Radical Recombination Reactions Research Articles

Theoretical studies of radical recombination reactions. 4. An AM1/CI study of reactions of benzylic and allylic radicals. An intrinsic barrier to bond formation

Theoretical studies of radical recombination reactions. 4. An AM1/CI study of reactions of benzylic and allylic radicals. An intrinsic barrier to bond formation

Qualitative analysis of chlorolignins and lignosulphonates in pulp mill effluents entering the river Rhine using pyrolysis—mass spectrometry and pyrolysis—gas chromatography/mass spectrometry

Macromolecular chlorolignins and lignosulphonates have been analysed in effluents from five pulp mills discharging on the river Rhine. Analytical techniques used were ultrafiltration, pyrolysis—mass spectrometry and pyrolysis—gas chromatography / mass spectrometry. The average organically bound chlorine content was 19 mg/l, with a maximum value of 68 mg/l for the pulp mill Cellulose de Strasburg. The annual discharge of organically bound chlorine (AOX) and of dissolved organic carbon (DOC) of these five mills was calculated to be 2100 tonnes AOX and 33 000 tonnes DOC, respectively. This would account for 90% of the AOX load and 10% of the DOC load of the river Rhine. Evidence for the existence of two chlorolignin classes, aromatic chlorolignins and aliphatic chlorolignins, was obtained. Monochloroguaiacol is the most abundant structural specific aromatic chlorolignin pyrolysis product. Methyl chloride is a major chlorine containing pyrolysis product, probably originating from aliphatic chlorolignins. Sulphur dioxide is a major pyrolysis product of lignosulphonates, and possibly can be used for the specific quantitative determination of lignosulphonates in river water. Radical recombination reactions of methyl radicals and other primary pyrolysis radicals play an important role in these samples, leading to several secondary pyrolysis products. The presence of lignosulphonates in the salt or acid form has an important effect on the pyrolysis process. Methyl chloride can probably be quantified with the reference compound tetramethylammonium chloride. The macromolecular compositions of these effluents show many similarities, but also significant differences, presumably reflecting the use of different wood types, pulping and bleaching conditions at the time of sampling.

Interaction of poly(methyl methacrylate) and nafions

AbstractThe thermal degradation of Nafion–H, Nafion–K, and the mixture of each of these with poly(methyl methacrylate) (PMMA) has been studied. A mechanism is proposed for the thermal degradation of Nafion–H that involves an initial cleavage of the carbon–sulfur bond leading to SO2 and OH radical and a carbon‐based radical. This carbon‐based radical then undergoes further degradation. The addition of PMMA to Nafion–H inhibits further degradation of the Nafion carbon‐based radical and instead radical recombination reactions between PMMA fragments and this Nafion radical dominate the reaction scheme. The replacement of the hydrogen in Nafion–H with a potassium (i.e., Nafion–K) produces a much more robust material that does not degrade under conditions similar to those used for Nafion–H. Nafion–K has little effect on the degradation of PMMA.

Structure and ESR investigation of plasma‐polymerized film of hexafluoropropene

AbstractThe structure and constitution of plasma‐polymerized film of C3F6 (PPHFP) have been investigated by measuring its ESCA, 19F‐NMR, and IR spectra. It is found that the PPHFP film was a high degree of branching and 57% of quaternary carbons are linked to CF3 groups. It is also found that the constitution of PPHFP film is changed greatly at high power and a large amount of Si and O atoms are linked to the PPHFP chains. Furthermore, using element analysis, ESCA, and theoretical calculation by electronegative model, we found that the nitrogen atoms get into the PPHFP chains when using N2 as plasma gas. Besides, the radical concentration in the film is obtained to be 9.97 × 1019 spins/g and its half‐life is about half a year at room temperature in Ar gas. Having investigated decay process of the radicals in two gases of air and Ar, we calculated the activation energies and rate constants of radical recombination and oxidation reactions at different temperatures.

Recombination of CH 3 radicals at elevated pressures and temperatures

As an extension to previous studies the methyl radical recombination reaction, CH3+CH3(+M)→C2H6(+M) (1) was studied behind reflected shock waves over the temperature range 1200–1600 K at total pressures of 10 to 240 bar with a 216.5 nm UV absorption spectroscopic technique. The reaction was in the falloff region under the conditions employed and the experimentally determined extinction coefficients of CH3 were found to be slightly pressure dependent at a fixed temperature. The extrapolated high-pressure limit rate constants could be well represented by (in cm3/mol s), k1∞=1.15×1015T−0.6, or k1∞=1.45×1013exp(330 K/T), or k1∞=5.36×1016T−1.1exp(−320K/T). A negative temperature dependence of k1∞ was therefore confirmed.

7345. The importance of free radical recombination reactions in CF 4/O 2 plasma etching of silicon : M Dalvie and K F Jenson, J Vac Sci Technol, A8, 1990, 1648–1653

7345. The importance of free radical recombination reactions in CF 4/O 2 plasma etching of silicon : M Dalvie and K F Jenson, J Vac Sci Technol, A8, 1990, 1648–1653

Effect of Damköhler number on superequilibrium OH concentration in turbulent nonpremixed jet flames

Simultaneous, spatially resolved measurements of mixture fraction and absolute hydroxyl radical concentration are obtained for the first time in nonpremixed, turbulent, hydrogenair flames. This is accomplished by combining spontaneous Raman scattering with linear, laser-induced fluorescence (LIF). The Raman scattering data define the instantaneous, local collisional quenching environment of the OH molecules, allowing quenching corrections to be applied for each laser shot and making the linear LIF measurements quantitative. The effect of Damköhler number on OH superequilibrium is determined by performing measurements at selected locations in two argon-diluted hydrogen flames (Reynolds numbers 8,500 and 17,000). Results demonstrate that departures from chemical equilibrium in these flames are a consequence of the fact that time scales for turbulent transport are competitive with time scales for three-body radical recombination reactions. Due to the slow characteristic time for the radical recombination, convective histories, as well as instantaneous local conditions, determine hydroxyl concentrations. Damköhler numbers are not sufficiently low for the rapid bimolecular reactions to be strongly affected. Comparison of turbulent flame data with results from strained laminar flame calculation incicates that partial equilibrium of the bimolecular reactions is a good approximation near the stoichiometric mixture fraction. Comparisons of the experimental data with predictions by Monte Carlo simulations using a partial equilibrium chemistry model show good overall agreement. However, simulations predict a smaller variance of OH concentration than is measured for a given value of mixture fraction, lower OH concentrations at off-stoichiometric conditions, and a more rapid decay toward equilibrium with streamwise distance.

ChemInform Abstract: 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.

The Benzylphenylether thermolysis mechanism: Insights from phase behavior

AbstractThe thermolysis of benzylphenylether (BPE) was examined at 320°C in liquid, vapor, and supercritical methanol phases as a probe of the reaction mechanism. Toluene and phenol were the major products in all cases, but observed selectivities were dependent upon the reaction phase. Isotopic labeling experiments demonstrated that methanol participated in the reaction network as a hydrogen donor through a free‐radical mechanism. These results are consistent with free‐radical steps for neat BPE primary pyrolysis that include BPE fission followed by hydrogen abstraction and radical recombination reactions. A detailed set of 21 free‐radical steps quantitatively explains the dependence of the selectivity and conversion on the phase behavior.

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

AbstractThe 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 𝓀 = 6.2 × 1012 exp(−17 kcal/RT) cm3 mol−1 s−1.

The importance of free radical recombination reactions in CF4/O2 plasma etching of silicon

The importance of free radical recombination reactions of F in CF4/O2 plasma etching of Si is explored in a combined experimental and modeling study of a single wafer etcher. The F atom concentration profiles in the reactor, as well as the silicon etch rate profiles, are strongly influenced by the recombination reactions. The surface recombination reaction is more important than the gas phase recombination reactions in determining overall system behavior. The effects of the homogeneous as well as heterogeneous recombination reaction diminishes with increasing flow rate. This is due to combined residence time and transport effects.

Mechanisms of cytochrome P‐450 catalysis

Cytochrome P-450 (P-450) enzymes catalyze the oxidation of a wide variety of substrates. Although a large number of P-450s have been characterized in different species and tissues, the mechanisms of catalysis of oxygenation may be understood in terms of a few basic principles. The chemistry is dominated by the ability of a high-valent formal (FeO)3+ species to carry out one-electron oxidations through the abstraction of hydrogen atoms, abstraction of electrons in n or pi orbitals, or the addition to pi bonds. A series of radical recombination reactions then completes the oxidation process. The protein structures are postulated to provide the axial thiolate ligand to the heme, to control the juxtaposition of the substrate (and therefore the regio- and stereoselectivity of oxidation), to alter the effective oxidation potential of the (FeO)3+ complex, and possibly to participate in specific acid/base catalysis in the oxidation of some substrates.

Combined Experimental and Modeling Study of Spatial Effects in Plasma Etching: CF 4 / O 2 Etching of Silicon

A combined experimental and modeling study of uniformity in plasma etching of silicon is presented. The model includes detailed treatment of transport (including diffusion) and chemical kinetics. Experimental measurements of spatial variations in active species concentrations are made by spatially resolved actinometry; spatial variations in etch rates are also measured. The effect of flow rate and feed composition on etch rate uniformity is examined. Model predictions are in good agreement with observations. Diffusion and free radical recombination reactions play an important role in commercial size systems. The insight gained through the model is used to point out a pitfall in the interpretation of actinometry data. The applicability of the model as a design tool is demonstrated.

Photochlorination of simple alkanes in cryogenic solids

Photochemical reactions have been induced by laser photolysis of amorphous binary solids composed of chlorine mixed with propane, n-butane, isobutane, n-pentane, cyclopentane, n-hexane, or cyclohexane at 10-150 K. In each case, the laser dissociates the chlorine into atoms which react with the alkane to form HCl and the corresponding chloroalkane. Quantum yields for these reactions are in most cases less than or approximately unity. The results are consistent with a radical recombination reaction mechanism.

Toward a quantitative understanding of elementary combustion reactions

The modelling of combustion requires data bases of reliable rate coefficients which are represented in an accurate and economic way. The combination of well defined laboratory measurements, quantum-chemical calculations of potential energy surfaces, statistical and dynamic kinetic theories, as well as optimum thermodynamic data, continuously improves such data bases. The present contribution discusses selected aspects of this active field of research. First, unimolecular reactions and recombination reactions are considered. In particular, the implementation of nonthermal determinations of collisional energy transfer properties, of selected excited state rate constants, and of reaction product identifications are described. The relation between potential energy surfaces and high pressure unimolecular bond fissions or radical recombination reactions is analyzed in terms of explicit statistical adiabatic channel calculations. It follows a detailed analysis of more complex bimolecular reactions, treated in the framework of statistical unimolecular rate theory. Particular emphasis is given to the reaction H+O2⇌HO+O which passes through an HO2* intermediate. Good agreement between simple statistical calculations and the most recent experiments is found. The contribution of electronically excited HO2 is also discussed. The accuracy of the theoretical results in this case may soon exceed that of the experiments. After the “loose entrance-loose exit” system HO2, the reaction H+HCO⇌H2+CO, passing through an H2CO* intermediate, is chosen as an example for “loose entrance-rigid exit” systems. Rotational effects, in particular “rotational channel switching,” are of great importance. The additional effects of collisions on complex-forming bimolecular reactions are modelled in detail. The relationship to the reverse multichannel unimolecular dissociation reactions for the corresponding complex molecule is illustrated. The theoretical methods described provide simple possibilities for a “sensitivity test of the rate calculations” with respect to the various molecular contributions in a rate coefficient. In this sense they complement “sensitivity tests of the modelled mechanisms” with respect to the various elementary reactions and their rate parameters.

Fate of free radicals generated during one-electron reductions of 4-alkyl-1,4-peroxyquinols by cytochrome P-450.

Free radicals resulting from the one-electron reduction and subsequent homolytic cleavage of oxygen-oxygen bonds by heme proteins are likely to be responsible for some aspects of the toxicity of organic hydroperoxides. In the present work, effects of the 4-alkyl substituent of 2,6-di-tert-butyl-4-alkyl-4-hydroperoxycytohexa-2,5-dienones (1,4-peroxyquinols) on radical production were investigated with microsomal cytochrome P-450 from rat liver. Quinoxy radicals from homolysis of the peroxyquinols underwent beta-scission to produce a quinone and an alkyl radical, and this process occurred with increasing frequency as the stability of the alkyl radical increased. The fate of benzyl and 2-phenylethyl radicals generated from the appropriately substituted peroxyquinols was investigated also. The former was converted to benzyl alcohol, benzaldehyde, and toluene and the latter to 2-phenylethanol, phenylacetaldehyde, ethylbenzene, styrene, and benzaldehyde. Oxygen-18 labeling studies demonstrated that 80-85% of the benzyl alcohol incorporated oxygen from the hydroperoxide and the balance from molecular oxygen. This indicates that the predominant reaction pathway involved recombination between the benzyl radical and the iron-bound hydroxyl radical of the P-450 intermediate complex. By contrast, about 50% of 2-phenylethanol from the 2-phenylethyl radical incorporated oxygen from water and the balance from O2. Two alternative mechanisms are proposed to explain the formation of 2-phenylethanol that contained oxygen from water and the concurrent formation of styrene: (a) oxygen exchange of the P-450 intermediate with water, followed by hydrogen abstraction and radical recombination reactions with the P-450 complex, or (b) oxidation of the radical to the 2-phenylethyl cation followed by proton elimination and hydration.

Kinetics of the reversible recombination of substituted 2-(p-dimethylaminophenyl)indane-1,3-dion-2-yl radicals

1. We established linear relationships between Δ log 2k−1, Δ log 2k1, and Σσ of the Gammet constants with a positive ρ value, equal to 0.32 and 1.20, respectively. 2. We determined the kinetic and thermodynamic parameters of the recombination reaction of radicals and of the dissociation of dimers in the 2-arylindanedione-1,3 series. 3. The recombination rate constant of 2-arylindane-1,3-dion-2-yl radicals in benzene is activational. In the binary benzene-MO mixture at η≥2sP, the recombination rate constant is fully limited by molecular mobility. In the case of recombination of 5-iodo-2-(p-dimethylaminophenyl) indane-1,3-dion-2-yl radicals, we found that 2k−1 ∼ η−0.5.

Simulation of OH radical profiles in premixed atmospheric-pressure flat flames

OH profiles measured in the postflame gases of three propane-air (/phi/ = 0.63, 1.17, 1.46) flames and one (/phi/ = 1.01) methane-air flame at atmospheric pressure are compared with computer-simulated profiles generated by using a 15-reaction chemical mechanism. The simulated profiles deviate from the measured data by less than 15% for all four flames. Sensitivity analyses verify that reactions of the HO/sub 2/ radical determine the shape of the OH decay profile for the lean flame. For stoichiometric and rich flames, the radical recombination reaction (H + OH + M) strongly influences the OH radical profile. Experimental measurements of the CO, H/sub 2/, and O/sub 2/ profiles in lean and stoichiometric flames are also simulated correctly by the mechanism. For the stoichiometric flame, the experimental results verify that the H/sub 2/, O/sub 2/, and OH densities are in partial equilibrium in the postflame gases as predicted. These results provide a set of reaction rate constants and diffusion coefficients which correctly simulate the observed data over a wide range of fuel-air equivalence ratios.

Luminescence research and its relation to ultraweak cell radiation.

The fundamental laws of photochemistry and the essential results of experimental research on ultraweak cell radiation are presented. By comparing all the facts it can be concluded that the phenomena discussed may arise from a variety of possible reactions and sources. Recombination reactions of certain radicals actually do release sufficient energy to generate UV-photons of the intensity under consideration. On the other hand, stimulated emission cannot be excluded in view of the distinct deviation of the radiation field from thermal equilibrium. There exist, however, various other candidates, such as direct emitters like flavins, indoles, porphyrins, carbonyl derivatives and aromatic compounds, and molecular oxygen and its various species, as well as collective molecular interactions, e.g. dimole or exciplex transitions, triplet-triplet annihilation, collective hydrolysis, electric field effects in membranes, etc. Careful biochemical and biophysical experiments are still necessary to find answers to all the questions that remain; not only individual problems have to be solved, but it is important to keep in mind the interrelationships between certain reactions.

Angular momentum conservation in unimolecular and recombination reactions

AbstractIt is shown that the master equation describing fall‐off effects in unimolecular and recombination reactions, with angular momentum (J) conservation taken into account, can be solved exactly if the assumption is made that the probability of collisional energy transfer in J is independent of initial state; this assumption is shown to be physically acceptable (from general conservation considerations and from trajectory calculations) for typical neutral radical recombination and decomposition reactions. This leads to a J‐averaged master equation which can be readily solved by standard means. Illustrative computations using this treatment are presented.