Electronic structures of monosubstituted benzenes and X-ray emission spectroscopy 5. Nitrobenzene

The electronic structure of nitrobenzene was studied by X-ray emission spectroscopy. The O-Kα, N-Kα, and C-Kα spectra of the title compound in the gas and solid phases were obtained. Based on the results of quantum chemical MNDO calculations, theoretical spectra were constructed. An interpretation of the experimental spectra is given. π-Interaction between phenyl fragment and nitro group in nitrobenzene is weak. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1297–1300, August, 2006.     

Relationship between the energy characteristics of formation of fluorozirconates

A correlation is established between the mean energy of fluorine addition in a particular coordination polyhedron and the fluorine content in fluorozirconates by quantum chemical model cluster calculations and analysis of structural data for crystalline fluorozirconates. A new energy approach to studies of complex fluorides is suggested. It is shown that the mean energy of fluorine addition varies within a narrow range for any structural unit met in real fluorozirconate crystals. Based on the energy approach to analysis of vibrational spectra it is found that the coordination number of zirconium in a series of barium fluorozirconate glasses is 7 or 8. Translated fromZhurnal Strukturmoi Khimii, Vol. 41, No. 2, pp. 255–262. March–April, 2000.     

X-ray spectra and electronic structure of solid ammonia

Intermolecular interactions in solid ammonia are investigated in a combined X-ray spectral and quantum chemical study. Theoretical NK_α spectra are constructed on the basis of MNDO calculations of the ammonia molecule and (NH₃)₇ and (NH₃)₁₃ clusters modeling solid ammonia; the spectra are in satisfactory agreement with the experimental X-ray spectra. Fragment analysis of the clusters with respect to the central ammonia molecule is carried out. It is shown that intermolecular electronic interactions in solid ammonia are most effective in MOs of e symmetry (σ-binding of nitrogen and hydrogen atoms). The fragment 2a₁ orbital contributes to the MO structure of the clusters to the least extent. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 721–726, July–August, 1996. Translated by L. Smolina     

X-ray spectral and quantum chemical analyses of the electronic structure of poly(monofluorocarbon)

The electronic structure of poly(monofluorocarbon) has been studied by X-ray spectral and quantum chemical methods. Calculations were performed in terms of the MNDO method, with the fluorographite layer modeled by clusters of different sizes. The high-resolution CK_a and FK_a spectra have been obtained; the calculated spectra are consistent with the experimental ones. It has been shown that carbon and fluorine are bonded mainly through the σ bonds. The p orbitals of fluorine atoms that are perpendicular to the C-F bond are not involved in the chemical bond, while the transitions from the molecular orbitals consisting of these p orbitals are responsible for the main maximum in the FK_a spectrum. Deceased. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 630–635, July–August, 1995. Translated by I. Izvekova     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of the phenol molecule in the gas phase was studied by X-ray emission spectroscopy (using the O-Kα and C-Kα spectra). MNDO calculations were performed, which made it possible to construct theoretical spectra and interpret experimental spectra. The structure of the molecular orbitals of phenol was compared with those of benzene and water. The π-interaction of the phenyl fragment with the oxygen-containing substituent was investigated. The contribution of the 2p atomic orbital of the oxygen atom to the π-HOMO of phenol is considerably less than that to lower-lying orbitals. For Part 3, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2187–2193, December, 1997.     

Novel methods for calculating the fine vibronic spectra of polyatomic molecules

Novel approximate methods for calculating the vibrational structure of the electronic spectra of polyatomic molecules—a method for the direct calculation of the overlap integrals of vibrational wave functions for the electronic states involved in a transition and a variational method for the solution of the vibrational problem for the excited states—are discussed. The methods are based on the consideration of the displacement and entanglement of normal coordinates, the quasiorthogonality of the Dushinsky transformation, and the classification of the states by total vibrational quantum numbers. Matrix perturbation theory is employed. It is shown that the accuracy of these methods compares well with the accuracy of the available “exact” techniques (the errors are ∼1 cm⁻¹ for frequency and 10% for relative intensity). At the same time, calculations by the new methods are performed more than two orders of magnitude faster than by the previously known methods. K. A. Timiryazev Agricultural Academy, Moscow. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 217–230, March–April, 1995. Translated by I. Izvekova     

NMR studies of the electronic structure of coumarins

¹H and¹³C NMR spectra of a series of coumarin dyes used as active laser media are studied. Electron densities and ring currents are calculated. Formulas for calculating¹³C NMR chemical shifts from quantum mechanical data for aminocoumarins are suggested. NMR data and electronic structure calculations are used to explain the generation properties of coumarins. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 3, pp. 495–502, May–June, 1997.     

Quantum chemical and x-ray spectral studies of the structures of superstoichiometric fluorocarbons

The possibility of hole formation in the structures of superstoichiometric fluorocarbons is studied. Different geometries are modeled by removing one, two, or six CF groups from the stoichiometric fluorocarbon lattice. The positions of fluorine atoms in the internal CF₂ groups are optimized using the semiempirical MNDO method. The quantum chemical calculations of fluorocarbon clusters containing holes of different geometries suggest the preferential formation of six-center hole structures in fluorocarbon lattices. The X-ray emission CK_α-spectra of the superstoichiometric CF_x (x=1.20 and 1.33) samples are obtained. Based on the cluster calculations, theoretical CK_α-spectra of CF_x are constructed. A comparison of the theoretical and experimental results shows that the spectra of the superstoichiometric fluorocarbons are characterized by a short-wave maximum, whose intensity increases with x. Deceased Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1072–1080, November–December, 1996. Translated by I. Izvekova     

Conformational structure of divinylacetylene

Ab initio quantum chemical calculations of the divinylacetylene molecule with different mutual orientations of vinyl groups are carried out using the 6–31G*/MP2 basis set. The torsional potential is approximated by a Fourier series. It is shown that the second term of the series dominates. The enthalpies of the cis- and trans-isomers are nearly equal; the maximum corresponds to the gosh-orientation (180.4 cm⁻¹ with respect to the cis-form). The vibrational spectra of the compounds with cis-, gosh-, and trans-orientations of vinyl groups are analyzed based on the ab initio calculations. It is concluded that the experimental data available in the literature agree with the hypothesis that divinylacetylene exists as cis- and trans-isomers. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 610–617, July–August, 1998.     

X-ray and quantum chemical study of the electronic structure of glycine and its metal complexes

OK_α spectra of glycine and some transition metal complexes with glycine ligands were obtained. The electronic structure of the glycine zwitterion is calculated by a quantum chemical method, and a theoretical X-ray spectrum of the glycine molecule is constructed. The nature of the metal-ligand bond in the compounds is discussed on the basis of experimental spectra and theoretical calculations. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Moscow State Academy of Light Industry, Novosibirsk Branch. Moscow State Academy of Light Industry. Novosibirsk State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 112–116, July–August, 1994. Translated by L. Smolina     

Quantum chemical calculations and35Cl NQR study of the molecular structure of trichloroacetyl halides and trichloroacetaldehyde

An optimal geometry of CCl₃C(O) X molecules (X=H, F, Cl, Br, and I) is determined by the MNDO quantum chemical method. According to the results of calculations, the molecules in the free state have an eclipsed conformation with a syn-peri-planar position of the oxygen atom and one of the chlorine atoms of the trichloromethyl group. However, as follows from the³⁵Cl NQR spectra at different temperatures, in the crystal state of CCl₃C(O)X this conformation is distorted under the influence of intermolecular interactions due to the torsional rotation of the CCl₃ group around the C-C bond. Indicatory potentialities of the NQR method with respect to the structural features that are due to crystallographic effects are discussed. Perm State University. Translated fromZhurnal Strukturmoi Khimii, Vol. 37, No. 3, pp. 494–500, May-June, 1996.     

Investigation of electronic interactions in solid hydrogen fluoride

Intermolecular interactions in solid hydrogen fluoride are studied by the combined quantum chemical and X- ray diffraction method. The structure of crystalline HF is modeled by (HF)_n chains (n =2, 3,...,20, by an (HF)₄₅ cluster consisting of five (HF)₉ chains, and by an (HF)₁₀₈ cluster consisting of twelve (HF)₉ chains with nearly zero dipole moment. The quantum chemical calculations of the clusters are performed by the semiempirical AM1 method, which is most suitable for electronic structure investigations of hydrogen fluoride, as shown by comparing the X- ray experimental and theoretical spectra. The theoretical X- ray spectra are also compared with the experimental FK_α spectra of gaseous and solid hydrogen fluoride. For more detailed studies of electronic interactions in crystalline HF, fragrnent analysis of MOs of the clusters with respect to the MOs of the central molecule is carried out. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 686–695, July–August, 1997.     

Calculation of the vibronic spectra of adenine

The vibrational structure of the absorption spectra of the first two π-π* singlet transitions of adenine is calculated in the Franck-Condon approximation including Herzberg-Teller interactions. The effect of excitation-induced changes in molecular angles on the intensities of the vibrational components is estimated. Structural models of the adenine molecule in the excited states are constructed. The theoretical and absorption spectra of the first π-π* transition are compared. The results of the electronic structure calculations of adenine by different CNDO/S methods are discussed. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 334–344, March–April, 1997.     

Vibrational spectra and stereochemistry of mono- and polysaccharides. III. β-methyl-d-glucoside, α-methyl-d-glucoside,and β-methyl-d-xyloside

The IR and Raman spectra of β- and α-methyl-D-glucosides and β-methyl-D-xyloside are compared. Experimental data are correlated with theoretical calculations of the frequencies of normal vibrations. Predominant contributions (20% and more) of particular CiOi and CiC(i+1) bonds to the potential energy distribution of normal vibrations (i.e., localization of vibrations on these bonds, which form skeletons of monosaccharide molecules) are estimated. This approach is used to interpret the main distinctions between the IR and Raman spectra of the test compounds. The use of vibrational spectra in selective analysis of certain functional groups is discussed. It is shown that replacing the hydroxyl group at C1 has a specific effect on the predominant localization of vibrational modes in particular skeletal bonds (mainly in C1O1) of the monosaccharide molecules. B. I. Stepanov Institute of Physics, Belarus Academy of Sciences. V. Tshebyatovski Institute of Low Temperatures and Structural Studies, Polish Academy of Sciences. Translated fromZhurmal Strukturnoi Khimii, Vol. 36, No. 3, pp. 443–455. May–June, 1995. Translated by I. Izvekova.     

Parametric method in the theory of vibronic spectra of complex molecules. Excited state structure and fluorescence spectrum of stilbene

The parameters of the adiabatic model of the stilbene molecule in the excited state and the vibrational structure of the fluorescence spectrum are calculated by the parametric method of the theory of vibronic spectra of complex molecules. The first and second orders of theory are used. The molecular models are constructed by the fragment method. Satisfactory agreement with experiment is obtained. The parameters of molecular fragments are shown to be highly transferable. The model and the spectrum max be refined in the second order of theory using only one additional parameter for CCC angles (optimal values were obtained). The parametric method under discussion makes it possible to predict variations of structural parameters due to excitation and to calculate the vibrational structure of the electronic spectra of complex molecules, including characteristic variations of the spectra in series of related molecules (stilbene-di-phenylpolyenes). This method surpasses all previous procedures (in particular, the correlation method) in completeness and accuracy of results. Translated fromZhurnal Strukturnoi Khinii, Vol. 41, No. 2, pp. 369–378, March–April, 2000.     

X-ray spectral study of the nature of electronic interactions in transition metal dithiocarbamates

SK_β spectra are studied for a series of nickel(II), zinc(II), and cadmium(II) dithiocarbamate complexes. Model quantum chemical calculations of the electronic structure of planar chelate rings are reported. It is shown that the metal-ligand interaction forming a coordination bond between the dithiocarbamate ligand and the metal mainly involves the nonbonding n-electrons and the weakly bonding π-electrons localized on the sulfur atoms. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1121–1126, November–December, 1998.     

Parametric method in the theory of the vibronic spectra of complex molecules. Absorption spectrum of decapentaene

The absorption spectra of decapentaene are analyzed and calculated using the available efficient methods of calculating the matric elements of the vibronic problem, a new parametric method for determining the potential surfaces of excited molecules, and a fragmentation approach to the design of molecular models. Good agreement for the fine vibrational structure of the calculated and experimental spectra (differences between the main band frequencies and intensities less than 20 cm⁻¹ and 10%, respectively) is obtained without correcting the parameters of the method. The mechanisms of excitation-induced structural changes in linear polyenes and the effect of bond angle changes on the vibrational structure of the spectrum are revealed. A new interpretation of the absorption spectrum of decapentaene in an argon matrix is proposed [J. Mol. Spectrosc.,114, 54–59 (1985)]. It is shown that the parametric method allows quantitative prediction of fine structure for the vibronic spectra of complex molecules. K. A. Timiryazev Moscow Agricultural Academy. Translated fromZhurnal Strukturmoi Khimii, Vol. 37, No. 6, pp. 1031–1039, November–December, 1996. Translated by I. Izvekova     

Parametric method for computing the structure of the excited states and the vibronic spectra of complex molecules. Absorption and fluorescence spectra of perylene

The structures of the perylene molecule in the first excited 1¹ 0B_2u state and the band shape (vibrational structure) of its fluorescence and absorption spectra are computed by the parametric method. A fragmentary approach and the molecular fragments H/1C= with the parameters obtained for acenes and polyenes are used to form molecular models in the excited state. It is shown that a model that corresponds to the choice of fragments with the parameters of acenes is the most optimal. The theoretical spectra satisfactorily reproduce both qualitatively and quantitatively, the basic specific features of the vibrational structure of the experimental spectra. Calculation results show high degree of transfer of the parameters of the method in a series of related molecules not only for acenes with “linear” arrangement of the rings (benzene, naphthalene, anthracene, etc.) but also for more complex structures (perylene). It is shown that the parametric method developed is efficient for predicting the vibronic spectra and the structure of the excited states of complex molecules. Translated fromZhurnal Struktumoi Khimii, Vol.40, No. 2, pp. 242–250, March–April, 1999.     

Computational modeling of amorphous silica. 1. Modeling the starting structures. A general conception

This paper presents concepts underlying computational modeling of dispersive silicas. An object for modeling is polyatomic nanometer-sized clusters. A chemical software package adapted for the PC AT 386(486)/387 is utilized. A structure-sensitive method of vibration spectroscopy is suggested to verify the results. Ultradispersive materials are considered to be an adequate object to establish feedback between computational and real experiments. The program is realized on a set of dispersive silicas; an algorithm for construction of large clusters is proposed, which is based on the concept of radical difference of dispersive silicas. From analysis of vibration spectra we suggest that aerosils, silica gels, and aerogels should be regarded as amorphous tecto-, cyclo-, and polysilicas, respectively, in line with Liebau's classification of crystal silicates. The category of silica is determined by its production procedure. Institute of Surface Chemistry, Ukrainian Academy of Sciences. Russian University of Peoples' Friendship. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 74–84, March–April, 1994. Translated by O. Kharlamova     

Computational structural determination and energy landscape analysis of the hepatic carcinogen 2-(acetylamino)fluorene

 2-(Acetylamino)fluorene (AAF), a potent mutagen and a prototypical example of the mutagenic aromatic amines, forms covalent adducts to DNA after metabolic activation in the liver. A benchmark study of AAF is presented using a number of the most widely used molecular mechanics and semiempirical computational methods and models. The results are compared to higher-level quantum calculations and to experimentally obtained crystal structures. Hydrogen bonding between AAF molecules in the crystal phase complicates the direct comparison of gas-phase theoretical calculations with experiment, so Hartree–Fock (HF) and Becke–Perdew (BP) density functional theory (DFT) calculations are used as benchmarks for the semiempirical and molecular mechanics results. Systematic conformer searches and dihedral energy landscapes were carried out for AAF using the SYBYL and MMFF94 molecular mechanics force fields; the AM1, PM3 and MNDO semiempirical quantum mechanics methods; HF using the 3-21G*and 6-31G* basis sets; and DFT using the nonlocal BP functional and double numerical polarization basis sets. MMFF94, AM1, HF and DFT calculations all predict the same planar structures, whereas SYBYL, MNDO and PM3 all predict various nonplanar geometries. The AM1 energy landscape is in substantial agreement with HF and DFT predictions; MMFF94 is qualitatively similar to HF and DFT; and the MNDO, PM3 and SYBYL results are qualitatively different from the HF and DFT results and from each other. These results are attributed to deficiencies in MNDO, PM3 and SYBYL. The MNDO, PM3 and SYBYL models may be unreliable for compounds in which an amide group is immediately adjacent to an aromatic ring. Received: 26 May 2002 / Accepted: 12 December 2002 / Published online: 14 February 2003