[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

No abstract available

[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] 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

[en] Ab initio calculation methods (RHF/6-31G, MP2(full)/6-31G and MP2(full)/6-311++G) ) are used to investigate thermodynamic stability, energy and structural characteristics of different position isomers of isoelectron B₃N₄H₈ (1) and Be₄O₄H₈ (2) systems with cubane structure. High thermodynamic stability of these system is shown. Decomposition of structure (1) into two 1,3,2,4-diazadiborethidine molecules or four molecules of iminoborane HBNH is an endothermal process and needs 10.1 (RHF/6-31G), 39/6 (MP2(full)/6-31G) Cal/mol and 140.6 (RHF/6-31G), 161.4 (MP2(full)/6-31G) Cal/mol accordingly. Decomposition of structure (2) into two 1,3,2,4-dioxydiberyllotidine or four molecules of HBeOH is an endothermal process too and needs 22.1 (RHF/6-31G), 39.8 (MP2(full)/6-31G) Cal/mol and 127.1(RHF/6-31G), 155.2 MP2(full)/6-31G) Cal/mol accordingly. Geometrical characteristics of simple BeH₂, Be₂, Be₂H₂, Be₂H₄, BeO, Be₂O₂ molecules are calculated

No abstract available

[en] The geometrical, energy and electron characteristics of the simplest boron hydrides and carboranes are studied with ab initio (RHF/6-31G^**, MP2(full)/6-31G^**, MP2(full)/6-31++G^** and CCD(full)/6-311++G^**) calculations in the isolated states B_mH_n^k- (m=1, n=4, k=-1; m=2, n=2, 4, 6, k=0, -1, -2), BCH_n^k- (n=2-6, k=0, -1) and in the complexes B_mH_n · Li_k (m=1, n=4, k=1; m=2, n=2, 4, 6, k=1, 2), BCH_n · Li (n=2-6). Ir is shown that the geometrical characteristics of the boron hydrides and carboranes ions are extremely close and analogous to the fragments in the complexes with lithium. At the same time the calculated complex-formation energies and electron distributions in the complexes indicate, that these complexes are covalent compounds, but not ion pairs

[en] The reaction pathways for the 1,5-sigmatropic displacement of the nitroso group along the ring perimeter of 5-nitroso-1,3-cyclopentadiene and its valence isomerization were studied using the semiempirical MINDO/3 method. The mechanism for the 1,3-shift of the nitroso group in the 1,5-sigmatropic carousel reaction is as probable as the ordinary 1,2-shift mechanism. In contrast to nitroso derivatives of other [n]-annulenes (n = 3, 4, 6), the calculations for 5-nitroso-1,3-cyclopentadiene did not reveal other isomeric π-complex structures with perpendicular or parallel orientation of the nitroso group relative to the ring plane. The transition state were localized for the thermally forbidden concerted isomerizations of 5-nitroso-1,3-cyclopentadiene to pyridine N-oxide and 1,2-oxazepin as well as of the isomerization of pyridine N-oxide to the unknown, kinetically stable isomer, bicyclo[4.1.0]-1-aza-7-oxa-2,4-heptadiene

[en] Previously unknown stable structures with D_3h and C_2v symmetry B₂H₅⁺ molecules, which contain three and two bridge bonds B-H-B, respectively, while there is a small activation barrier of regrouping between them (5.41 kcal/mol, calculation SCF/3-21G), have been predicted using ab initio method (SCF/3-21G and DZP). 14 refs.; 2 figs

[en] Using ab initio methods it is shown that interhalogen complexes XF₃ (where X = Cl, Br, I) in gaseous phase from stable dimers with C_2h symmetry. Dimerization energy of XF₃ increases abruptly in the series Cl-Br-I and for the later one reaches the values of 21-34 kcal/mol, which is comparable with the covalent bond energies. In dimers (XF₃)₂ an essential elongation of axial X-F bonds participating in formation of intermolecular X...F bonds is observed. It gives rise to a rapid synchronous double fluorine exchange with low energy barriers ∼ 2-10 kcal/mol