Kinetics and modeling of the H2?O2?NOx system

The addition of NO (0 to 400ppm) to mixtures of H₂ (ca. 1%) and O₂ (0.7 to 22%) has been studied over the temperature range 700 to 825 K, in a flow reactor at atmospheric pressure. The overall effect of NO is to promote the oxidation of H₂ but high concentrations of O₂ actually inhibit the NO-promoted oxidation of H₂. A detailed kinetic mechanism has been constructed and found to describe the experimental observations. The promotion of the oxidation of H₂ arises through the catalytic cycle The ability of R.34 to reactivate chains normally terminated by the formation of HO₂ is a key feature of this system. The predictions are highly sensitive to the rate of the reaction R.5 and the rate constants for this reaction is the only adjustable parameter required in the model. The value of k_5,N2 found to describe all the results has an absolute uncertainty <35%. The uncertainty relative to other important rate constants in the H₂? O₂ system is less than 10%. © 1995 John Wiley & Sons, Inc.     

P?N ligands in lanthanide chemistry

The coordination chemistry of inorganic amides in Group 3 and lanthanide chemistry is discussed. Three different ligand systems (phosphino‐amides, bis(phosphino)amides, and bis(phosphinimino)methanides) that consist of one or more P N units were used. In this series the steric demand of the ligands is increased in a stepwise fashion and the negative charge is delocalized over more atoms. These properties were used in the design of new lanthanide complexes. For all three compounds the synthesis of the alkali metal derivatives is reported first, followed by the reaction of the alkali metal salts with various lanthanide trichlorides. Further reactions of the obtained lanthanide complexes as well as their application as catalysts are discussed. Most of the reported complexes show a dynamic behavior in solution. In phosphinoamide and bis(phosphino)amide complexes, in which the phosphorus atom is in oxidation state +3, there is always a weak coordination of the phosphorus atom to the lanthanide atom observed. In bis(phosphinimino)methanide complexes, in which the phosphorus atom is in oxidation state +5, no such interaction is noticeable. Instead a weak coordination of the methine atom to the center metal can be seen in the solid state. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:514–520, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10096     

CF3Br?H2 reaction in shock waves

CF₃Br? H₂ mixtures highly diluted with Ar were studied by using a time-resolved IR-emission of HBr and a gas-chromatography for reaction products. The temperature range covered was 1000–1600 K and the total pressure behind the reflected shock waves used was 1.2–2.6 atm. CF₃H, C₂F₆, and C₂F₄ were produced and the yields of these products were determined as a function of temperature. The main product under our experimental conditions was CF₃H. The mechanism and the rate constants of CF₃Br? H₂ reaction at high temperatures were discussed. The experimental data was satisfactorily modeled using a 14-reaction mechanism. Reaction (5) played an important role in the formation of CF₃H together with reaction (4). The rate constant expression k₅ = 2.2 × 10¹³ exp(?12 kcal/RT) cm³ mol^?1 s^?1 gave the best agreement between the calculated and observed results. © 1993 John Wiley & Sons, Inc.     

Intramolecular Pnicogen Interactions in PHF?(CH2)n?PHF (n=2–6) Systems

A computational study of the intramolecular pnicogen bond in PHF? (CH₂)_n? PHF (n=2–6) systems was carried out. For each compound, two different conformations, (R,R) and (R,S), were considered on the basis of the chirality of the phosphine groups. The characteristics of the closed conformers, in which the pnicogen interaction occurs, were compared with those of the extended conformer. In several cases, the closed conformations are more stable than the extended conformations. The calculated interaction energies of the pnicogen contact, by means of isodesmic reactions, provide values between ?3.4 and ?26.0 kJ mol^?1. Atoms in molecules and electron localization function analysis of the electron density showed that the systems in the closed conformations with short P ??? P distances have a partial covalent character in this interaction. The calculated absolute chemical shieldings of the P atoms showed an exponential relationship with the P ??? P distance. In addition, a search in the Cambridge crystallographic database was carried out to detect those compounds with a potential intramolecular pnicogen bond in the solid phase.     

Anodic film formation on binary Al?Mg and Al?Ti alloys in nitric acid

Nitric acid is commonly used for surface treatments of aluminium alloys. It is used to clean the surfaces after alkaline etching; it has application in chemical polishing and is also used for electrograining. The majority of these treatments undergo the application of anodic polarisation that results in formation of anodic oxide film. However, little is known about the behaviour of aluminium containing magnesium or titanium in solid solution under such conditions. To reveal the effects of magnesium and titanium alloying additions on anodic film formation in nitric acid, Al‐1800 ppm Mg and Al‐800 ppm Ti alloys were investigated. It was found that porous alumina film developed on the surfaces with reduced efficiency of 40%, due to the reactive nature of nitric acid to alumina. The presence of magnesium and titanium in aluminium had little influence on the efficiency of film growth, as confirmed by the relatively similar thicknesses of oxide formed on binary alloys and aluminium. However, incorporation of magnesium ions into the alumina film led to development of a high‐population density of localised voids near the alloy/film interface. An increased titanium content was found in the film regions close to the alloy/film interface, indicating its oxidation. Copyright © 2009 John Wiley & Sons, Ltd.     

Ni?Co?Mo nanoparticles as an efficient electrocatalyst for methanol electro-oxidation in alkaline media

Using the electrospinning approach, various percentages of less expensive metal alloy-decorated nanofiber catalysts have been successfully made as a substitute for platinum in direct methanol fuel cells (DMFC). This work focuses on the synthesis and characterization of catalysts with metal fixed ratio of 20% wt for DMFC applications, specifically Ni/CNFs, Co/CNFs, and Ni Co Mo/CNFs. The catalysts are characterized using various techniques, including x-ray diffraction, scanning electron microscope, transmission electron microscopy, energy dispersive x-ray, and electrochemical measurements. All the prepared samples, regardless of the metal concentration, had good nanofiber form and a distinct nanoparticle appearance, according to the scanning electron microscope (SEM). Chromatography, scan rate, response time, and cyclic voltammetry all were used to examine the samples' ability to perform methanol electrocatalysis. When Mo is added to Ni with Co, the electrooxidation reaction's activation energy and electrode stability both increase. With a starting potential of 0.22 V, the maximum current density in the Ni Co Mo/CNF sample was 99.8 mA/cm² at 0.6 V. To electrooxidize methanol, our electrocatalysts combine diffusion control with kinetic-limiting processes. This work has shown how to create an effective Ni Co Mo based methanol electrooxidation catalyst using a special technique.     

Catalytic C?C,C?N,and C?O Ullmann‐Type Coupling Reactions

Copper‐catalyzed Ullmann condensations are key reactions for the formation of carbon–heteroatom and carbon–carbon bonds in organic synthesis. These reactions can lead to structural moieties that are prevalent in building blocks of active molecules in the life sciences and in many material precursors. An increasing number of publications have appeared concerning Ullmann‐type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

Ethylenedithio?Tetrathiafulvalene?Helicenes: Electroactive Helical Precursors with Switchable Chiroptical Properties

Electroactive fused ethylenedithio? tetrathiafulvalene? [4]helicene and ‐[6]helicenes have been synthesized through a strategy that involved the preparation of 2,3‐dibromo‐helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single‐crystal X‐ray analysis, are 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. Their solid‐state architectures show interplay between S???S and π???π intermolecular interactions. The chiroptical properties of the enantiopure EDT? TTF? [6]helicene derivatives have been investigated and supported by TDDFT calculations. Remarkable redox switching of the circular dichroism (CD) signal between the neutral and radical‐cation species has been achieved.     

Studies on the Synthesis and Characterization of Pd?TiO2?SiO2 Nanocomposite for Electroanalytical Applications

A nanocomposite of Pd? TiO₂? SiO₂ is developed through a sol‐gel process from the reaction products of titanium isopropoxide followed by mixing the same with palladium linked 3‐glycidoxypropyltrimethoxysilane. The reaction product is sonicated and calcinated to obtain the nanocomposite of Pd? TiO₂? SiO₂. The calcination at 600 °C yielded an amorphous structure whereas at 900 °C it resulted into a nanocrystalline structure. The nanocomposite of palladium was further characterized by TEM, XRD, IR and EDS. The material acts as an efficient electrocatalyst. Electrocatalysis of ascorbic acid is observed at 0.1 V vs. Ag/AgCl, shows linearity between 1 µM and 1 mM in 0.1 M phosphate buffer (pH 7.0).     

The N?H Functional Group in Organometallic Catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst–substrate, ligand–ligand, ligand–catalyst, and catalyst–catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.     

B?H,C?H,and B?C Bond Activation: The Role of Two Adjacent Agostic Interactions

Tuning the nature of the linker in a L∼BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ‐C H and ε‐B_sp2 H, agostic interactions. Such a unique coordination mode stabilizes a 14‐electron “RuH₂P₂” fragment through connected σ‐bonds of different polarity, and affords selective B H, C H, and B C bond activation as illustrated by reactivity studies with H₂ and boranes.     

Nickel‐Catalyzed Enantioselective C?C Bond Formation through C?O Cleavage in Aryl Esters

We report the first enantioselective C C bond formation through C O bond cleavage using aryl ester counterparts. This method is characterized by its wide substrate scope and results in the formation of quaternary stereogenic centers with high yields and asymmetric induction.     

Oxindole Synthesis by Direct Coupling of C?H and C?H Centers

An sp ² /sp ³ get‐together : A novel and efficient method can be used to synthesize 3,3‐disubstitued oxindoles by the direct intramolecular oxidative coupling of an aryl C? H and a C? H center (see scheme; DMF=N,N‐dimethylformamide).

     

Density functional study of Fe2 ?N2

The interaction of two iron atoms with molecular nitrogen was studied by means of density functional techniques. Calculations were of the all-electron type, and both conventional local and gradient-dependent approximate (GDA) models were used. A ground state (GS) of linear structure was found for Fe₂-N₂ with 2S + 1 = 7; whereas a distorted tetrahedral structure, being also a septuplet, was located at 4.0 and 14.3 kcal/mol above the GS, at the local and GDA levels of theory, respectively. The N-N bond is moderately perturbed in the GS, but it is strongly activated in the tetrahedral mode: It has bond orders of 2.6 and 1.5, vibrational frequencies of 2148 and 1496 cm^-1, and equilibrium bond lengths of 1.14 and 1.24 Å, for the linear and tetrahedral geometries, respectively. These values are 3.0, 2359 cm^-1, and 1.095 Å, for free N₂. At GDA level of theory, the Fe₂-N₂ binding energy is 15 kcal/mol, which is bigger than that of Fe-N₂ (9 kcal/mol). The π-back donation, in the linear GS, is of 0.31 electrons, but the total charge transfer, from Fe₂ to N₂, is only 0.05 units. This is relevant in comparison with the tetrahedral mode, where the Fe₂ to N₂ total charge transfer is of 0.45 electrons, yielding a stronger activated N₂ moiety. © 1996 John Wiley & Sons, Inc.     

A theoretical investigation of Zn?Zn and Be?Be one electron bond

Attachment of one electron to 1,2-diBeX-benzene and 1,2-diZnX-benzene derivatives leads to the formation of stronger Be Be and Zn Zn interaction compared to the neutral one. This is reflected in the dramatic shortening of the Be Be and Zn Zn distance. The formation of these 2-center-1-electron bonds have also been confirmed by topological survey of electron density using quantum theory of atoms in molecules and electron localization function. The formation of these bonds is expected to render stability to these radical anions. These radical anions are stable toward electron detachment and computed bond dissociation energy values are also significant.