formed in the degradation, and sometimes subsequent weakening, of certain concretes. The connection between coordination number and chemical bonding in oxidic compounds generally is explored using the optical basicity concept to estimate the extent of negative charge provided by oxide(-II) donor atoms in the coordination sphere. A correlation is established between this charge and the heat of formation of silicate (per oxide(-II) atom), which indicates that reduction in charge decreases thermodynamic stability. Calculations for thaumasite show that 4-fold coordination would provide a very small charge, but that 6-fold provides a charge comparable with that for stable 4-fold coordinated silicates such as K2Si2O5.     

Quantum-chemical investigation of nature of neutral intermolecular hydrogen bond in complex H3PO...HF

Results are presented from modeling the complex H₃PO...HF by the ab initio Hartree-Fock-Roothaan method. A generalized method is proposed for the quantitative fragmentary analysis of molecular orbitals (MOs). It is shown that when the complex is formed, the chemical bond between the phosphine oxide and the HF molecule is formed by means of a shift of the electron pair of the -bond of the HF to a 2p orbital of the O atom and the formation of a bonding three-center MO localized on the O, H, and F atoms, and also through a shift of an electron pair from the O atom to a 2p orbital of the F atom and the formation of a nonbonding MO localized on the O and F atoms.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 38–41, January–February, 1992.