Combined method for calculating raman band intensities with consideration for the dependence of the raman tensor on the frequency of exciting radiation

It is shown that at certain assumptions the tensor of Raman bands can be represented as a sum of the Placzek term and the fast convergent series that contains information on the sensitivity of the tensor elements to the frequency of exciting radiation. The convergences of the corresponding series and the frequency dependences of the Raman band intensities for particular molecules are studied by computer experiments. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Moscow State Civil-Engineering University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 241–249, March–April, 1996. Translated by I. Izvekova     

Effective account of 2p–2h excitations in molecular calculations by the equation-of-motion method

The excitation energies and oscillator strengths of transitions in the helium atom (as a test system) and carbon dioxide and ethylene molecules are calculated by a modified equation-of-motion method with effective account of the most important polarization effects, including the 2p−2h (two particles-two holes) interactions, in the quasiparticle approximation of the density functional. Two different types of the effective potential of the partile-hole polarization interaction are used. The accuracy of the calculations is sensitive to the type of the potential. It is shown that the account of the polarization corrections for the 2p−2h effects is of fundamental importance since it contributes up to 30% to the values of the excitation energies and oscillator strengths. Odessa Institute of Hydrometeorology. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 615–622, July–August, 1995. Translated by I. Izvekova     

Crystal-chemical mechanism in the fundamental absorption of MgO crystal

Correlations between structural, thermal, and optical properties of MgO are shown as linear dependences of the energy ε of the photons absorbed by the crystal on the enthalpy of sublimation ΔH of the crystal to molecules, atoms, ions, and electrons. This dependence holds true only if in different schemes of crystal sublimation the values of ΔH are calculated for the cell MgO rather than its formula unit. M. V. Lomonosov Moscow State University, Geological Faculty. IGEM, Moscow. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 283–288, March–April, 1996. Translated by I. Izvekova     

Variational method for calculating the anharmonic vibrations of polyatomic molecules using a combined morse-anharmonic basis taking into account the fragmentary structure of molecules

A method of variational solution of anharmonic vibration problems using a mixed Morse—anharmonic basis is proposed. The basis functions are the products of the Morse oscillator eigenfunctions for vibrations of peripheral bonds, the harmonic oscillator eigenfunctions for almost harmonic skeletal and deformation vibrations, and the anharmonic basis functions for essentially anharmonic skeletal and deformation vibrations. The anharmonic basis wave functions are taken as a linear combination of the Morse and harmonic oscillator eigenfunctions. The introduction of the combined Morse—anharmonic functions allows one to factorize the solution of a problem into a series of individual blocks according to the fragmentary structure of molecules. Volgograd Pedagogical University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 231–238, March–April, 1995. Translated by I. Izvekova     

Rank-1 approximation to the van der Waals interaction

We present and discuss a variational single-product approximation to the van der Waals dispersion interaction leading to a simple formula for C ₆ that seems capable to give more than 99% of the ‘exact’ value. The formula is derived from Hylleraas’ variational principle in the tensor product space of the interacting molecules and therefore enjoys bounding properties. The formula has been tested by computing the C ₆ dispersion constants of H–H, and, at Full CI level, of the following systems: He–He, He–Li, Li–Li, LiH–LiH, HF–HF. Connections with the London formula are discussed. Contribution to the Fernando Bernardi Memorial Issue.     

Vibrational structure of the electron transition to the second excited singlet state of pyridine N-oxide

The vibrational structure of the electron transition to the second singlet excited state of pyridine N-oxide has been studied. The frequency of the 0–0 transition is 34502 cm⁻¹. A computer-aided technique for the assignment of the frequencies of the normal vibrations of polyatomic molecules in the excited electronic states is proposed. The frequencies of the totally symmetric vibrations of pyridine N-oxide in the second singlet electronically excited state are assigned. N. G. Chernyshevskii Saratov State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 350–355, March–April, 1995. Translated by I. Izvekova     

Dependence of the Daman intensities of combinations and overtones on the frequency of incident light

is shown that the combined method for calculating the Raman tensor elements suggested earlier [1, 2] may be extended to calculations of the intensities of second-order Raman bands (overtones and combinations). The behavior of the intensities of the first- and second-order Raman bands is studied in a wide range of frequencies of incident light, including the resonance region. The resulting equations for the Raman scattering tensor elements are convenient from computational viewpoint; this is especially important for the intermediate frequencies, which are most difficult for calculations. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 3, pp. 465–469, May–June, 1997.     

Modified tight-binding equations for electron wave functions in crystals with point defects

A system of tight-binding equations for the electron wave function in infinite, semi-infinite, or interface crystals with point defects is transformed to a finite system for the coefficients of specially constructed converging and diverging waves. The solution of this finite system allows the construction of the electron wave function over all space. This makes it possible to calculate the electronic structure of a defect and its “images” in the various experimental methods used for studying defects in the bulk, on the surface, and at the interface of crystals, without conventional structural simplifications. The proposed method is applied to the calculation of point defects in a two-dimensional square lattice. G. K. Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 985–993, November–December, 1996. Translated by I. Izvekova     

Force fields of halide-substituted methane derivatives

The paper describes methods for determining the force fields of large sets of molecules which share the common property that in their constituent groups of molecules the equivalent force constants always change monotonically with the properties of substituent atoms belonging to the same subgroup of the periodic table. The force fields of the CH_4−nX_n (n=0–4, X=F, Cl, Br, I) molecules of the methane series are obtained. The applications of these force fields are discussed. In particular, they can serve as a basis for a data bank of the force fields of fragments suitable for constructing the force fields of more complex molecules. Russian Scientific Center “Applied Chemistry”, St. Petersburg. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 2, pp. 250–256, March–April, 1996. Translated by I. Izvekova     

Effect of structural characteristics of water on the infrared absorption and raman spectra

An analysis of band contour in the IR absorption (overtone region) and Raman spectra of water suggests that the high-frequency components of spectral bands correspond to a single continuous distribution in energy of H-bonds. A local increase in the probability density function at high frequencies, which leads to an increase in the population of states with weak hydrogen bonds, is caused by the specific behavior of the angular dependence of the energy of hydrogen bonds between water molecules. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 467–472, May–June, 1995. Translated by I. Izvekova     

Semiempirical parametric method in the theory of vibronic spectra of polyatomic molecules

A semiempirical parametric method for calculating the vibrational structure of the electronic spectra of polyatomic molecules is developed; the method is based on the adiabatic molecular model and uses a single parametric system for all excited states. Within the model approach, simplified analytical expressions for potential surface variation during molecular excitation are derived; the expressions include the principal terms according to the order of magnitude. The first and second derivatives of Coulomb and resonance one-electron integrals with respect to natural coordinates in a basis set of hybrid atomic orbitals are used as parameters. It is shown that the parameters possess distinct locality, are transferable in molecular series, and may be easily ranked according to absolute values; describing a molecular model requires few most significant parameters. Excitation-induced variations of potential surfaces and absorption spectra of some molecules (butadiene, hexatriene, octatetraene) are calculated using only two parameters, which are the same for all molecules. The results of calculations are in good agreement with the experimental data. Supported by RFFR grant No. 95-03-08808. V.I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhumal Struckturnoi Khimii, Vol. 37, No. 3, pp. 419–431, May–June, 1996.     

Fragment calculation of electronic structures of polyatomic molecules in the ground state. II. A method of delocalized states of fragments

A fragment method is proposed to calculate the electronic structures of polyatomic molecules in the ground state. Localization and delocalization of the electronic states of molecular fragments are calculated simultaneously. The compact formulation of the method allows algorithmically efficient calculations of the electronic structures of interacting molecular fragments as well as of the whole molecules. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 3, pp. 395–400, May–June, 1995. Translated by I. Izvekova     

To the theory of electron scattering by polyatomic molecules

Within the Born-Oppenheimer adiabatic perturbation theory, an equation was obtained for the intensity of fast electron scattering by polyatomic molecules. All of its parameters are explicitly defined by vibronic interaction in a molecule; due to this, quantum chemical calculations are fully applicable to electron diffraction studies of molecular geometries. Engels Anti-Aircraft Missile Higher Military School. Translated fromZhurmal Strukturnoi Khimii, Vol. 35, No. 2, pp. 40–45, March–April, 1994. Translated by L. Smolina     

Calculation of two-center two-electron integrals in the slaters,p,d-basis set for the Ono electron-electron potential

This paper deals with the problems of extending semiempirical MNDO methods to compounds with d-elements. The problem is solved by estimating two-center two-electron integrals (TTI) with the Ono potential modeling interaction between two electrons in a molecule. A scheme for calculating TTI is suggested which uses the expansion of the Ono potential in a series of pairwise products of spherical harmonics centered on two atoms in the molecule. The scheme is stable and efficient for calculations in arbitrary Slater basis sets (including the s,p,d-basis set) and seems to be useful for development of NDDO methods. Scientific Research Institute of Chemistry at N. I. Lobachevskii Nizhnii Novgorod State University. Translated fromZhurnul Strukturnoi Khimii Vol. 35, No. 4, pp. 24–27, July–August, 1994. Translated by L. Smolina     

Investigation of azomethine group vibrations in aromatic schiff bases and their N-oxides by vibrational spectroscopy methods

The effects of fluorine substitution in benzene rings and quaternization of the nitrogen atom in Schiff bases of the general formula R−CH=N−R′ on the vibration characteristics of the azomethine group are analyzed. Normal vibrations and intensities of IR bands are calculated. It is shown that the spectroscopic behavior of the C=N bond is almost independent of the electronic effects of substituents in the benzene rings of the molecules studied, and the changes in the intensities of ν_C=N bands are caused by interactions between vibrations. This points to the stability of this bond in the series of molecules under consideration. For diphenylnitrones, N→O vibrations are identified. L. M. Litvinenko Institute of Physical Organic Chemistry and Carbon Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 310–315, March–April, 1995. Translated by L. Smolina     

Structure reinterpretation and determination of the clathrate nature of the compounds of the general formula MX2·6Py

Unit cell parameters and space group (Ccca) were determined by X-ray diffraction analysis for 11 compounds of the general formula [MPy₄X₂]·2Py (M=Cd, Co, Cu, Ni, Zn; X=Br, I, NO₃, HCOO). It was found that these compounds, together with the five known compounds, form a large group of clathrates, in which [MPy₄X₂) (host) complex molecules are packed with pyridine (guest) molecules held in the voids of the crystal framework by van der Waals forces. The results of the study and literature data are analyzed, and it is concluded that there are at least several dozens more clathrates of the molecular formula MX₂·6Py that are known but not defined structurally. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Institute of Physical Chemistry, Polish Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 1070–1073, November–December 1995. Translated by L. Smolina     

Algorithm for determination of the local similarity of molecules

An algorithm for seeking common structural fragments in compounds of different classes is developed. The algorithm allows for the molecular geometry and atomic characteristics, It may be used for recognition of compounds with properties associated with the local similarity of molecules such as ligand complementarity to a receptor. An example of seeking common structural elements for nondrug analgesics (para-acetamidophenol, acetylsalicylic acid, and amidopyrine) is given. I. I. Polzunov Altai State Technical University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 1083–1087, November–December, 1995. Translated by L. Smolina     

Construction of basis sets of hybrid atomic orbitals for describing the nonequivalent bonds of an atom with four neighboring atoms (sp 3 hybridization)

Methods for constructing the basis sets of sp³ hybrid atomic orbitals for atoms surrounded with four neighboring atoms are discussed in detail. The results obtained are generalized as a theorem of sp³ hybridization. A general method is proposed for constructing basis sets of orthogonal sp³ hybrid atomic orbitals for atoms with distorted tetrahedral environments. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp.969–975, November–December, 1995. Translated by I. Izvekova     

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     

Synthesis and structure of the product of bis-condensation of 4-methyl-2,6-diformylphenol with dimethyl N′,N′-hydrazine diacetate

Bis-condensation of 4-methyl-2,6-diformylphenol with dimethyl N′,N′-hydrazine diacetate gave product I. The structure of I was determined by X-ray analysis. The conformations of the two side chains of dimethyl N′,N′-hydrazine diacetate are significantly different. The position of one chain is fixed by the intramolecular H-bond of O−H.…N type. Due to its conformation, compound I is partly ready for complex formation with d-metals. Institute of Applied Physics, Academy of Sciences, Moldova Republic. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Struktumoi Khimii, Vol. 35, No. 3, pp. 91–98, May–June, 1994. Translated by L. Smolina