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     

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     

New algorithm and software for analyzing the vibronic spectra of polyatomic molecules

A new algorithm for automatic assignment of the resolved vibrational structure of the electronic absorption spectra of diatomic and polyatomic molecules is suggested. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 2, pp. 363–368, March–April, 1997.     

Ab initio and infrared spectral study of 4′-substituted phenylthiolbenzoates

The geometry of the 4′-cyano-(4′-CNPTB) and the 4′-methoxy-(4′-MePTB) phenylthiolbenzoates were obtained by ab initio calculations employing 6–31G basis set at Hartee-Fock level of theory. The results predict an extended form of the molecules and torsional angle between the phenyl rings at 90.85(6)⁰ and 90.87(3)⁰, respectively. On the basis of vibrational analysis the frequency assignment was carried out. The calculated frequencies were compared with the experimental IR spectral data in carbon tetrachloride, carbon disulfide solutions and in solid state.     

AB Initio calculations and vibrational spectra of CH3SC≡CH and CH3SC≡CSCH3

Methylthio- (MTE) and bis-methylthioethyne (BMTE) molecules are calculated by the SCF MO method (geometry optimization, basis set 6–31G*/MP2). The calculated internal rotation barriers of methyl groups are 7.12 kJ/lmole for MTE and 12.86 kJ/mole for BMTE (both groups are simultaneously rotated). The s-gosh-orientation of the thiomethyl fragments corresponds to a stable conformation of BMTE. The estimated values of the s-cis- and s-trans-barriers of mutual rotation of SCH₃ groups about the axis of the C≡C bond are 13.61 and 12.54 kJ/mole, respectively. Conformationally sensitive MOs and vibration frequencies are established. An analysis of the experimental IR absorption and Raman spectra and the calculated vibrational spectrum makes it possible to conclude that in the liquid phase the BMTE molecules also have an s-gosh-conformation. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 602–609, July–August, 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     

Effect of scaling of a quantum mechanical force field on the frequencies and forms of molecular vibrations

The effect of scaling of an ab initio quantum mechanical force field on the frequencies and forms of normal vibrations are studied in terms of first- and second-order perturbation theory. Scaling the force constant matrix according to Pulay using certain assumptions in first-order perturbation theory is equivalent to scaling vibration frequencies and does not modify the form of vibrations. In this case, the second-order corrections to the frequencies and forms of vibrations become zero. The first-order perturbation theory formulas are used to verify the assumptions by calculating the frequencies and matrices of transition to perturbed forms of vibrations of ethane, propane, ethylene, cyclopropene, and isobutene molecules from quantum mechanical force fields found with the 6-31G basis set. It is shown that the vibration frequencies calculated by the formulas of first-order perturbation theory are in good agreement with exact values; the matrix of transition to perturbed eigenvectors is rarefied, with only ≈1% of its elements being markedly nonzero. Moscow State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 2, pp. 210–216, March–April, 1998. This work was supported by RFFR grant No. 96-03-34085.     

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.     

Electronic structure and vibration spectrum of protonated semicarbazide

Quantum chemical optimization of the geometrical parameters of the protonated form of semicarbazide [H₂NCONHNH₃]⁺ is performed in the MNDO/H approximation. Effective charges on atoms are calculated as well as bond orders, formation enthalpy, and the ionization potential. An inverse spectral problem is solved for the protonated form of semicarbazide and its deutero analog. Frequencies, potential energy distribution over vibrational degrees of freedom, and force constants are calculated. Assignments of frequencies to vibrations of atomic groups are given. It is shown that protonation of the semicarbazide molecule at the nitrogen atom of the hydrazine fragment leads to weakening of all bonds except for CO and CN bonds of the carbamide group. D. I. Mendeleev Russian Chemical Technological University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 46–52, July–August, 1994. Translated by L. Smolina     

Comparison of the results of AB initio analysis and inverse vibrational problem solution for glyoxal

The vibration frequencies and their assignments for glyoxal conformers obtained by solving the direct vibrational problem with a scaled ab initio force field in an MP2/6-31G*//MP2-6-3IG* approximation are compared with the results of inverse vibmtional problem solution using the so-called Badger model of force field as an initial approximation. The latter approach is inapplicable to polyatomic molecules. Translated from Zhumal Struktumoi Khimii, Vol. 41, No. 1, pp. 157-163, January–February, 2000.     

Ab initio study of molecular structure and internal rotation in methyldicyanophosphine and methyldiisocyanophosphine. The Raman spectrum of methyldicyanophosphine and its interpretation with the use of scaling ofab initio force fields

The equilibrium geometric parameters and structures of transition states of internal rotation for the molecules of methyldicyanophospine MeP(CN)₂ and its isocyano analog MeP(NC)₂ were calculated by the RHF and MP2 methods with the 6–31G^* and 6–31G^** basis sets. At the MP2 level, the total energy of cyanide is −35 kcal mol⁻¹ lower than that of isocyanide and the barriers to internal rotation of methyl group for MeP(CN)₂ and MeP(NC)₂ are 2.2 and 2.7 kcal mol⁻¹, respectively. For both molecules, the one-dimensionalab initio potential functions of internal rotation approximated by a truncated Fourier series were used to determine the frequencies of torsional transitions by solving direct vibrational problems for a non-rigid model. The Raman spectrum of crystalline MeP(CN)₂ was recorded in the range 3500–50 cm⁻¹. The vibrational spectra of this compound were interpreted by scalingab initio force fields calculated by the RHF and MP2 method. The vibrational spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1703–1714, September, 1998.     

Methodology of using expert systems for structure elucidation of organic molecules by their spectra

Major axioms underlying structure elucidation of molecules by their characteristic spectral data are considered. The amount of information obtained by solving the problem on structure elucidation of molecules serves as a basis to evaluate the quality of solution. Conditions for computer-aided solution of problems using expert systems (ES) are analyzed. Reasons for incorrect or zero solutions and for inconsistencies leading to negative results are examined. The role of additional information as a necessary condition for correct solution is discussed. On the basis of this analysis, a general strategy for structure elucidation of molecules by their spectra using ES is suggested. The principal idea is to use the iteration method, gradually increasing the rigidity of constraints. Strategies named ARCHEOLOGIST and SCULPTOR are discussed. Examples are given. Russian Scientific Research Institute of Organic Synthesis. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 548–558, May–June, 1995. Translated by L. Smolina     

Vibrational spectra and stereochemistry of mono- and polysaccharides

A comparative study of the IR and Raman spectra of D-glucose anomers is reported. The spectra were found to differ between the anomers, despite the absence of symmetry elements in the anomer molecules. The results of theoretical calculations are systematized; it was found that the skeletal CiOi or CiO(i+1) (i=1, 2, 3, 4, 5) bond is characterized by a specific set of frequencies of normal vibrations, which have predominant contributions from the vibrations of atomic groups involving these bonds to the potential energy distribution (PED). The frequencies of normal vibrations with the major contribution to PED from CO and CC bonds are well separated and differ between the D-glucose anomers. The C5C6 bond has the greatest number of normal vibrations with predominant contributions to PED. Model calculations of vibrational spectra are reported for D-glucose anomers with modified (β→α and α→β) conformations of the CH₂OH group. B. I. Stepanov Institute of Physics, Belarus Academy of Sciences. Institute of Low Temperatures and Structural Studies, Polish Academy of Sciences, Wroclaw. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 322–329, March–April, 1995. Translated by L. Smolina     

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.     

Four-membered ring cyclic ketene –O,O–, –O,S–, –O,N–, –S,S–, –S,N–, and –N,N–acetals and their corresponding cations: a computational study

A systematic computational study of four-membered cyclic ketene –O,O–, –O,S–, –O,N–, –S,N– and –N,N-acetals as well as their protonated analogs have been performed at the second order M?ller Plesset level with a polarized triple zeta basis set. The main purpose of this study was to make predictions about the nucleophilicity of these systems and the variations in nucleophilicity with the hetero atoms. Our calculations suggest that all six target molecules are good nucleophiles, and that the N,N analog is the strongest and the S,S analog the weakest nucleophile. Our results include molecular geometries, bond lengths, proton affinities, vibrational frequencies, and calculated charges.     

Computational chemistry of the silicon nitride surface. 1. Water,ammonia, and water-ammonia complex

This paper deals with computational modeling of structure and properties of the silicon nitride surface zone using combined computational and real experiments. The computational experiment implies quantum chemical calculations of structure and vibrational spectra of polyatomic clusters. The real experiment suggests measurement and analysis of vibrational spectra. For quantum chemical calculations, semiempirical methods (MNDO and AM1) were chosen. In most calculations, the MNDO/H method was preferred because of the presence of many H-bonds in the surface zone. For verification of calculations, we calculated the structures and vibrational spectra of water and ammonia molecules and the water-ammonia complex and compared the results with experimental and ab initio (extended basis) data; MNDO/H proved to be an optimal method giving reliable results. Russian Peoples' Friendship University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 1, pp. 58–69, January–February, 1995. Translated by L. Smolina     

Spatial localization and dynamics of water molecules with good tetrahedral surroundings

The local structure of the molecular- dynamic model of water (729 particles at 300 K) is analyzed by isolating molecules whose surroundings differ slightly in configuration from a regular tetrahedron. These molecules are not randomly distributed in space but form nanometer clusters having a fractal structure. In these clusters, molecules are less mobile than the model molecules in general;their self- correlation function of rate and the density of vibrational states also differ from the average characteristics of the system. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 713–722, July–August, 1997.     

Structure of molecular complexes in liquid CO2 solutions of water

The calculated anharmonic frequencies and intensities of simultaneous vibrational transitions in CO₂−H₂O and (CO₂)₂−H₂O hydrogen-bonded complexes are compared with known experimental data. Hydrogen-bonded complexes form in liquid CO₂ solutions of water under normal conditions, unlike the gas phase in which the CO₂ and H₂O molecules are coupled by van der Waals bonds. The majority of these complexes are 2∶1 complexes. N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 375–379, March–April, 1995. Translated from E. Taskaeva     

Vibrational spectra and rotational isomerism of bis(N-2-chloroethyl)nitramine

The IR and Raman spectra of bis(N-2-chloroethyl)nitramine (BCENA) in the liquid and crystalline states and in CCl₄ and CH₃CN solutions are studied. The spectra are compared, and it is concluded that BCENA exists as a mixture of conformers of different polarities in the liquid state and as one less polar conformer in the crystalline state. To determine the conformations corresponding to the total electron energy minima and interpret the vibrational spectrum of BCENA, we performed an ab initio quantum chemical calculation of the BCENA molecule in the Hartree-Fock approximation using the 3–21G* and 6–31G* basis sets. Out of twelve possible conformations five are stable; the most stable conformer is C₂(GG). The frequencies and forms of normal vibrations of stable conformers are calculated using scaled quantum chemical force fields. The calculated and experimental frequencies are compared, and the relations between the frequencies of skeletal stretching and bending vibrations are analyzed. It is concluded that the BCENA crystal is formed by the C₂ (GG) conformer. The vibrational spectrum is interpreted, and the frequencies are assigned to vibrations of conformers differing in form. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 303–317, March–April, 1997.     

Crystallographic analysis of atomic structures and the phenomenological mechanism of crystallization

A theoretical study of the structure and vibrational spectrum of methyl-β-D-glucopyranoside is performed with allowance for the hydrogen bond effect on them. At the density functional theory level with the use of the B3LYP functional in the 6–31G(d) basis set the structural dynamic models of a free molecule of methyl-β-D-glucopyranoside and its simplest complexes with hydrogen bonding in the form of dimers with different structures are constructed. Energies are minimized; structures, electro optical parameters, force constants, and normal vibrational frequencies in the harmonic approximation and their intensities in IR spectra are calculated; the hydrogen bond energy is estimated. Based on the calculation, the conclusions are drawn about the structure of the methyl-β-D-glucopyranoside sample, the formation and interpretation of its IR spectrum, and the possibilities of the used density functional theory method.