R. M. Minyaev

A Quantum Chemical Study of Bis-(iminoquinonephenolate) Zn(II) Complexes

A computational DFT B3LYP*/6-311++G(d,p) study performed on bis-(iminoquinonephenolate) Zn(II) complex [Zn(II)(C(12)H(8)NO(2))(2)] has revealed a previously unexplored mechanism for valence tautomerism inherent in transition metal complexes with redox active (noninnocent) ligands. The occurrence of energy-close isomeric forms of the complex and their low energy barrier interconversion is caused not by the intramolecular electron transfer (IET) between the metal and ligand frontier orbitals, but the intersystem conversion within a redox active ligand without involvement of a metal center. This mechanism gives a new insight into the origin of the previously experimentally studied isomeric forms of bis-(iminoquinonephenolate) Zn(II) complexes that must be assigned to [Zn(II)((1)L(-1))(2)] (8) and [Zn(II)((1)L(-1))((3)L(-1))] (9) structures. The spin-forbidden transition between the two forms of the complex proceeds via a minimal energy crossing point (MECP) corresponding to the energy barrier of 8.9 kcal mol(-1) for the 9 --> 8 transformation in the gas phase.

Gradient line reaction path of HF addition to ethylene

Abstract The gradient line reaction path of the HF addition to ethylene is shown to consist of two distinct gradient lines. The first connects the reactant minimum via one transition state to another transition state. The second is associated with the internal rotation about the carbon-carbon bond in ethyl fluoride and this gradient line also passes through the second transition state. The main feature of this reaction path is that the first gradient line is not connected to the product minima directly.

Hypervalent Intramolecular X←N (X = C, Si, Ge) Coordination in Atranes: Quantum-Chemical Study

The structure and pentacoordination effect in atranes containig Group IVa element were studied ab initio [MP2(full)/6-31G**] and in terms of the density functional theory [B3LYP/6-311+G**]. Stabiliza- tion of these compounds is determined mainly by the secondary hypervalent (R)X←N bond (X = C, Si, Ge), whose strength increases in the series X = C, Si, Ge. Attractive (R)X←N interaction originates from donation of unshared electron pair on the nitrogen atom to the antibonding σ*XR orbital.

Stabilization of nonclassical types of valence bond orientation at the carbon atom in organoboron compounds

The complete topological structure of the potential energy surface (PES) of methane in the inversion region was studied by theab initio CCD(full)/6-311++G** method. The necessity of taking into account nuclear motions was shown. Penta- and hexacoordination of carbon atoms in boron-containing organic compounds was investigated by theab initio MP2(full)/6-31G** and MP2(full)/6-311++G** methods. The CB4H4Li2, CB6N2H2, and CB6O2 systems containing hexacoordinated carbon atoms correspond to rather deep minima on the relevant PES and can be the subject of synthetic studies. According to theab initio calculations, pyramidal boron-containing systems with hypercoordinated carbon atoms, which fulfill the “8e rule,” also correspond to rather deep PES minima and can be detected experimentally.

Intermolecular interactions between hypervalent molecules: Ph2IX and XF3 (X=Cl, Br, I) dimers

The intermolecular bonding in dimers of the T-shaped hypervalent title compounds is analyzed using a combination of density functional calculations and qualitative arguments. Fragment molecular orbital interaction diagrams lead us to the conclusion that the bonding in these species can be understood using the language of donor-acceptor interactions: mixing between occupied states on one fragment and unoccupied states on the other. There is also a strong electrostatic contribution to the bonding. The calculated strengths of these halogen–halogen secondary interactions are all less than 10 kcal mol-1. There is a very soft potential energy surface for the deformation that makes the bridge in the dimers asymmetrical.

Quantum-chemical study of valence tautomerism of a cobalt complex with phenoxybenzoquinone imine

319 An important way to search for structures for molecular switches and memory cells of molecular computers is to study valence tautomerism of transi tion metal complexes [1–4]. This involves the exist ence of isomers with different charge and spin density distributions and, hence, different optical, electrical, and magnetic properties. Transitions between tauto meric forms accompanied by reversible intramolecu lar electron transfer have been studied in Mn and Co complexes with phenoxybenzoquinone imine [5, 6], which can form complexes with metals in the mono (Cat–N–BQ1–) and dianionic (Cat⎯N⎯SQ2–) forms.

Pyramidane and isoelectronic pyramidal cations

Abstract Ab initio STO-3G, 4-31G and semi-empirical MINDO/3 calculations were performed on a series of C 4 H 4 X compounds to analyze the possibility of existence of their nonclassical C 4V pyramidal isomeric forms. It was shown that with the four-electron group X (C, N + , P + , O 2+ , S 2+ ) in an apex, C 4V pyramidal structures are indeed stable, i.e., conform to local minima on the corresponding potential energy surfaces. Relative energies, molecular geometries and charge density distribution for various isomeric forms of the C 4 H 4 X compounds are presented. The theoretically derived strategies for an experimental trapping of the pyramidane 1 , X = C and the C 4V pyramidal thiophene dication 1 , X = S 2+ are discussed.

Intramolecular hypervalent interaction in the conjugate five-membered rings.

The intramolecular hypervalent interaction between the electron abundant atomic centers X and Y belonging to the IV-VI groups and second and fourth periods has been computationally studied on a model quasi-cyclic conjugate five-membered ring system 9 using the CCSD/6-311+G** and DFT B3LYP/6-311+G** methods. Electronic and structural factors affecting the strength and geometrical characteristics of the hypervalent X←Y interaction were analyzed based on the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. The donor-acceptor n(Y)→σ*(XR) interaction has been shown to be the central factor correlating all important properties of the studied hypervalently bonded compounds 9.

Sandwich Compounds of Transition Metals with Cyclopolyenes and Isolobal Boron Analogues

A series of sandwich compounds of transition metals (M=Ni, Fe, Cr) with cyclic hydrocarbon (M(CH)(n)) and borane (M(BH(2))(n)), ligands (including mixed hydrocarbon/borane sandwiches) has been studied using density functional theory (B3LYP/6-311+G(df,p)). Multicenter bonding between the central metal atom and basal cycloborane rings provides stabilization to planar cycloborane species. Large negative NICS values allude to aromatic character in the cycloboranes similar to the analogous cyclic hydrocarbons. The ability of cycloborane sandwiches to stabilize attached carbocations, radicals and carbanions is also assessed.

Influence of structural factors on the magnetic properties of the binuclear copper complexes with salicylaldehyde hydrazone and bis(hydrazone)-2,6-diformylphenol: Quantum-chemical calculations

The structures and magnetic properties of the binuclear copper complexes of salicylaldehyde mono- and bis(hydrazone) derivatives were studied by the quantum-chemical density functional theory (B3LYP/6-311++g(d,p)) using the broken-symmetry technique. The change in the degree of deprotonation of the ligands was found to exert an insignificant effect on the magnetic properties, whereas the coordination of solvent molecules substantially weakened the antiferromagnetic interaction.