[en] Ab initio (RHF/6-31G) calculations were used to study the degenerated isomerization of a new system tetracyclo[4.4.4.4.]-1-boron-3,5-diazapentane. The calculations showed, that suggested structure corresponded to energy minimum, boron atom formed the fourth chemical bond due to dative (donor-acceptor) interaction between a free σ(p)-orbital of boron atom and nonshared electron pair of nitrogen atom. The length of donor-acceptor bond is equal to 1.533 A. Such sufficient reduction of B -> N bond is related with sterical forcing of boron and nitrogen centers to convergence in a tense tetracyclic system. Reaction of bond reversing proceeds through transient state with crossing the activation barrier, equal to 6.67 kcal/mol. 14 refs.; 1 fig.; 1 tab

[en] Structural modelling of mechanisms of inversion with central atom tetrahedral configuration in bischelate complexes of nontransitional metals, including Be, B, In, Cd and other is conducted. It is shown that in solutions the complexes mentioned undergo fast intramolecular stereoisomerization as well as reactions of intermolecular ligand exchange. Possible mechanism of these reactions are considered. 53 refs., 4 figs., 2 tabs

[en] Short note. 1 fig

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[en] The factors involved in the orbital stabilization of framework structures containing a flat cyclic system of nonbonded methylene groups and corners of single atoms and axially and parallel oriented σ or π bonds and π-conjugated rings were examined. It was established that the π/sub σ/-π/sub σ/ interactions have a stabilizing effect. The effects of the electronegativity of the apical groups, the ring sizes, and other factors on the geometric parameters and the stability of derivatives of bicyclo[1.1.1]pentane, [1.1.1.1.]peddlane, bis- and tetrahomotetrahedranes, and [n]asteranes was determined. The heats of formation and the molecular geometries of a series of known and not yet described framework structures of these types were calculated by the MINDO/3 method

[en] Electron structure and thermodynamic stability of (LiF)₃ trimer of D_3h symmetry have been considered on the basis of the results of calculations of the trimer plane structure by the MNDO and ab initio (basis 3-21G) methods. It is shown that in (LiF)_n monocyclic structures of D_nh symmetry, which are the limiting case of the structures with electronic density alternation, there are non indications of aroma and antiaroma, although formal analogy of π-level systems of the structures and cyclobutatiene (D_2h), benzene, cyclo-octatetraene (D_4h) structures is preserved

[en] The energy characteristics and electron distributions were calculated for the fundamental and transitional states in topomerization of configuration of sulfo-, selenium and tellurium-uranium compounds of the AX₄ type. Also mapping of the potential energies was made for the predominant topomerization mechanisms. The mechanisms were classified into rearrangement types according to exchange-responsible apical and equatorial ligands. The physical mechanisms of the topomerization were determined on the basis of symmetry requirements for transitional configuration with respect to non-interchanging ligands. The lowest activation barrier was characteristic for the Berry mechanism of pseudorotation; however, other mechanism (the lever and pyramidal type) can compete with the Berry mechanisms. The expanded method of Hukkel does not give correct values for the relative energy of the fundamental and transitional configurations for sulfouranium molecules and their analogs for two of the three exchange mechanisms examined. Tunnel ligand-exchange mechanism cannot compete with the kinetic path of psudorotation reaction even at very low temperatures

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