CH bond activation in the presence or absence of oxygen or nitrous oxide

The reaction of C₂H₆with lattice oxygen, O^2- (in the absence of gaseous oxygen), or “adsorbedℍ oxygen (in the presence of gaseous oxygen) over NiMoO₄ catalysts has been performed and compared to C₃H₈ activation. The results obtained indicate that adsorbed oxygen exhibits a higher reactivity to C₂H₆, while lattice oxygen is more reactive relative to C₃H₈. Kinetic studies of these two reactions in presence of molecular oxygen have indeed shown that the ethane oxidative dehydrogenation (ODH) is dependent on the oxygen partial pressure, whilst on the contrary propane ODH is not. In order to confirm the presence of “adsorbed” oxygen for ethane activation, ODH tests have been performed with N₂O. On increasing temperature, the O^- adsorbed species enhances the mild oxidation of ethane. The activation energy of ethane consumption E_C2H6, relative to propane (E_C3H8 = 133 kJ/mol) is 145 kJ/mol. A possible mechanism is proposed for the oxidative dehydrogenation of ethane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanism of inhibition of hydrogen/oxygen flames of various compositions by trimethyl phosphate

The effect of the catalytic recombination reactions of H and OH^? involving phosphorus-containing products of trimethyl phosphate (TMP) combustion on the burning velocity and the structure of H₂/O₂/N₂ flames at atmospheric pressure has been investigated. An earlier mechanism for inhibition of rich hydrogen/oxygen flames by organophosphorus compounds has been tested and modified by comparing experimental data with the results of simulation. The sensitivity analysis of the calculated flame speed to the rate constants of chain branching reactions and chain termination reactions involving phosphorus-containing compounds has revealed significant specific features of the inhibition mechanism of hydrogen flames with various stoichiometries and dilution ratios. Unlike the inhibition efficiency of hydrocarbon flames, in which the reactions of H and OH^? radicals with PO, PO₂, HOPO, and HOPO₂ play the key role, the inhibition efficiency of hydrogen flames at atmospheric pressure is determined by the interaction of hydrogen and oxygen atoms with TMP and with organophosphorus products of its decomposition in the low-temperature zone of the flame. The sensitivity analysis has demonstrated that, as the equivalence ratio (?) or the dilution ratio is increased, the ratio of the chain branching rate to the rate of chain termination via reactions involving phosphorus compounds decreases. As a consequence, the efficiency of inhibition of H₂/O₂/N₂ flames, as distinct from that of hydrocarbon flames, increases as ? is raised from 1.1 to 3.0 and as the mixture is progressively diluted with nitrogen.     

Iron‐Catalyzed Aerobic Oxidation of Aldehydes: Single Component Catalyst and Mechanistic Studies

An aerobic oxidation of aldehydes towards carboxylic acids in MeCN using 1 atm of pure oxygen or oxygen in air as the oxidant and a catalytic amount of single component catalyst, Fe(NO₃)₃ · 9H₂O, has been developed. Carboxylic acids with different synthetically useful functional groups were obtained at room temperature. Two mechanistic pathways have been proposed based on isotopic labeling, NMR monitoring, and control experiments. The practicality of this reaction has been demonstrated by conducting several 50 mmol‐scale reactions using pure oxygen or an air‐flow of ~30 mL/min.     

Oxidation of the 1-naphthyl radical C10H7• with oxygen: Thermochemistry,kinetics, and possible reaction pathways

The reaction of the 1-naphthyl radical C₁₀H₇• (A2•) with molecular (³O₂) and atomic oxygen, as part of the oxidation reactions of naphthalene, is examined using ab-initio and DFT quantum chemistry calculations. The study focuses on pathways that produce the intermediate final products CO, phenyl and C₂H₂, which may constitute a repetitive reaction sequence for the successive diminution of six-membered rings also in larger polycyclic aromatic hydrocarbons. The primary attack of ³O₂ on the 1-naphthyl radical leads to a peroxy radical C₁₀H₇OO• (A2OO•), which undergoes further propagation and/or chain branching reactions. The thermochemistry of intermediates and transition state structures is investigated as well as the identification of all plausible reaction pathways for the A2• + O₂ / A2• + O systems. Structures and enthalpies of formation for the involved species are reported along with transition state barriers and reaction pathways. Standard enthalpies of formation are calculated using ab initio (CBS-QB3) and DFT calculations (B3LYP, M06, APFD). The reaction of A2• with ³O₂ opens six main consecutive reaction channels with new ones not currently considered in oxidation mechanisms. The reaction paths comprise important exothermic chain branching reactions and the formation of unsaturated oxygenated hydrocarbon intermediates. The primary attack of ³O₂ at the A2• radical has a well depth of some 50 kcal mol⁻¹ while the six consecutive channels exhibit energy barriers below the energy of the A2• radical. The kinetic parameters of each path are determined using chemical activation analysis based on the canonical transition state theory calculations. The investigated reactions could serve as part of a comprehensive mechanism for the oxidation of naphthalene. The principal result from this study is that the consecutive reactions of the A2• radical, viz. the channels conducting to a phenyl radical C₆H₅•, CO₂, CO (which oxidized to CO₂) and C₂H₂ are by orders of magnitude faster than the activation of naphthalene by oxygen (A2 + O₂ → A2• + HO₂).     

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O₂ would successfully compete with H₂ for surface adsorption sites due to the grater attractive force acting on the O₂ molecule compared to H₂. The H₂ dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H₂ + O₂, other practical systems were considered, that is, H₂ + CO and CH₄. Calculations were performed for the CO chemisorption and H₂ dissociation on iron surface with predissociated CO, as well as, CH₄ surface dissociation. The results indicate that CO inhibition of H₂ dissociation proceeds via similar mechanism to O₂ induced inhibition, whereas CH₄ traces in the H₂ gas have no effect on H₂ dissociation. © 2014 Wiley Periodicals, Inc.     

Identification of corona discharge-induced SF6 oxidation mechanisms using SF6/18O2/H2 16O and SF6/16O2/H2 18O gas mixtures

The absolute yields of gaseous oxyfluorides SOF₂, SO₂F₂, and SOF₄ from negative, point-plane corona discharges in pressurized gas mixtures of SF₆ with O₂ and H₂O enriched with¹⁸O₂ and H₂ ¹⁸O have been measured using a gas chromatograph-mass spectrometer. The predominant SF₆ oxidation mechanisms have been revealed from a determination of the relative¹⁸O and¹⁶O isotope content of the observed oxyfluoride by-product. The results are consistent with previously proposed production mechanisms and indicate that SOF₂ and SO₂F₂ derive oxygen predominantly from H₂O and O₂, respectively, in slow, gas-phase reactions involving SF₄, SF₃, and SF₂ that occur outside of the discharge region. The species SOF₄ derives oxygen from both H₂O and O₂ through fast reactions in the active discharge region involving free radicals or ions such as OH and O, with SF₅ and SF₄.     

Electron transfer reactions of some hindered silicon-containing phenols with phenylmercuric hydroxide

Reactions between hindered silicon-containing phenols and C₆H₅HgOH have been studied. These reactions, as shown by ESR-spectra, proceed via the intermediate formation of phenoxyls ansing from an electron transfer from the phenol to the phenylmercury cation. In the reactions of C₆H₅HgOH with silicon-containing phenols [2,6-bis(trimethylsilyl)-4-tert-butyl-, 2,4-bis(trimethylsilyl)-6-tert-butyl-, and 2-phenyldimethylsilyl-4,6-di-tert-butyl-phenols] which have ortho-triorganosilyl group capable of migrating to the phenoxyl oxygen, mercurated products have been formed. In the interaction between C₆H₅HgOH and 2,6-di-tert-butyl-4-trimethylsilylphenol, which has a para-trimethylsilyl group which is unable to migrate to the oxygen, no mercurization occurs; phenol oxidation only was observed.     

Mechanistic Study of Ozone-Water Reaction and Their Reaction Products with Gas Phase Elemental Mercury:A Computational Study

The interaction of oxygen of water and central oxygen of ozone produces stable H₂O‐O₃ complex with no barrier. With decomposition of this complex through H‐abstraction by O₃ and O‐abstraction by H₂O, four possible product channels have been found. The reaction of mercury and the products of water‐ozone reaction have been studied. All geometrical and AIM parameters of intermediate, transition states, and the products of reactions are calculated and thermodynamic parameters are obtained. The negative value of free energy show that channels Hg+H₂OO, Hg+H₂O₂ and Hg+H₂O₄ in hydrogen tetroxide form (HTO) may be the main reaction channels.     

Mass spectrometric fragmentation of saturated six membered heterocyclic systems containing two chalcogen family atoms

The mass spectra of six-membered saturated heterocycles containing oxygen, sulphur, selenium and tellurium in the 1,4-positions have been measured. The differing fragmentation modes have been characterized using high resolution, low voltage and metastable ion scan techniques. The important decomposition reactions of the molecular ions involve elimination of C₂H₄ and CH₂X (X is a chalcogen atom) and formation of [C₂H₄X]⁺ and C₂H₅⁺. The propensities of these reactions vary systematically as a function of the ability of the chalcogen to stabilize a positive charge.     

An in depth study of the formation of new tetrathiafulvalene derivatives from 1,8-diketones

A detailed study of the reactions of phosphorus pentasulfide and Lawesson's reagent with a series of 4,5-bis(RCOCH₂S)-1,3-dithiole-2-thiones (R=Ph, 4-MeOC₆H₄, 4-Br C₆H₄, Me) has been carried out. These reactions lead to fusion of either an unsaturated 1,4-dithiin ring or a thiophene to the dithiole; the former in higher yield, while the latter is a significant product in the reactions with Lawesson's reagent; as well as small amounts of minor products. A mechanistic rationalization of these products is discussed in some detail. The new fused dithioles have been converted to novel series of fused TTF derivatives.     

Structure and mechanism of fragmentation of [C2H5O]+

The decomposition reactions of [C₂H₅O]⁺ ions produced by dissociative electron-impact ionization of 2-propanol have been studied, using ¹³C and deuterium labeling coupled with metastable intensity studies. In addition, the fragmentation reactions following protonation of appropriately labeled acetaldehydes and ethylene oxides with [H₃]⁺ or [D₃]⁺ have been investigated. In both studies particular attention has been paid to the reactions leading to [CHO]⁺, [C₂H₃]⁺ and [H₃O]⁺. In both the electron-impact-induced reactions and the chemical ionization systems the fragmentation of [C₂H₅O]⁺ to both [H₃O]⁺ and [C₂H₃]⁺ proceeds by a single mechanism. For each case the reaction involves a mechanism in which the hydrogen originally bonded to oxygen is retained in the oxygen containing fragment while the four hydrogens originally bonded to carbon become indistinguishable. The fragmentation of [C₂H₅O]⁺ to produce [CHO]⁺ proceeds by a number of mechanisms. The lowest energy route involves complete retention of the α carbon and hydrogen while a higher energy route proceeds by a mechanism in which the carbons and the attached hydrogens become indistinguishable. A third distinct mechanism, observed in the electron-impact spectra only, proceeds with retention of the hydroxylic hydrogen in the product ion. Detailed fragmentation mechanisms are proposed to explain the results. It is suggested that the [C₂H₅O]⁺ ions formed by protonation of acetaldehyde or ionization of 2-propanol are produced initially with the structure [CH₃CH?\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}H] (a), but isomerize to [CH₂?CH? \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}H₂] (e) prior to decomposition to [C₂H₃]⁺ or [H₃O]⁺. The results indicate that the isomerization a → e does not proceed directly, possibly because it is symmetry forbidden, but by two consecutive [1,2] hydrogen shifts. A more general study of the electron-impact mass spectrum of 2-propanol has been made and the fragmentation reactions proceeding from the molecular ion have been identified.     

A mild oxidative aryl radical addition into alkenes by aerobic oxidation of arylhydrazines

A mild and practical oxyarylation of alkenes by oxidative radical addition has been developed by using aerobic oxidation of hydrazine compounds. The use of a catalytic amount of potassium ferrocyanide trihydrate (K₄[Fe(CN)₆]?3 H₂O) and water accelerated this radical reaction to give peroxides or alcohols from simple alkenes in good yields. The environmentally friendly and economical radical reactions were achieved at room temperature in the presence of iron catalyst, oxygen gas, and water. A method involving aniline as a radical precursor is also described.     

Different hydrogen-bonding patterns in two [Zn(ENTPP)] complexes with water or methanol as ligands

We have synthesized zinc complexes of H₂ENTPP (5-(8-ethoxycarbonyl-1-naphthyl)-10,15,20-triphenyl porphyrin) as a model to study hydrogen-bonding interactions. When water or methanol is a ligand, crystals of [Zn(ENTPP)(CH₃OH)] or [Zn(ENTPP)(H₂O)]?·?C₆H₅CH₃ were obtained. In both structures, the ligand has hydrogen-bonding interactions, but in different patterns. In [Zn(ENTPP)(CH₃OH)], the methanol oxygen and carboxylate oxygen in the naphthyl group form an intermolecular hydrogen bond. In [Zn(ENTPP)(H₂O)]?·?C₆H₅CH₃, there are two independent molecules A and B. In molecule B, there is an intramolecular hydrogen bond between the water oxygen and the carboxylate oxygen, while in molecule A, besides the intramolecular hydrogen bond, there is an intermolecular hydrogen bond between the water oxygen and the carboxylate oxygen. ¹H NMR spectra suggest the binding of methanol or water to zinc are equilibrium processes in solution. Equilibrium constant has been determined by UV-Vis measurements, and it suggests the binding affinity of zinc to methanol has been moderately increased.     

Syntheses, structure and intercalation properties of low-dimensional phenylarsonates, A(HO3AsC6H5)(H2O3AsC6H5) (A = Tl, Na, K and Rb)

Four new low-dimensional phenylarsonates, A(HO₃AsC₆H₅)(H₂O₃AsC₆H₅) (A = Tl(1), Na(2), K(3) and Rb(4)), have been synthesized and characterized by X-ray diffraction, spectroscopic and thermal studies. They crystallize in triclinic unit cells and have approximately planar arrangement of A⁺ ions, coordinated to oxygen atoms of phenylarsonates, on both sides. Structure of thallium phenylarsonate as determined by single crystal X-ray diffraction, is one-dimensional, whereas those of alkalimetal analogues are two-dimensional. Successful intercalation reactions of compounds1 and2 with primaryn-alkyl amines have been demonstrated. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Quantum Dot Assembly for Light‐Driven Multielectron Redox Reactions,such as Hydrogen Evolution and CO2 Reduction

Light‐driven multielectron redox reactions (e.g., hydrogen (H₂) evolution, CO₂ reduction) have recently appeared at the front of solar‐to‐fuel conversion. In this Minireview, we focus on the recent advances in establishing semiconductor quantum dot (QD) assemblies to enhance the efficiencies of these light‐driven multielectron reduction reactions. Four models of QD assembly are established to promote the sluggish kinetics of multielectron transfer from QDs to cocatalysts, thus leading to an enhanced activity of solar H₂ evolution or CO₂ reduction. We also forecast the potential applications of QD assemblies in other multielectron redox reactions, such as nitrogen (N₂) fixation and oxygen (O₂) evolution from H₂O.     

Quantum Dot Assembly for Light‐Driven Multielectron Redox Reactions,such as Hydrogen Evolution and CO2 Reduction

Light‐driven multielectron redox reactions (e.g., hydrogen (H₂) evolution, CO₂ reduction) have recently appeared at the front of solar‐to‐fuel conversion. In this Minireview, we focus on the recent advances in establishing semiconductor quantum dot (QD) assemblies to enhance the efficiencies of these light‐driven multielectron reduction reactions. Four models of QD assembly are established to promote the sluggish kinetics of multielectron transfer from QDs to cocatalysts, thus leading to an enhanced activity of solar H₂ evolution or CO₂ reduction. We also forecast the potential applications of QD assemblies in other multielectron redox reactions, such as nitrogen (N₂) fixation and oxygen (O₂) evolution from H₂O.     

DFT Studies on the addition reaction between germylene and furan

The mechanism of the addition reaction of germylene H₂Ge and furan has been investigated with B3LYP/6-311+G* method, including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. From the surface energy profile, it can be predicted that there are four reactions (1)–(4). Reactions (1) and (2) are similar, which are reactions between H₂Ge and C=C of isolated furan. Furthermore, H₂Ge can react with oxygen atom of furan to form a stable complex H₂Ge-Furan. Reactions (3) and (4) are similar, which are reactions between H₂Ge and C=C of the complex H₂Ge-Furan. All the reactions consist of two steps: the two reactants first form an intermediate (INT) through a barrier-free exothermic reaction; this intermediate then isomerizes to a product (P) via a transition state (TS). The article is published in the original.     

三维有序大孔钙钛矿型氧化物LaFeO3的合成及甲烷化学链重整性能

采用无皂乳液聚合法制备了聚苯乙烯(PS)聚合物微球,并采用胶晶模板法制备了三维有序大孔3DOM LaFeO₃钙钛矿型氧化物.通过扫描电镜、X射线衍射和傅里叶变换红外光谱等手段对氧化物的性能进行了表征.利用程序升温还原和多次氧化还原循环反应评价了氧化物的反应性,并在固定床反应器上研究了其甲烷氧化性能.结果表明,与离心法和蒸发法相比,垂直沉积法获得的PS微球模板排列更均匀有序;前驱物溶剂及浓度对最终的三维有序大孔材料的结构有显著影响,利用乙醇为前驱物溶剂所制备的样品比利用乙烯为溶剂的样品具有更好的三维有序大孔结构,前驱物乙醇溶液浓度在1.0 mol/L为宜.甲烷氧化实验表明,3DOM-LaFeO₃钙钛矿型氧化物中存在两种氧物种:表面吸附氧和体相晶格氧.表面吸附氧主要在反应初期将甲烷完全氧化为CO₂和水蒸汽,而体相晶格氧主要将甲烷部分氧化为H₂和CO.在甲烷部分氧化反应中,三维有序大孔LaFeO₃钙钛矿型氧化物比相同质量的纳米LaFeO₃氧化物提供了更多的氧,并且可使甲烷在较宽的反应阶段生成H₂和CO摩尔比为2:1的合成气,从而更有利于后续的费托合成等工艺.     

Rates of reaction of atomic oxygen with C2H3F,C2H3Cl,C2H3Br, 1,1-C2H2F2, and 1,2-C2H2F2

Rate constants for the reactions of atomic oxygen (O³P) with C₂H₃F, C₂H₃Cl, C₂H₃Br, 1,1-C₂H₂F₂, and 1,2-C₂H₂F₂ have been measured at 307°K using a discharge-flow system coupled to a mass spectrometer. The rate constants for these reactions are (in units of 10¹¹ cm³ mole^?1 s^?1) 2.63 ± 0.38, 5.22 ± 0.24, 4.90 ± 0.34, 2.19 ± 0.18, and 2.70 ± 0.34, respectively. For some of these reactions, the product carbonyl halides were identified.     

Hydrogen peroxide-containing supramolecular structures based on Zn-Al layered double hydroxides and their modified analogues

Optimum conditions have been found for the synthesis of peroxide-containing supramolecular structures through the reactions of Zn-Al layered double hydroxide (LDH) and its modified analogues with hydrogen peroxide solutions. The supramolecular structures based on Zn-Al LDH are stable and contain 15% active oxygen (31.8% H₂O₂). A peroxide formation mechanism is suggested.