Based on the results of a critical analysis of theoretical data on the structure and vibration frequencies of monomer and dimer molecules of yttrium tribromide, the molecular constants are chosen and the thermodynamic functions are calculated for YBr3 and Y2Br6. The data in the literature on the ratio of the partial pressures of dimer and monomer molecules are used to determine (according to the third law of thermodynamics) the enthalpy of the YBr3(cr, l) + YBr3(g) = Y2Br6(g) reaction and to calculate the composition of yttrium bromide vapor in the temperature range of 1218–1492 K, for which the data on the total pressure are known. It is established that the Pd/Pm ratio increases from 0.06 to 0.15 in this temperature range. The values of the sublimation enthalpy of yttrium tribromide in the form of monomer and dimer molecules and the formation enthalpy of YBr3(g) and Y2Br6(g) are calculated based on the found partial pressures. The obtained values are added to the IVTANTERMO database.

Treatment of the literature data on the vapor pressure of yttrium trifluoride using the new thermodynamic functions of YF 3 molecules gave a refined value of the sublimation enthalpy of yttrium trifluoride in the form of a monomer and the enthalpy of formation of YF 3 (g). The quantum-chemical calculations of YF 3 and Y 2 F 6 molecules were performed, and the energy of dissociation of the dimer molecules into two monomer molecules was calculated. Using these data, the enthalpy of sublimation of yttrium trifluoride in the form of a dimer was found, and the formation enthalpy of Y 2 F 6 (g) was calculated. The composition of the yttrium trifluoride vapor was calculated: the pressure ratio of Y 2 F 6 and YF 3 , P d / P m , in the range 1400–3000 K increased from ≈2 × 10 –4 to ≈2 × 10 –2 . The obtained thermochemical values ​​were entered into the database of IVTANTERMO software.

New calculations of the functions for YF 3 and Y 2 F 6 in the gas phase using quantum-chemical calculations by MP2 and CCSD(T) methods are performed in connection with the ongoing work on obtaining reliable thermodynamic data of yttrium halides. The obtained values are entered in the database of the IVTANTERMO software complex. Equations approximating the temperature dependence of the reduced Gibbs energy in the Т = 298.15–6000 K range of temperatures are presented.

The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO2 group. The equilibrium eclipsed configuration of the C s symmetry molecule has the following experimental bond lengths and valence angles: r e(N=O) = 1.226//1.226(8) Å, r e(C–N)//r e(C–C) = 1.520//1.515/1,521(4) Å, ∠еC–C–N = = 109.1/106,1(8)°, ∠еO=N=O = 124.2(6)°, ∠eC–C–Havg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C–C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.

The equilibrium structure of the urotropine molecule is characterized by means of gas electron diffraction (GED) with the involvement of quantum chemistry and vibrational spectroscopy. A structural analysis of the GED data is performed based on the parameters of the intramolecular potential function using of the program complex SYMM/DISP/ELDIFF/LARGE. The quadratic and cubic force constants of the urotropine molecule were obtained earlier on the basis of calculations at the MP2(full)/cc-pVTZ level and assuming molecular symmetry T (d) . The values of the equilibrium geometric parameters r (e) of the urotropine molecule are found. The experimental structural parameters are in good agreement with those calculated at the MP2(full)/cc-pVTZ level.

The results of quantum-chemical calculations of the formation energy, equilibrium structure, and potential surface sections along the nonrigid degrees of freedom of the silver trifluoroacetate dimer are presented. Calculations were performed by the B3LYP method with the cc-pVTZ correlation-coherent basis for C, O, and F atoms using the basis and relativistic effective core potentials Stuttgart 1997 RSC for Ag atoms, and, for comparison, by the HF method in the 6-31G(d) basis and MP2 method in the 6-311G(df) basis for C, O, and F atoms using the basis and relativistic effective core potentials SBKJC for Ag atoms. The eight-membered ring is a rigid planar fragment with a bond order of 0.2 between the silver nuclei. The nearly free internal rotation of the CF3 group affects the geometrical parameters of the ring. It was substantiated that in electron diffraction experiments, the difficulties of interpretation could be explained not only by the presence of decomposition products in the sample, but also by possible oligomerization of silver trifluoroacetate.

In this paper, the equilibrium structural parameters of the 2-nitropropane molecule and the barrier of internal rotation of the nitrogroup are determined from the gas electron diffraction data, with the use of quantum chemistry calculations and experimental vibrational frequencies, in the framework of the large-amplitude motion model for internal rotation.Quantum chemistry calculations at the MP2 and B3LYP levels of theory with various Pople and Dunning basis Sets unambiguously predict the same minimum energy molecular conformation, with relatively close values of internal rotation barrier (375-525 cm(-1)).The results of present analysis show that the minimum of the potential function of the nitrogroup internal rotation is located in syn-H position when one of the oxygen atoms eclipses hydrogen atom that does not belong to any of CH(3) groups (dihedral angle H-C-N-O is zero). It has also been found that internal rotation is hindered, with the barrier height in the range of 220-560 cm(-1) (0.6-1.6 kcal/mol) with the most probable Value near 380 cm(-1) (1.1 kcal/mol).The main equilibrium Structure parameters in syn-H Configuration are as follows (values in parentheses correspond to 3 times standard deviations): r(e)(C-C) = 1.516(5) angstrom, r(e)(C-N) = 1.501(5) angstrom, r(e) (N = O) = 1.225(4) angstrom, angle C-C-N = 108.7( 1.0)degrees. angle O=N=O = 124,8(0.4)degrees. We also provide thermally averaged parameters for comparison with the results of previous studies. (C) 2009 Elsevier B.V. All rights reserved.

The results of B3LYP quantum-chemical calculations of the equilibrium structures of [(CX3COOCu)2]3, [(CX3COOCu)2]2, and (CX3COOCu)2 oligomers (X = H, F) using the cc-pVTZ correlation-consistent basis for C, O, and F atoms and the Stuttgart 1997 RSC basis and relativistic effective core potential for Cu(I) atoms are presented. The differences in the structures of the free dimer and dimer units in oligomers were studied. The hexamer structure was chosen as the model of a fragment of the crystalline phase. Good agreement was obtained between the experimental and calculated differences between the geometrical parameters of the structures in the “gas phase-crystal” and “acetate-trifluoroacetate” series. Based on the calculated data, the increase in the Cu(I)-Cu(I) bond length in the silver acetate crystal compared with the gas phase can be explained by the effect of the neighboring dimer units of the polymer ribbon, while the increase in the Cu(I)-Cu(I) bond length in gaseous trifluoroacetate compared with acetate, by the acceptor effect of fluorine atoms.

A novel software package for extracting the intensity distributions from radial symmetric diffractograms is described. The software provides advanced features for diffraction pattern processing and a kit of interactive user-friendly tools allowing visualization of each stage of processing. The tool supports the whole procedure of data extraction, starting with the digital image and creating a set of data ready to be used in structural analysis. The software has been tested during numerous molecular structure studies based on the gas electron diffraction technique and is used in experimental investigations at the Laboratory of Electron Diffraction of the Chemical Faculty of Moscow State University. This work was supported by the Russian Foundation for Basic Research (grant N 05-03-33034).

Studies of the molecular structure and spectra of the lower nitroalkanes are complicated due to hindered internal rotation of the nitro group around the CN bond. In the present work, the structure of nitroethane molecule is determined from electron diffraction (ED) data supplemented by results of quantum chemical calculations within the model accounting for the large-amplitude motions and anharmonicity of vibrations in the rigid fragments.

This work presents a way to select the optimal form of the sector lobe and practical algorithms to determine the sector function of the device in use. Knowledge of the sector function enhances the quality of drawing a background line and ensures the division of multiplicative and additive noise components, thus increasing the reliability of structural parameters determined by gas electron diffraction.