银配合物与联氨定向反应的研究II. 酸度与温度对定向反应影响

本文研究了酸度和温度对银配合物与联氨定向反应的影响, 在不同的pH和温度下, 微量Cu(II)离子的加入均能有效地提高N2H4的四电子定向反应率; N2H4的四电子反应率随体系pH和温度的提高吾有序增加, 无Cu(II)离子存在时, N2H4的四电子反应率与银配合物logβ2呈线性关系。Cu(II)离子存在时, N2H4单电子反应率的对数与1/logβ2呈线性关系。     

Synthesis and properties of azobenzocrown ethers with π-electron donor, or π-electron donor and π-electron acceptor group(s) on benzene ring(s)

New azobenzocrown ethers of differentiated size and with substituted benzene residues have been synthesized. These crown ethers possess π-electron donor, or π-electron donor-π-electron acceptor pair of functional group(s) in benzene ring(s) in the para position to azo-grouping. Their metal ion complexation abilities in solution were studied using UV-vis spectrophotometry. The X-ray structure of a 19-membered crown ether with 4-dimethylamino-4′-nitroazobenzene fragment has been solved.     

Kinetics and mechanisms of substitution at paramagnetic metal centers in organometallic complexes

The mechanism of ligand substitution in 17- and 19-electron organometallic radicals is discussed. These species substitute ligands by an associative process some 10⁶ to 10¹⁰ faster than analogous 18-electron complexes. When 17-electron species can be generated by bond homolysis or electron transfer reactions of 18-electron complexes, they can act as intermediates in radical chain reactions of 18-electron complexes. A 17–19 electron rule is proposed to explain transformations of organometallic radicals just as the 16–18 electron rule finds use for closed shell organometallic complexes. The origin of this rule is the favorable two-center three-electron bond that can form when an odd electron in a sterically accessible metal d-orbital interacts with an electron pair on an entering nucleophile. Besides simple substitution, these radicals can disproportionate, dimerize, and undergo insertion or atom abstraction reactions.     

Dynamical screening by Σ-electrons in π-electron transitions: Treatment in the equations-of-motion method

Dynamical screening in π-electron systems is studied by the equations of motion method. By using a partitioning technique on the equations of motion we can obtain simple expressions for the effect of dynamical screening directly on the transition energies and transition moments in π-electron systems. These results are used to study the effect of such screening in the N → V transition in ethylene. This procedure can be used to extend the equations-of-motion method to larger π-electron systems.     

An iridium-stabilized, formally uncharged te(10) molecule with 3-center-4-electron bonding

Te for 10: A tricyclic Te(10) molecule is stabilized in an iridium complex. Bonding analysis reveals 3-center-4-electron bonds in the linear Te(3) fragment. The tellurium atoms act as 2-electron donors to the transition-metal atoms.     

Unusual Formation of the Paramagnetic Complex (η4-C4Me4)CoI2(PhTeI) and Specific Features of Its Electronic,Molecular, and Crystal Structures

Russian Journal of Coordination Chemistry - The 18-electron (η4-C4Me4)Co(CO)2TeI2Ph compound noticeably decomposes upon prolonged storage and also unusually transforms into the 17-electron...     

Graph theory and molecular orbitals. The loop rule

Graph theory is applied to the study of the dependence of total π-electron energy, π-electron charge distribution and free valency indices of conjugated hydrocarbons on molecular topology. It is shown that the number of loops in the molecular graph determines these quantities.     

Bond orders in 1,5-benzodiazepinium cations compared to those in biphenylenes. Another example of bonds of low bond order which isolated π-electron systems.

The bonds which link separate stablishing π-electron systems in 1,5-benzodiazepinium cations are shown from X-ray analysis and MNDO calculations to be of very low bond order, like the bridging bonds in biphenylenes, thereby inhibiting destabilising interaction between the π-electron systems.     

Interaction of a bulky N-heterocyclic carbene ligand with Rh(I) and Ir(I). Double C-H activation and isolation of bare 14-electron Rh(III) and Ir(III) complexes

Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-heterocyclic carbene (NHC) ligands are presented. These systems are capable of intramolecular C-H bond activation and lead to coordinatively unsaturated 16-electron complexes. The resulting complexes can be further unsaturated by simple halide abstraction, leading to 14-electron species bearing an all-carbon environment. Saturation of the vacant sites in the 16- and 14-electron complexes with carbon monoxide permits a structural comparison. DFT calculations show that these electrophilic metal centers are stabilized by pi-donation of the NHC ligands.     

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO–LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.     

Four- and two-electron rules for diatropic and paratropic ring currents in monocyclic pi systems

The pi ring current in an even-eletron monocycle is dominated by the HOMO-LUMO transition, and hence corresponds to circulation of four electrons in a diatropic (4n + 2)-electron, but two in a paratropic (4n)-electron cycle.     

The role of odd-electron intermediates and in-cage electron transfer in ultrafast photochemical disproportionation reactions in Lewis bases

Femtosecond visible pump-IR probe studies of Cp2W2(CO)6 in P(OMe)3 and CH2Cl2 have allowed direct observation of a 19-electron intermediate and of disproportionation into CpW(CO)3- and CpW(CO)3P(OMe)3+ on the ultrafast time scale. A new disproportionation mechanism involving in-cage electron transfer between a 19-electron intermediate and a 17-electron radical has been proposed.     

π-Electron currents in fixed π-sextet aromatic benzenoids

In view of different patterns of π-electron density currents in benzenoid aromatic compounds it is of interest to investigate the pattern of ring currents in various classes of compounds. Recently such a study using a graph theoretical approach to calculating CC bond currents was reported for fully benzenoid hydrocarbons, that is, benzenoid hydrocarbons which have either π-sextets rings or “empty” rings in the terminology of Clar. In this contribution we consider π-electron currents in benzenoid hydrocarbons which have π-electron sextets and C=C bonds fully fixed. Our approach assumes that currents arise from contributions of individual conjugated circuits within the set of Kekulé valence structures of these molecules.     

The total π-electron energy as a problem of moments: application of the Backus–Gilbert method

The total π-electron energy problem can be formulated as a classical problem of moments. This observation allows us to apply general methodologies developed in the field of moment’s theory to solve the total π-electron energy problem. In the present article, we apply the Backus–Gilbert method to obtain analytical expressions for the total π-electron energy in terms of its spectral moments.AMS subject classification: 30E05, 05C50, 92E10, 78M05     

Atomic and molecular impurities in4He clusters

Recoil-ion charge distributions produced in single collisions of 8 MeV/u Kr¹³⁺ and Kr³²⁺ projectiles with Xe atoms have been measured using time-of-flight spectroscopy. The post-collision charge states of the projectile ions were determined by magnetic dispersion onto a position sensitive microchannel plate detector. The recoil-ion distributions for ionization accompanied by electron loss from Kr¹³⁺ projectiles were bell-shaped with averages that ranged from 7.4 for 1-electron loss to 13.9 for 5-electron loss. The recoil-ion distributions for ionization accompanied by electron capture to Kr³²⁺ projectiles were also bell-shaped, but had much higher average charges that ranged from 20.4 for 1-electron capture to 28.5 for 4-electron capture. The large difference in the average charges produced in the two types of collisions is mainly attributable to charge magnification by Auger decay. A simple model quantitatively explains the variation of the capture-ionization charge distribution width and average charge as a function of the number of captured electrons.     

π-Electron ring currents and magnetic properties of porphyrin molecules in the MO LCAO SCF method

The coupled variant of double-parameter perturbation theory in the MO LCAO SCF method in the London approximation has been used for the calculation of π-electron current distributions in the molecules of porphin and its derivatives. The chemical shifts of¹H-NMR have been computed on the basis of calculations of ring currents and charge distributions. It is shown that π-electron ring currents are responsible for the dominant contribution to the shielding of protons. The theoretical and experimental values of proton chemical shifts are in a good agreement. Chemical shifts of the¹³C and¹⁵N nuclei have also been estimated. Two aromaticity scales are proposed for the compounds under study based on the calculations of the π-electron contribution to the diamagnetic susceptibility and of π-electron currents, respectively.     

Preferential site of solvation of the furan ring in acidic solvents

A study of the correlation of solvent effects on the chemical shifts of the methyl protons of sylvan, toluene and α-picoline led to the conclusion that the preferential site of solvation of the furan ring in acidic solvents is the π-electron cloud and not the n-electron orbital of the oxygen atom.     

Macrocyclic π-systems with C2-symmetry as the sum of π-systems with hückel and möbius topology

The π-system of a macrocycle with twofold rotation symmetry and 2N conjugated π-electron can conveniently be regarded as the sum of two cyclic N π-electron systems having Hückel and Möbius topology, respectively.     

Gold nanoparticle decoration of insulating boron nitride nanosheet on inert gold electrode toward an efficient electrocatalyst for the reduction of oxygen to water

Overpotential for oxygen reduction reaction (ORR) at Au electrode is reported to be reduced by 0.27 V by the modification with boron nitride nanosheet (BNNS) but oxygen is reduced only to H₂O₂ by 2-electron process at Au electrode. Here we demonstrate that the decoration of BNNS with gold nanoparticles (AuNP) not only reduces the overpotential for ORR further by ca. 50 mV, but also opens a 4-electron reduction route to water. Both rotating disk electrode experiments with Koutecky–Levich analysis and rotating ring disk electrode measurements show that more than 50% of oxygen is reduced to water via 4-electron process at Au–BNNS/Au electrode while less than 20 and 10% of oxygen are reduced to water at the BNNS/Au and bare Au electrodes, respectively. Theoretical analysis of free energy profiles for ORR at the BN monolayer with and without Au₈ cluster placed on Au(111) shows significant stabilization of adsorbed oxygen atom by the Au₈ cluster, opening a 4-electron reduction pathway.     

Graph theoretical approach to the topological spin hamiltonian applied to conjugated molecules

Approximate formulae for the total π-electron energy calculated within the topological spin Hamiltonian are derived using the connected moments expansion (CMX) technique up to the third order. This provides the first reported graph theoretical approach to the topological Hamiltonian other than the Hückel one. The formulae, valid for alternant, singlet ground state hydrocarbons, are used to estimate the total π-electron energy of some large molecules