T. V. Timofeeva

Distortion of “tetrahedral” C3v coordination in the R3CO moiety due to the reduction of molecular symmetry: X-ray,. conformational, and quantum-chemical study

Abstract Distortions of the ideal configuration of the R3AX moiety in R3AXR′ type molecules are established by X-ray diffraction (R, R′ = organic substituent, A  element of maingroups IV or V of the periodic table, X = non-linearly coordinated O, S or N atom): the RAX bond angle (φ) in the transoid RAXR′ moiety is essentially smaller than the tetrahedral value of 109.5°. Decrease of the φ angle in Me3COME (I) and Me3COOCMe3 (II) molecules was obtained in molecular mechanic (φI = 103.9°, φΠ = 106.5°) and CNDO/2 (φI = 105.2°, φΠ = 104.3°) calculations and substantiated by the low-temperature X-ray study. The effect of symmetric coordination distortion has evidently, an electronic nature and is also observed in planar trigonal, trigonal-bipyramidal and octahedral environments.

Hydrogen bonds and conformations of permethylated α-metallocenylcarbinols (M = Os, Ru, Fe)

Abstract IR spectral data show that, in contrast to α-ferrocenylcarbinols which form OH⋯π and OH⋯M intramolecular hydrogen bonds, α-ruthenocenyl- and α-osmocenylcarbinols, as well as their permethylated analogues, involve hydrogen bonds of only one type, OH⋯M. The relative stabilities of the cyclic chelates closed by the OH⋯M hydrogen bonds are consistent with the sequence of metal basicities (Fe

Binuclear and mononuclear di-η5-cyclopentadienylniobium chemistry: a new insight. Crystal and molecular structure of [Cp2NbH2]−[Na · benzo-15-crown-5]+

Abstract A mononuclear di-η 5 -cyclopentadienylniobium complex Cp 2 NbH 2 Na (I) has been found to be a by-product of the reaction of Cp 2 NbCl 2 with NaH, the principal products being (η 5 : η 1 -C 5 H 4 ) 2 Cp 2 Nb 2 H 2 ) 2 (II) in THF and (η 5 : η 5 -C 5 H 4 C 5 H 4 )Cp 2 -Nb 2 (η-H) 2 (III) in DME. Cp 2 NbH 2 Na wasalso obtained by reaction of Cp 2 NbH 3 or the mixture of Cp 2 NbBH 4 + Et 3 N with NaH. Crystal and molecular structure of [Cp 2 NbH 2 ] − [Na · NbH 2 − [Na · B15C5] + (Ia) (B15C5 = benzo-15-crown-5) was established by an X-ray diffraction study (4208 reflections, R = 0.022; monoclinic, at −120° C, a 17.345(3), b 11.742(3), c 22.965(5) A, β 98.52(1)°, Z = 8 space group C 2/ c ). The structural data indicate substantial ionic character of the (Cp 2 NgH 2 ) − … (Na · B15C5) + interaction in the solid.

THE MM3 FORCE FIELD FOR 12-VERTEX BORANES AND CARBORANES

Abstract A model and parameters were developed which allow conformational calculations to be carried out on the 12-vertex boranes, carboranes and their derivatives, within the framework of the MM3 force field. In order to avoid description of all the bond angles, the molecule was represented as a superposition of two rings and two cap groups. The valence bonds between these moieties were described by a Buckingham potential. The statistical analysis of carborane molecules found in the Cambridge Structural Database has been carried out in order to find the mean values of the bond lengths and angles for these molecules. A parameter set was developed and used for the calculations on substituted carboranes, and the results show good agreement with the experimental data.

MOLECULAR MECHANICS CALCULATIONS ON CARBORANES AND METALLOCARBORANES

Abstract A new approach has been developed within the framework of the MM3 force field which allows us to calculate the molecular conformations of organometallic molecules containing 7-vertex carborane and metallocarborane residues. A carborane cage in the molecular mechanics approximation is treated as a superimposition of a 5-membered ring and apical (cap) groups. Chemical bonds between these fragments are described by a Hill-like potential. The fitting of the force field parameters was based on the results of ab initio calculations of geometry and energy-distance dependences of several simple 7-vertex carborane molecules: 2,4-C 2 B 5 H 7 , 2,3-C 2 B 5 H 7 , l-(SiH 2 )-2,3-C 2 B 4 H 6 and 1-Mg-2, 3-C 2 B 4 H 6 . The elaborated force field was applied in calculations of conformational features of a series of sandwich metallocarboranes of the [RR′C 2 B 4 H 2 ] 2 M type. It was shown that the bent sandwich structure of the Sn derivative is due to nonbonded interligand interactions, as in the case of the metallocenes.

The crystal structure of 4-difluoromethoxybenzylidene-4′-(1″,1″,2″,2″,3″,3″,4″,4″-octafluorobutyl)aniline and modelling of the 4-methoxybenzylidene-4′-n-butylaniline crystal structure

Abstract The crystal and molecular structure of a non-mesogenic member of the series of mesogenic fluorinated 4-methoxybenzylidene-4′-n-butylaniline (MBBA) derivatives is described. Calculations based on atom-atom potentials were carried out to test the variability of the hypothetical crystal structures based on the MBBA derivatives studied by the authors and to build the variable crystal structures for the classical nematogen MBBA.

Hydrogen bonds and conformations of α-carbinol derivatives of cyclopentadienyltricarbonyls of manganese and rhenium

Abstract On the basis of IR spectra and molecular mechanics calculations, the conformations stabilized by hydrogen bonding in α-carbinol derivatives of cyclopentadienyltricarbonyl manganese and rhenium have been determined. Intramolecular H-bonds with Mn and Re atoms are found in the tertiary carbinols. Relative stability of the OH ⋯ M hydrogen bonds increases in accordance with metal basicity (Mn

Molecular structure and electron density distribution in the crystals of 3,6-dimethoxy-1,2,4,5-tetrazine and 3-phenyl-1,2,4,5-tetrazine based on X-ray diffraction data at 120 and 150 K. “Bent” bonds in the six-membered heterocycle

Characteristic features of the molecular geometry and electron density distribution in the crystals of 3,6-dimethoxy-1,2,4,5-tetrazine and 3-phenyl-1,2,4,5-tetrazine have been studied by X-ray structural analysis and quantum-chemical (RHF and MP2) and molecular mechanics (MM3) calculations. An unusual shift of the maxima of the deformation electron density from the N-N bonds toward the center of the heterocycles was found, which may be interpreted as a “bending” of the corresponding bonds. This “bending” was confirmed by calculations of characteristics of the electron density distribution within the multipole model.

Molecular structure of dicumyl peroxide

ConclusionsIt has been shown by x-ray diffraction analysis that the peroxide group in. dicumyl peroxide has a planar transoid conformation as in the crystals of its analogs (bis(triphen-ylmethyl) peroxide and di-tert-butyl peroxide); and so this conformation is the preferred one for peroxides with two tertiary hydrocarbon substituents. This conclusion is supported by the results of conformational calculations carried out using MNDO and molecular mechanics.

Structure Investigations of Thermotropic Liquid Crystalline Monomer and Polymer Organosilicon Compounds

Structures of monomolecular siloxane derivatives forming mesophase due to the presence of the one-dimensional system of hydrogen bonds are described on the basis of X-ray investigations of LC and TC phases. X-ray investigations of related polymer compounds was used as a ground for polymer structure modeling on an atomic level.