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.     

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.     

ToF‐SIMS multivariate characterization of surface modification of polymers by N2?H2 atmospheric pressure dielectric barrier discharge

Time of flight secondary ion mass spectrometry (ToF‐SIMS) is a powerful tool for the surface characterization of plasma‐modified surface. However, the SIMS fragmentation patterns of the resulting surface are quite complex and a full interpretation may be prohibitive. As a result, many studies are turning to multivariate statistical methods to simplify the interpretation. In this study, a principal component analysis (PCA) was used to obtain a more detailed interpretation of the surface modification of polymers by an atmospheric pressure plasma. The dataset was obtained from three polymers with different chemical compositions [namely, polyethylene, polyvinylidene fluoride, and poly(tetrafluoroethylene)], where each material was treated with an atmospheric pressure dielectric barrier discharge (DBD) in an atmosphere composed of different N₂/H₂ ratios. The results are discussed in terms of the suitability of ToF‐SIMS analysis combined with PCA for the discrimination between the three polymers and the possibility to create a predictive model that would describe the plasma surface modification, independent of the polymer substrate chemical composition. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

M4(CH2)4 Cubane‐Type Rare‐Earth Methylidene Complexes: Unique Reactivity toward Unsaturated C?O,C?N,and C?S Bonds

The reactivity of the cubane‐type rare‐earth methylidene complex [Cp′Lu(μ₃‐CH₂)]₄ ( 1 , Cp′=C₅Me₄SiMe₃) with various unsaturated electrophiles was investigated. The reaction of 1 with CO (1 atm) at room temperature gave the bis(ketene dianion)/dimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₂(μ₃,η²‐O‐C?CH₂)₂] ( 2 ) in 86 % yield through the insertion of two molecules of CO into two of the four lutetium–methylidene units. In the reaction with the sterically demanding N,N‐diisopropylcarbodiimide at 60 °C, only one of the four methylidene units in 1 reacted with one molecule of the carbodiimide substrate to give the mono(ethylene diamido)/trimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₃{iPrNC(=CH₂)NiPr}] ( 3 ) in 83 % yield. Similarly, the reaction of 1 with phenyl isothiocyanate gave the ethylene amido thiolate/trimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₃{PhNC(S)=CH₂}] ( 4 ). In the case of phenyl isocyanate, two of the four methylidene units in 1 reacted with four molecules of the substrate at ambient temperature to give the malonodiimidate/dimethylidene complex [Cp′₄Lu₄(μ₃‐CH₂)₂{PhN=C(O)CH₂(O)C?NPh}₂] ( 5 ) in 87 % yield. In this reaction, each of the two lutetium–methylidene bonds per methylidene unit inserted one molecule of phenyl isocyanate. All the products have been fully characterized by NMR spectroscopy, X‐ray diffraction, and microelemental analyses.     

Preparation of α,ω‐telechelic hexyl acrylate polymers with ?OH, ?COOH,and ?NH2 functional groups by RAFT

The design, synthesis, and use of two new, stable, functionalized chain transfer agents (CTA's) containing OH and amine end groups for the RAFT polymerization is reported: 2‐hydroxyethoxy‐carbonylphenylmethyl dithiobenzoate and 2‐(2‐(tert‐butoxycarbonyl)ethylamino)‐2‐oxo‐1‐phenylethyl benzodithioate, respectively. The RAFT polymerization of n‐hexyl acrylate (HA) using those CTA's, were compared to several other functionalized dithiobenzoate esters reported in the literature containing COOH and Ester groups. The performances of the dithiobenzoates were compared in terms of kinetics and molecular weight distribution control. Good control in polymerization of n‐hexyl acrylate with a linear increase of M_n with conversion mantaining polydispersity indices (PDI) below 1.1 was obtained by use of the new functionalized CTA's developed and also by use of some other CTA's tested, to produce well‐defined linear polymers with one specific chain‐end functionality: ? OH, ? COOH or Amine. Using a postpolymerization reaction with functionalized azocompounds in a 5 to 1 ratio, α,ω‐telechelic polymers, with ? OH or ? COOH as functional group at the second end were obtained. By using this synthetic strategy α,ω‐homotelechelic and heterotelechelic polymers were readily prepared. The chemical availability of functional end‐groups in the telechelics was demonstrated by reaction with methacrylic anhydride. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3033–3051, 2010     

Modeling the H bond donor strength of ?OH, ?NH,and ?CH sites by local molecular parameters

A quantum chemical model is introduced to predict the H‐bond donor strength of monofunctional organic compounds from their ground‐state electronic properties. The model covers ? OH, ? NH, and ? CH as H‐bond donor sites and was calibrated with experimental values for the Abraham H‐bond donor strength parameter A using the ab initio and density functional theory levels HF/6‐31G** and B3LYP/6‐31G**. Starting with the Morokuma analysis of hydrogen bonding, the electrostatic (ES), polarizability (PL), and charge transfer (CT) components were quantified employing local molecular parameters. With hydrogen net atomic charges calculated from both natural population analysis and the ES potential scheme, the ES term turned out to provide only marginal contributions to the Abraham parameter A, except for weak hydrogen bonds associated with acidic ? CH sites. Accordingly, A is governed by PL and CT contributions. The PL component was characterized through a new measure of the local molecular hardness at hydrogen, η(H), which in turn was quantified through empirically defined site‐specific effective donor and acceptor energies, EE_occ and EE_vac. The latter parameter was also used to address the CT contribution to A. With an initial training set of 77 compounds, HF/6‐31G** yielded a squared correlation coefficient, r², of 0.91. Essentially identical statistics were achieved for a separate test set of 429 compounds and for the recalibrated model when using all 506 compounds. B3LYP/6‐31G** yielded slightly inferior statistics. The discussion includes subset statistics for compounds containing ? OH, ? NH, and active ? CH sites and a nonlinear model extension with slightly improved statistics (r² = 0.92). © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

Protonation of 2H‐Azaphosphirene Complexes: P?N Bond Activation and Ring‐Expansion Reactions

P? N bond activation of 2H‐azaphosphirene complexes 1 and 2 by using triflic acid led to ring expansion in the presence of nitriles. In the absence of nitriles, the reaction surprisingly afforded two haptomeric N‐protonated 1‐aza‐3‐phospha‐butadiene complexes in the case of complex 1 , whereas the N‐protonated 2H‐azaphosphirene complex [H‐ 2 ]⁺ was characterized by NMR spectroscopy.

     

Matrix Infrared Spectroscopy and Quantum‐Chemical Calculations for the Coinage‐Metal Fluorides: Comparisons of Ar?AuF,Ne?AuF,and Molecules MF2 and MF3

The reactions of laser‐ablated Au, Ag, and Cu atoms with F₂ in excess argon and neon gave new absorptions in the M? F stretching region of their IR spectra, which were assigned to metal‐fluoride species. For gold, a Ng? AuF bond was identified in mixed neon/argon samples. However, this bonding was much weaker with AgF and CuF. Molecules MF₂ and MF₃ (M=Au, Ag, Cu) were identified from the isotopic distribution of the Cu and Ag atoms, comparison of the frequencies for three metal fluorides, and theoretical frequency calculations. The AuF₅ molecule was characterized by its strongest stretching mode and theoretical frequency calculations. Additional evidence was observed for the formation of the Au₂F₆ molecule.     

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).     

Bifunctional transition metal‐based molecular catalysts for asymmetric C?C and C?N bond formation

This paper describes the recent advances in the conceptually new bifunctional Ir and Ru catalysts for asymmetric catalytic reactions. These reactions include the enantioselective Michael addition of 1,3‐dicarbonyl compounds to cyclic enones and nitroalkenes, and the enantioselective direct amination of α‐cyanoacetates with diazoesters. The outcome of these reactions in terms of reactivity and selectivity was delicately influenced by the catalyst structures and the reaction conditions including the solvents used. Even with a 1 : 1 molar ratio of donors to acceptors, the reactions proceeded smoothly to give the corresponding chiral adducts with an excellent yield and enantiomeric excess (ee). Preliminary mechanistic studies showed that the key stage of the catalytic cycle is the interaction of the bifunctional catalyst with a pronucleophilic reagent that leads to stereoselective formation of C‐, O‐, or N‐bound complexes. The resulting protonated catalyst bearing metal‐bound nucleophiles readily reacts with electrophiles to provide C? C and C? N bond formation products in a highly stereoselective manner. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 106–123; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20172