Über die Struktur von Bis-(dihydroxoboroxalendiamiddioximato)-nickel(II)-tetrahydrat

The space group of the sparingly soluble brown coloured bis-(dihydroxobor-oxalene-diamide-dioximato)-nickel(II)-tetrahydrate (C₄H₁₂N₈O₈B₂Ni · 4 H₂O) is P21/c with a = 9.25, b = 13.27, c = 6.61 Å and β = 92.7°. The structure was solved on single crystals. There are two molecules in the unit cell. Within the square planar complex the nickel atom is bonded to the four nitrogen atoms of the plane system of the two dioximato ligands (Ni? N = 1.84 ± 0.02 Å). Each of the boron atoms is tetrahedrally surrounded by two oxygen atoms belonging to the oxime groups and two hydroxyl groups (B? O = 1.50 ± 0.06 Å). Thus the complex has the conformation of a chair form. The water molecules participate in intermolecular hydrogen bonds. There is no interaction between Ni atoms.     

Ethylenic compounds reactivity: bromination—XLI: Relative reactivities of R1CMeCH2 and R1CHCMe2 series as a criterion for distinguishing between bridged and acyclic transition states. Degree of symmetry of the bromonium ion-like transition state

Reactivities of homogeneous series of ethylenic compounds R¹CHCH₂1, trans R¹CHCHMe 2, R¹MeCCH₂3 and R¹CHCMe₂4 have been measured in methanol at 25°C (R¹ Me, Et, n-Pr, -CH₂C₆H₅-CH₂OCOCH₃, -CH₂Cl). The criterion developed to decide between a bridged and a carbonium ion-like transition state is based on an internal comparison of the series and avoids resorting to external structural scales. Even in the case of alkenes 3 and 4 which are very dissymmetric with respect to polarity, the sensitivity to polar effects remains constant and the transition state behaves like a symmetric entity; only a slight secondary effect, attributed to hyperconjugation, is detectable.     

Exchange of halogen bonded on aromatic core. Heterogeneous catalysis—I : Reaction mechanism of iodine exchange in o-iodobenzoic acid

Reaction mechanism of halogen exchange between arylhalogenide and halogenide anion is studied in a case when the aromatic core is inactivated with respect to the S_NAr substitution by other substituents. The exchange of iodine between o-iodobenzoic acid and ¹³¹I labelled NaI serves as a modelling reaction. The reaction was found to proceed heterogeneously on a glass surface. In the case of a rapid course of the surface reaction the reaction mechanism is more complicated and the equation derived by McKay cannot be applied for expressing the time dependence of the degree of isotopic exchange. Reaction mechanism was proposed for this reaction and compared with experimental data.     

The adsorption and dissociation of O2 on Cu low-index surfaces

The extended LEPS of O(2)-Cu single crystal plane systems is constructed by means of 5-MP (the 5-parameter Morse potential). Both the adsorption and dissociation of O(2) on Cu low-index surfaces are investigated with extended LEPS in detail. All critical characteristics of the system that we obtain, such as adsorption geometry, binding energy, eigenvalues for vibration, etc., are in good agreement with the experimental results. Our calculated results suggest there are many differences between O(2)-Cu (110) and O(2)-Pd (110) systems. On a Cu (110) surface, O(2) adsorbs in a tilted configuration and there are two lowest energy dissociation channels along the [001] and [10] directions, respectively. We speculate that the adsorption geometry of O(2) on the metal surfaces relates to the lattice constant of metal. Meanwhile, We use the concepts of the molecular dissociation limit and the surface dissociation distance to analyze again the dissociation mechanism of the O(2) on the low-index surfaces.     

Über Erdalkalimetalloxothallate. II. Darstellung und Kristallstruktur von Ba2Tl2O5

On Alkaline Earth Metal Oxothallates. II. Preparation and Crystal Structure of Ba₂Tl₂O₅ Ba₂Tl₂O₅ was prepared and investigated by X-ray single crystal methods (space group D? Pcmn, a = 6.264, b = 17.258, c = 6.05 Å) Ba₂Tl₂O₅ is isotypic with Ca₂Fe₂O₅.     

Regioselectivity of Birch reductive alkylation of biaryls

[reaction: see text] The regioselectivity of the Birch reductive alkylation of polysubstituted biaryls has been investigated. Results indicate that regioselectivity is affected by the electronic nature of substituents on both aromatic rings. The electron-rich 3,5-dimethoxyphenyl moiety is selectively reduced and then alkylated, while phenols and aniline are not dearomatized under these conditions. Biaryls possessing a phenol moiety are alkylated on the second ring, providing that the acidic proton has been removed prior to the Li/NH3 reduction.     

Assessment of the Fe-Sn-Zn phase diagram at 450°C

The Fe-Sn-Zn system is of interest because Sn is one element added to the Zn galvanizing bath to overcome the drawbacks due to the presence of Si in semi-killed steels. This work has been undertaken with the aim to understand the tin effect on the microstructure and the layers growth in batch galvanized coatings on low alloyed steels. Various experimental techniques such as metallography, scanning electron microscopy (SEM) coupled with X-ray energy dispersive spectroscopy (EDX) are used in order to characterize the microstructure and the properties of such coatings elaborated in a zinc bath enriched with tin. Solidification phenomena and layers growth mechanisms during galvanization are explained by means of the ternary phase diagram Fe-Sn-Zn at 450°C. The Calphad method allows to obtain this phase diagram from the three optimized binary phase diagrams Fe-Sn, Fe-Zn and Sn-Zn and some experimental data inside the ternary Fe-Sn-Zn system.     

CO吸附在过渡金属铂表面的微观动力学研究

建立了处理双原子分子-表面相互作用的推广的LEPS势.借助推广的LEPS势,系统研究了一氧化碳分子在铂低指数表面吸附的动力学特性,重现了低指数表面的分子吸附热、吸附几何及本征振动等实验数据；鉴定了某些不合理的文献信息,预测了实验尚未探测到的重要信息:预测到Pt(100)表面四重洞位的C－O伸缩振动频率为1 962.60 cm－1;预测到Pt(110)表面吸附态的C－O及C－Pt键长分别为115.1、147 pm.     

Calix[6]arene-based cuprous "funnel complexes": a mimic for the substrate access channel to metalloenzyme active sites

Two calixarene-based model systems (a and b) for monocopper enzymes are compared. Both present a tris(pyridine) coordination site for Cu that mimics the imidazole-rich neutral binding site in enzymes. Upon reaction with 1 equiv of copper(I), the tridentate ligands gave rise to ill-defined unsymmetrical complexes. However, in the presence of an organonitrile RCN (R = Me, Et, Ph), tetrahedral species were obtained, with the nitrilo ligand included in the calixarene hydrophobic cone. System b presents a larger cavity than system a, with a wider opening thanks to the removal of three tBu groups from the calixarene structure. As a result, the recognition pattern for MeCN vs PhCN is inverted, and the relative affinity constants differ by 3 orders of magnitude. The mechanism of the acetonitrile exchange at the cuprous centers was studied by (1)H NMR spectroscopy. Thermodynamic and kinetic data show that it follows a dissociative pathway in both cases. The main differences between systems a and b stem from the presence of a door that entraps the guest in case a. In system b indeed, the removal of three calixarene tBu groups led to a 100-fold acceleration of the MeCN exchange rate. Hence, these supramolecular systems provide a rare and interesting model for the hydrophobic substrate channel giving access to a metalloenzyme active site.     

On the Criteria of Instability for Electrochemical Systems

Both cyclic-voltammetry-based and impedance-based experimental criteria that have been developed recently for the oscillatory electrochemical systems are critically appraised with two typical categories of oscillators.Consistent conclusions can be drawn by the two criteria for the category of oscillators that involve the coupling of charge transfer mainly with surface steps(e.g.ad-and desorption)such as in the electrooxidation of C1 organic molecules.Whereas,impedance-based criterion is not applicable to the category of oscillators that involve the coupling of charge transfer mainly with mass transfer(e.g.diffusion and convection) such as in the Fe(CN)6^3- reduction accompanying periodic hydrogen evolution.The reason is that the negative impedance cannot include the feedback information of convection mass transfer induced by the hydrogen evolution.However,both positive and negative nonlinear feedbacks,i.e., the diffusion-limited depletion and convection-enhanced replenishment of the Fe(CN)6^3- surface concentration,that coexist between the bistability,i.e.,Fe(CN)6^3- reduction with and without hydrogen evolution at lower and higher potential sides respectively,are all reflected in the crossed cyclic voltammogram(CCV).It can be concluded that the voltammetry-based criterion(in time domain)is more intuitive,less time-consuming and has a wider range of applications than the impedancebased one (in frequency domain).