Improved method for calculating vibronic spectra of complex molecules

The previously suggested approximate method for calculating the overlap integrals of vibrational wave functions is considerably improved for the purpose of maximally accurate calculation of excitation-induced mixing of normal coordinates. A general formula is obtained for all types of overlap integrals as a finite power series of the potential surface shift parameters; the coefficients are derivatives of the corresponding generating Junctions represented as polynomials of the shift vector elements of the normal coordinates and the mixing matrix. The spectra of model molecules of decatetraene and tetra- and hexadecaheptaene were calculated using the expressions derived in this work and a semiempirical parametric method for determination of excitation-induced changes in the potential surface of molecules. The calculations confirmed the high efficiency of both the parametric method and the new technique. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 2, pp. 240–247, March–April, 1997.     

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.     

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.     

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 calculating the vibronic spectra of complex molecules. Diphenylpolyenes

The absorption and fluorescence spectra of diphenylpolyenes (diphenylbutadiene, diphenylhexatriene, diphenyloctatetraene, and 1,6-di(4′-methylphenyl)-1,3,5-hexatriene) are calculated by a recently proposed parametric method using the fragmentation approach for designing molecular models. The parameters of the^H>C=molecular fragment (derivatives of the Coulomb and resonance integrals with respect to internal coordinates in the HAO basis set) obtained in calculation for polyenes were transferred to the molecular models of diphenylpolyenes without changes (∂H_e/∂q⁽⁰⁾=0.055 and ∂²H_e/∂q _k ⁽⁰⁾ ∂q _l ⁽⁰⁾ =0.1 au). The theoretical spectra are sufficiently adequate to quantitatively and qualitatively reproduce the main features of the vibrational structure of the experimental absorption and fluorescence spectra, and the parameters of the models of the potential surfaces of the excited states of diphenylpolyenes are consistent with the previous estimations. It is shown that this method allows predictive calculations of the vibronic spectra of complex molecules and the developed parametrization posesses all needed properties: locality, transferability, invariance to minor changes in electron density, ranking according to magnitude, small number of parameters for molecular fragments, etc. K. A. Timiryazev Moscow Agricultural Academy. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 6, pp. 1040–1049, November–December, 1996. Translated by I. Izvekova     

Determination of the potential functions of molecular inversion from experimental data: Current state and problems

Various approximations of the method of determining two-well potential functions of molecular inversion from experimental data (geometrical parameters and inversion level energies) are considered. The potential of the method is illustrated by reference to carbonyl molecules in the lowest excited states. Some of the current problems are discussed. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 5, pp. 947–961, September–October, 1998     

Quantitative prediction of absorption maxima in the electronic spectra of unsaturated compounds: Extrapolation from fragments to molecules in quantum chemical calculations

The possibility of quantitative prediction of maxima in the electronic absorption spectra of unsaturated organic compounds is studied by the Pariser-Parr-Pople method with fitted parameters. The quantum chemical methods are classified according to the type of extrapolation: from elementary particles (physical constants) to molecules, from atoms to molecules, and from fragments to molecules. N. G. Chernyshevskii Saratov State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1016–1022, November–December, 1996. Translated by I. Izvekova     

Models of slow rotational mobility of paramagnetic probes in glassy media

Experimental ESR spectra of the 2,2,6,6-tetramethyl-4-oxopiperidinoxyl (TEMPON) radical probe in the glycerol, polystyrene, and polyvinylbutyral matrices measured in the temperature range 77–373 K were quantitatively compared with the ESR spectra calculated using the known theoretical models of rotational mobility. It was shown that simulation of ESR spectra by the nonlinear least-squares method is an efficient procedure for discriminating between theoretical models. The temperature ranges were determined in which it is possible to achieve quantitative agreement between experimental and theoretical spectra as well as the ranges in which theoretical models are insufficient to quantitatively describe the experimental results. It was established that the widely used model of Brownian diffusion in isotropic medium is inadequate to describe the ESR spectra in the case of slow motions of small probe molecules. It was found that specific interactions (formation of weak complexes) between the probe molecules and the molecules of the medium results in strongly anisotropic molecular rotational motions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1065–1073, June, 2000.     

Double exchange in polynuclear mixed-valence clusters

A method for calculating the energies of the tunnel states of mixed-valence, clusters is proposed. It is based on the secondary quantization technique and allows one to obtain analytical expressions for all matrix elements that appear in double exchange theory as well as to determine the microstructures of the parameters of this theory. The energy spectra of trimeric systems are calculated with allowance made for the three-center transfer integrals, and the magnetic properties of these systems are investigated. Moldova State University. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 4, pp. 644–653, July–August, 1995. Translated by I. Izvekova     

Pressure effect in the vibronic spectra of perylene inn-Octane

The effect of external pressure on the fluorescence spectra of perylene in n-octane at helium temperatures is investigated. It is shown that isobaric treatment of the sample considerably decreases the electronphonon interaction of the impurity molecules with the surroundings and gives rise to doublet structure of the spectra. The latter is explained in terms of the two-well adiabatic potential model Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 318–323, March–April, 1997.     

Model for the description of the dynamic and electrooptic properties of hydrogen bonds

A semiempirical one-parameter model is proposed to describe the dynamic and electrooptic properties of hydrogen bonds. The model allows one to determine all parameters that are necessary for calculating the vibrational spectra of complexes using the strength of intermolecular binding. The experimental IR spectra of some hydrogen-bonded systems are compared to the corresponding spectra calculated by this model. It is shown that the model is applicable to calculating the first-to-third order vibrations of any hydrogen-bonded complex. N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 255–268, March–April, 1995.     

Vibrational spectra, conformations, and force constants of bis(dimethylamino)chlorophosphine and 2-chloro-1,3-dimethyl-1,3,2-diazaphospholane

Conformational analysis of the molecules of bis(dimethylamino)chlorophosphine and 2-chloro-1,3-dimethyl-1,3,2-diazaphospholane was carried out by the molecular mechanics method. According to calculations, both compounds are conformationally homogeneous, which is in agreement with the data of vibrational spectroscopy. The structural parameters found were used to calculate the frequencies and modes of normal vibrations, to analyze the IR and Raman spectra, and to estimate force constants for both diaminophosphines. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 391–394, March, 1998.     

Semiempirical calculations of the electronic spectra of organometallic compounds: Estimation of solvent and counterion effects

The electronic spectra of the [Ru(NH₃)₅pz]²⁺, [Ru(NH₃)₅pz]³⁺, and [Ru(CN)₅pz]³⁻ complexes are calculated by the CI INDO method. The effect of solvation on the spectra is considered in a point charge approximation and in terms of the parametric model of a charged sphere enveloping the metal fragment. Interrelation of these approaches is discussed. Data on the molecular electrostatic potentials created by the complexes are presented to substantiate the models. St. Petersburg State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 603–618, July–August, 1996. Translated by I. Izvekova     

Calculation of intensity in the resonance Raman and two-photon absorption spectra of polyatomic molecules

A direct method for calculating the resonance Raman and two-photon absorption spectra of polyatomic molecules is described in detail The method is based on the adiabatic model and uses Herzberg-Teller’s approximation. Relations ruling out direct summation over vibrational quantum numbers of excited electronic states and representing the matrix elements of the Green function of a multidimensional oscillator as functions of vibration frequencies and Dushinsky transformation parameters are derived. The relations are convenient for constructing algorithms. Translated from Zhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 248–255, March–April, 1997.     

Phase and structure transformations in water microclusters

The behavior of 12- and 14-molecule water microclusters with ST2 potential in the range of 50–325 K is studied by the molecular dynamics method. Structure and phase transitions, including evaporation of molecules from the clusters and transition through the percolation threshold via hydrogen bonds, were found. Topological, temporal, and energy characteristics of the bond network are analyzed based on the dynamic criterion of hydrogen bonding. Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences. Translated fromZhurmal Strukturnoi Khimii, Vol. 35, No. 1, pp. 64–70, January–February, 1994. Translated by L. Chernomorskaya     

Optimization of molecular asymmetry estimation by the dissymmetry function method

The dissymmetry fonction (DF) method and related techniques are critically analyzed. Three modifications of the DF method are suggested: molecular asymmetry evaluation relative to the S_n (n > 6) and C_n axes; optimization of “quasisymmetry” element orientations; using the edge model of molecules. It is shown on a series of model structures that realization of all these improvements as a new technique-asymmetry function (AF) method-increases the potential of the DF method and permits higher accuracy of molecular asymmetry evaluation. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 4, pp. 795-804, July-August, 2000 This work was supported by INT AS grant INTAS-UA 97-17 30.     

Optimization of molecular asymmetry estimation by the dissymmetry function method

The dissymmetry fonction (DF) method and related techniques are critically analyzed. Three modifications of the DF method are suggested: molecular asymmetry evaluation relative to the S_n (n > 6) and C_n axes; optimization of “quasisymmetry” element orientations; using the edge model of molecules. It is shown on a series of model structures that realization of all these improvements as a new technique-asymmetry function (AF) method-increases the potential of the DF method and permits higher accuracy of molecular asymmetry evaluation. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 4, pp. 795-804, July-August, 2000 This work was supported by INT AS grant INTAS-UA 97-17 30.     

Effect of repulsive interaction on the structural and dynamic features of liquid water. The role of molecular polarizability

The effect of variation of the potential function on the structural and dynamic properties of model water when the surface of pair interactions is invariant is studied by the molecular dynamics method. Variations for the rigid and polarizable models are compared. Translated fromZhurnal Strukturnoi Khimii, Vol.40, No. 2, pp. 296–303, March–April, 1999.     

Vibrational spectra and structure of 4-cyano-4′-pentalkoxybiphenyl in different physical states

The polymorphism, conformation mobility and structure of 4-cyano-4′-pentalkoxybiphenyl (5OCB) in different physical states are studied by IR spectroscopy. The spectra were measured in the frequency range 400–4000 cm⁻¹ at temperatures from 300 to 350 K. The IR spectra of 5OCB are modeled using the concept of conformational mobility of these molecules. An analysis of the experimental and theoretical spectra reveals absorption bands whose spectroscopic parameters are sensitive to variations of temperature (experiment) and conformation (theory); a relationship between these changes is established. It is concluded that the polymorphism of 5OCB is of conformation type. In the solid crystalline state, 5OCB molecules have conformations with a planar biphenyl fragment; the angle of orientation of the plane of the carbon framework of the alkyl radical relative to the biphenyl fragment decreases as the temperature increases from 35° in the solid crystalline state to 10° in the liquid crystalline and isotropic liquid states. In both of these states the biphenyl fragment becomes nonplanar. The angle between the phenyl rings is up to 30°. Saratov State University. Institute of Physics, Ukrainian Academy of Sciences. Samarkand State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 1, pp. 55–60, January–February, 1998. This work was supported by RFFR grant No. 97-03-32175a.