Reactions of alkoxy and peroxy radicals with carbon monoxide

Experimental rate constants of the reactions HO^· + CO → H^· + CO₂, RO^· + CO → R^· + CO₂, HO ₂ ^· + CO → HO^· + CO₂, and RO ₂ ^· + CO → RO^· + CO₂ are analyzed in the framework of the intersecting-parabolas model. The transition states of the additions of the methoxy and methylperoxy radicals to carbon monoxide were calculated by quantum-chemical methods. The reactions occur in two consecutive steps: first the HO^· (RO^·, RO ₂ ^· ) radical adds to CO and then the resulting unstable intermediate radical decomposes to evolve CO₂. The kinetic parameters of these reactions are calculated by two methods (using the intersecting-parabolas model and the quantum-chemical method). The activation energies and rate constants of a series of R _i O^· + CO and R _i O ₂ ^· + CO reactions are calculated. A comparison of the kinetic parameters suggests close similarity between the transition states in the additions of the O-centered radicals to CO and olefins.     

Mechanism of propagation and degenerate chain branching in the oxidation of polypropylene and polyethylene

Samples of polypropylene with adjacent and isolated hydroperoxide groups have been prepared. The rate constants of free-radical formation from solid hydroperoxides were measured by the inhibitor method. It was found that the free radicals yielded by adjacent hydroperoxide groups are formed more rapidly. The main reaction of free-radical formation in oxidized polypropylene is of the type: ROOH + ROOH → RO + H₂O + RO₂^˙. The average yield of free radicals from polypropylene hydroperoxide is 2–4%. Oxygen has no effect on the yield of free radicals. However, the pressure of oxygen Po₂ affects the rate of degenerate chain branching in polypropylene. The number of adjacent hydroperoxide groups and the rate of initiation increase with Po₂. Consequently, a reaction of the type, R^˙, + RH → RH + R^˙, plays an important part in transport of free valence through solid polymer. This reaction is very fast in polyethylene, and no adjacent hydroperoxide groups are formed. The free radicals from polyethylene hydroperoxide are found to form by a reaction of the type: ROOH → RO^˙ + HO^˙.     

Free-radical decarboxylation of carboxylic acids as a concerted abstraction and fragmentation reaction

Two variants of the reaction of radicals with the carboxyl group of carboxylic acids, namely, RO ₂ ^? + R _i COOH → ROOH + R _i CO ₂ ^? and RO _i ^? + R _j COOH → ROOH + R _i ^? + CO₂ are theoretically analyzed. It is demonstrated by the intersecting-parabolas method that if the reaction proceeded via the formation of an intermediate carboxyl radical, it would be much slower than is actually observed. Quantum-chemical calculations carried out by the density functional method using the nonempirical functional PBE have shown that the reactions of the methyl radical with the carboxyl group of acetic, butyric and vinylacetic acids include concerted H atom abstraction and C-C bond breaking. In the framework of the intersecting-parabolas model, an algorithm has been developed to calculate the activation energy and rate constant for X^? + R _i COOH → XH + CO₂ + R _i ^? reactions, where X = R^?, RO^?, HO^?, ArO^?, Ar₂N^? or H^?     

Elementaey rate constants of radical reactions in cyclohexanol undergoing oxidation

Conclusions The elementary rate constants of the reactions of RO₂ ^. + RH and RO₂ ^. +RO₂ ^. in cyclohexanol undergoing oxidation and in mixtures of cyclohexanol with chlorobenzene were measured by the method of the photochemical aftereffect.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 1737–1743, October, 1966.     

Autocatalysis by the nitroxyl radical in the cyclic mechanism of chain termination in polymer oxidation

The activation energy and rate constant of the reaction between the nitroxyl radical and N-alkoxyamine as a concerted abstraction–fragmentation reaction have been calculated using the intersecting parabolas model. This reaction proceeds fairly rapidly and leads to nitroxyl radical autoregeneration as a result of the following consecutive reactions:AmO^? + AmOR → AmOH + >C=O + Am^?, RO ₂ ^? + AmOH → ROOH + AmO^?, Am^?+ O₂ → Am ₂ ^? , and 2AmO ₂ ^? → 2AmO^? + O₂. Thus, the nitroxyl radical is an effective radical catalyst for its own regeneration from N-alkoxyamine. The rates of regeneration of the nitroxyl radical from its N-alkoxyamine under the action of alkyl, alkoxyl, peroxyl, nitroxyl, and hydroperoxyl radicals under conditions of polypropylene oxidation inhibited by the nitroxyl radical are compared. It is demonstrated that only peroxyl, hydroperoxyl, and nitroxyl radicals are involved in AmO^? regeneration from AmOR.     

Effect of Mg2+ Ions on the Stability of PolyA/2PolyU Three-Stranded Helices in Aqueous Solutions

The influence of Mg²⁺, Na⁺ and temperature on the conformational state of three-stranded helical polyA/2polyU (A2U) has been studied by the thermal denaturation method. At Na⁺ concentrations of 0.01–0.1 M , on heating the transition A2U→AU+U (the 3→2 transition) and then AU→A+U transition (the 2→1 transition) are observed. (AU is double helix polyA/polyU; A and U are single-stranded polyA and polyU, respectively.) With 0.01 M and 0.03 M Na⁺ these transitions occur at Mg²⁺ concentrations within (0 ÷ 0.003) M . At these ionic concentrations, there is a narrow temperature region (3 ÷ 5°C) at which double-helical AU formed by the 3→2 transition is resistant to heating. In 0.1 M Na⁺, a rise in the Mg²⁺ concentration leads to a continuous decrease in the temperature range of this region, and above a critical concentration of Mg²⁺ (ca. 3.6×10^–5 M )_cr there is only one transition (the 3→1 transition) instead of the successive transitions 3→2→1. The constants of Mg²⁺ ion association with polyU, polyA and A2U were calculated using equilibrium binding theory. The data obtained helped explain the reasons for the different phase diagrams for A2U + Mg²⁺ complexes in solution at high and low Na⁺ concentrations.     

Experimental study of the D + H2(ν = 1) reaction

The D + H₂(ν = 1) reaction, D + H₂(ν = 1) → K_a HD(ν = 1) + H, → K_n HD(ν = 0) + H, → K_r D + H₂(ν = 0) has been studied. The measurements were made in a flow-tube apparatus at 300 K. Vibrationally excited H₂ was generated in a furnace and D atoms in a microwave discharge. EPR and thermometric techniques were used for the detection of D and H atoms and H₂(ν = 1). The product branching rate constants (in CM³/Molecule s) were found to be K_a = (10.7 ± 4.1) × 10^?13. K_n = (5.4 ± 2.7) × 10^?13, K_r, < 2.7 × 10^?13.     

Field measurement of the organic peroxy radicals by the low-pressure reactor plus laser-induced fluorescence spectroscopy

A low-pressure reactor (LPR) was developed for the measurement of ambient organic peroxy (RO₂) radicals with the use of the laser-induced fluorescence (LIF) instrument. The reactor converts all the RO_x (= RO₂ + HO₂ + RO + OH) radicals into HO₂ radicals. It can conduct different measurement modes through altering the reagent gases, achieving the speciated measurement of RO₂ and RO₂^# (RO₂ radicals derived from the long-chain alkane, alkene and aromatic hydrocarbon). An example of field measurement results was given, with a maximum concentration of 1.88 × 10⁸ molecule/cm³ for RO₂ and 1.18 × 10⁸ molecule/cm³ for RO₂^#. Also, this instrument quantifies the local ozone production rates directly, which can help to deduce the regional ozone control strategy from an experimental perspective. The new device can serve as a potent tool for both the exploration of frontier chemistry and the diagnosis of the control strategies.     

EPR kinetic study of the reactions of CF3Br with H atoms and OH radicals

Reactions of CF₃Br with H atoms and OH radicals have been studied at room temperature at 1–2 torr pressures in a discharge flow reactor coupled to an EPR spectrometer. The rate constant of the reaction H + CF₃Br → CF₃ + HBr (1) was found to be k₁ = (3.27 ± 0.34) × 10^?14 cm³/molec·sec. For the reaction of OH with CF₃Br (8) an upper limit of 1 × 10^?15 cm³/molec·sec was determined for k₈. When H atoms were in excess compared to NO₂, used to produce OH radicals, a noticeable reactivity of OH was observed as a result of the reaction OH + HBr → H₂O + Br, HBr being produced from reaction (1).     

Stereochemistry and transformation of dihydrothebain in the bain synthesis from labelled thebaines

The complete stereochemistry of the Δ⁶-dihydrothebaine → 7-bromodihydrocodeinone dimethyl ketal → codeinone dimethyl ketal → thebaine sequence of transformations has been elucidated by NMR studies of the isotopically labeled compounds prepared using diimide-d₂, methyl hypobromite-[¹⁴C], and methyl hypobromite-d₃. Final elimination of methanol from codeinone dimethyl ketal proceeds in a stereospecific manner: cis under acid catalysis with POC1₃, and trans under alkaline catalysis with EtO^?. As a result of these reactions, variously isotopically labeled thebaines can be prepared.     

Spectroscopic studies of charge-transfer complexation of iodine with a new benzo-substituted macrocyclic diamide in chloroform, dichloromethane and their 1:1 mixture

Charge-transfer complexation of iodine with a new benzo-substituted macrocyclic diamide 5,6,7,8,9,10-hexahydro-2H-1,13,4,7,10-benzodioxatriazacyclopentadecine-3,11(4H,12H)-dione (L) with iodine was studied spectrophotometrically in chloroform, dichloromethane and their 1:1 (v/v) mixture. The observed time dependence of the charge-transfer band and subsequent formation of I₃ ^- ion are related to the slow formation of the initially formed 1:1 L.I₂ outer complex to an inner electron donor-acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion, as follows: L + I₂ → L.I₂ (outer complex), fast L.I₂ (outer complex) → (L.I⁺)I^- (inner complex), slow (L.I⁺)I^- (inner complex) + I₂ → (L.I⁺)I₃ ^-, fast The pseudo-first-order rate constants for the transformation process were evaluated in different solvent systems. The stability constants of the resulting EDAr complexes were also evaluated and the solvent effect on their stability is discussed. The resulting complexes were isolated and characterized by FTIR and ¹H NMR spectroscopy.     

Hot-atom initiated chemical laser study of the H* + SF6 reaction

Chemical laser methods have been used to study the “hot-atom” reaction H* + SF₆ → products at KE_rel < 102 kcal mole^?1. Collision-induced dissociation of SF₆ by H* is the dominant reaction channel. Internal excitation of SF₆ may be required to promote the abstraction reaction: H + SF₆ → HF + SF₅.     

Radical reactions in the coordination field of transition metals. VII—H-transfer between phenolic and aryl-aminic H-donors and their radicals

An ESR method for studying the mechanism of H-transfer reactions between H-donors of different reactivity (A₁H, A₂H…) and their free radicals (A₁; A₂^.…) in non-polar solvents at ambient temperature is presented. The new technique is based on a pulsed initiation of various secondary phenoxy or nitroxy radicals in binary mixtures of hindered phenols, unhindered phenols, partially hindered thiobisphenols and diphenylamine, employing a high concentration of free RO₂^. and coordinated (Co^III)RO₂^. tert-butyl peroxy radicals generated in the redox-reaction of Co(acac)₂ with tert-butyl hydroperoxide. The consecutive H-transfer reactions proceed to equilibrium until the most stable radicals are formed. In this way criteria are obtained for ranking the compared free and coordinated phenoxy radicals according to their relative stabilities. The secondarily generated phenoxy radicals from unhindered phenols after coordination to Co^III are stabilized and cannot take part in further H-transfer reactions.     

Influence of the preparation history of α-Fe2O3 on its reactivity for hydrogen reduction

TG experiments on the hydrogen reduction of α-Fe₂O₃ were carried out to elucidate the influence of the preparation history of the oxide on its reactivity. α-Fe₂O₃ samples were prepared by the thermal decomposition of seven iron salts in a stream of oxygen, air or nitrogen at temperatures of 500–1200°C for 1 h. Thirteen metal ions such as Cu²⁺, Ni²⁺, etc. were used as doping agents. The reactivity of the oxide was indicated by the initial reduction temperature (T_i. α-Fe₂O₃ prepared at lower temperatures showed lower T_i values and the reduction proceeded stepwise (Fe₂O₃ → Fe₃O₄ → Fe). T_i values increased with the rise in the preparation temperature of the oxide. The oxides prepared at higher temperatures showed that two reduction steps (Fe₂O₃ → Fe₃O₄ → Fe) proceed simultaneously. the preparation in oxygen gave higher T_i than that in air or nitrogen. The doping by metal ions, except Ti⁴⁺, lowered the T_i of α-Fe₂O₃. The Cu²⁺ ion showed the lowest T_i, while Ti⁴⁺ showed the highest T_i and the inhibition effect.The reduction process was expressed by two equations; Avrami—Erofeev's equation for α-Fe₂O₃ → Fe₃O₄ and Mampel's equation for Fe₃O₄ → Fe.     

Highly selective oxidation of amines to imines by Mn2O3 catalyst under eco-friendly conditions

Enhancing the selectivity of imines for the oxidative self-coupling of primary amines was found to be challenging in the heterogeneous catalysis. Three different manganese oxides (M-3, M-4, M-5) were synthesized by controlling the calcination temperature using a simple template-free oxalate route. The prepared manganese oxides were systematically characterized using XRD, N₂ sorption, SEM, TEM, XPS, H₂-TPR techniques. M-4 gave 96.2% selectivity of imine at 100% conversion of benzylamine, which was far more superior than other existing protocols. Mn³⁺/Mn⁴⁺ ratio was found to affect the selectivity of the imines. The probable reaction pathway for amines oxidation catalyzed by manganese oxides was proposed for the first time.     

The Formation of OH*(<Superscript>2</Superscript>Σ<Superscript>+</Superscript>) Radical in the Reaction of Hydrogen with Oxygen behind a Shock Wave in Nonequilibrium Conditions

The mechanism of formation of the electronically excited radical OH*(A²Σ⁺) has been studied by analyzing calculations quantitatively describing the results of shock wave experiments carried out in order to determine the moment of maximum OH* radiation at temperatures T < 1500 K and pressures P ≤ 2 atm in the H₂ + O₂ mixtures diluted by argon when the vibrational nonequilibrium is a factor determining the mechanism and rate of the overall process. In kinetic calculations, the vibrational nonequilibrium of the initial H₂ and O₂ components, the HO₂, OH(X²Π), O₂*(¹Δ) intermediates, and the reaction product H₂O were taken into account. The analysis showed that under these conditions the main contribution to the overall process of OH* formation is caused by the reactions OH + Ar → OH* + Ar, H₂ + HO₂ → OH* + H₂O, H₂ + O*(¹D) → OH* + H, HO² + O → OH* + O² and H + H²O → OH* + H², which occur in the vibrational nonequilibrium mode (their activation barrier is overcome due to the vibrational excitation of reactants), and by H + O₃ → OH* + O₂ and H + H₂O₂ → OH* + H₂O, which are reverse to the reactions of chemical quenching.     

Experimental and kinetic modeling study of the effect of sulfur dioxide on the mutual sensitization of the oxidation of nitric oxide and methane

New experimental results were obtained for the mutual sensitization of the oxidation of NO and methane in a fused silica jet‐stirred reactor operating at 10⁵ Pa, over the temperature range 800–1150 K. The effect of the addition of sulfur dioxide was studied. Probe sampling followed by online FTIR analyses and off‐line GC‐TCD/FID analyses allowed the measurement of concentration profiles for the reactants, stable intermediates, and final products. A detailed chemical kinetic modeling of the present experiments was performed. An overall reasonable agreement between the present data and modeling was obtained. According to the present modeling, the mutual sensitization of the oxidation of methane and NO proceeds via the NO to NO₂ conversion by HO₂ and CH₃O₂. The conversion of NO to NO₂ by CH₃O₂ is more important at low temperatures (800 K) than at higher temperatures (850–900 K) where the production of NO₂ is mostly due to the reaction of NO with HO₂. The NO to NO₂ conversion is favored by the production of the HO₂ and CH₃O₂ radicals yielded from the oxidation of the fuel. The production of OH resulting from the oxidation of NO accelerates the oxidation of the fuel: NO + HO₂ → OH+ NO₂ followed by OH + CH₄→ CH₃. In the lower temperature range of this study, the reaction further proceeds via CH₃ + O₂→ CH₃O₂; CH₃O₂+ NO → CH₃O + NO₂. At higher temperatures, the production of CH₃O involves NO₂: CH₃+ NO₂→ CH₃O. This sequence of reactions is followed by CH₃O → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂ → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂. The data and the modeling show that unexpectedly, SO₂ has no measurable effect on the kinetics of the mutual sensitization of the oxidation of NO and methane in the present conditions, whereas it frequently acts as an inhibitor in combustion. This result was rationalized via a detailed kinetic analysis indicating that the inhibiting effect of SO₂ via the sequence of reactions SO₂+H → HOSO, HOSO+O₂ → SO₂+HO₂, equivalent to H+O₂?HO₂, is balanced by the reaction promoting step NO+HO₂ → NO₂+OH. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 406–413, 2005     

High temperature pyrolysis of methane in shock waves. Rates for dissociative recombination reactions of methyl radicals and for propyne formation reaction

The pyrolysis of 2% CH₄ and 5% CH₄ diluted with Ar was studied using both a single–pulse and time–resolved spectroscopic methods over the temperature range 1400–2200 K and pressure range 2.3–3.7 atm. The rate constant expressions for dissociative recombination reactions of methyl radicals, CH₃ + CH₃ → C₂H₅ + H and CH₃ + CH₃ → C₂H₄ + H₂, and for C₃H₄ formation reaction were investigated. The simulation results required considerably lower value than that reported for CH₃ + CH₃ → C₂H₄ + H₂. Propyne formation was interpreted well by reaction C₂H₂ + CH₃ → P-C₃H₄ + H with ?? = 6.2 × ¹⁰12 exp(?17 kcal/RT) ^cm3 mol^?1 s^?1.     

Chemiluminescent ion-molecule reactions in the system C+ + H2

The optical emission resulting from collisions between C⁺ ions and H₂ gas was measured in the energy range 2 to 20 eV_c.m.. The observed spectrum consists mainly of the CH⁺ A ¹Π → X ¹Σ⁺ band system; CH⁺ (A ^fΠ) is shown to be formed in the chemiluminescent reactio: C⁺(²P⁰) + H₂ → CH⁺(A ¹Π) + H(²S). The energy dependence of the emission cross section was measured. The occurrence of this reaction is discussed in terms of a electronic state correlation diagram for the system.     

Water photoelectrolysis using nickel titanate and niobate as photoanodes

Nickel titanate (NiTiO₃) and nickel niobate (NiNb₂O₆), both with a cationic valence and conduction band, were examined for their photoelectrochemical properties. Applied as photoanode in a photoelectrochemical cell for water electrolysis, reduced pellets of these oxides show a photoresponse when irradiated in the optical band gap. The corresponding absorption is due to Ni²⁺ → Ti⁴⁺ and Ni²⁺ → Nb⁵⁺ charge-transfer transitions, respectively. These are situated at lower energy than the O^2? → Ti⁴⁺ and O^2? → Nb⁵⁺ charge-transfer transitions. The flatband potentials of both compounds were determined from photocurrent versus applied potential measurements. During reduction both compounds showed superficial decomposition. The effect of this decomposition on the photocurrents is discussed.