The rules governing hydrogen combustion close to the lower ignition limit in the kinetic region of chain termination

The kinetic scheme of hydrogen combustion near the lower self-ignition limit in the kinetic region can be augmented by the reactions of bimolecular nonlinear termination O + OH → H + O₂ and heterogeneous propagation of chains with the participation of adsorbed hydrogen atoms H_s. The literature data on the experimental determination of the probability of hydrogen atom trapping by the surface of quartz were analyzed.     

The lower self-ignition limit condition for hydrogen-oxygen mixtures in the kinetic region with taking into account surface modification by chain combustion products

The classic condition of the lower self-ignition limit in the kinetic region at a constant heterogeneous decay coefficient ɛ₀ was modified taking into account the degree of surface coverage by hydrogen atoms H_s and effective heterogeneous decay coefficient ɛ_eff. These values were determined by the ratio between the sticking χ and recombination γ coefficients. The condition γ = ɛ₀/2 was always fulfilled.     

Modeling the Effects of Chain Length on the Termination Kinetics in Multivinyl Photopolymerizations

A model is presented that predicts photopolymerization kinetics over several orders of magnitude change in initiation rate. The model incorporates polymerization features that have long been assumed negligible when examining multivinyl photopolymerizations. The assumption that radical termination is chain‐length‐independent is relaxed by incorporating a chain‐length‐dependent termination (CLDT) parameter based on Random‐walk theory into the kinetic model. Experiments and modeling of multivinyl free‐radical photopolymerizations clearly demonstrate that CLDT is important at low conversions, where a deviation from the classical square‐root relationship between polymerization rate (R_p) and initiation rate (R_i) is observed (R_p ∝ R _i^α, α = ¹/₂, classically). At moderate conversions, when reaction diffusion dominates termination, a transition region is observed from a chain‐length‐dependent to a chain‐length‐independent region. During this transition, long chain – long chain termination is reaction diffusion controlled while the short chain – short chain termination event remains translational and segmental diffusion controlled. The scaling exponent, α, gradually increases throughout this region until achieving the classical value, where once attained, a plateau is observed. Chain‐length effects were also examined by including chain‐transfer (CT) reactions into the kinetic expressions. Upon CT agent addition, a transition region is still observed; however, at low conversion, α adheres more closely to the classical predictions. Most importantly, the model clearly demonstrates a transition from a CLDT region at low conversion to reaction diffusion controlled termination region at high conversion, where chain length is unimportant.     

Interaction of a low-pressure gas flame and heterogeneous catalytic processes on the reactor surface

Conclusions (1) It was shown that the Semenov theory in various modes of chain termination quantitatively describes oxyhydrogen combustion near the first ignition limit with allowance for the interaction between the flame and the catalytic processes on the reactor wall. (2) Numerical modeling of oxyhydrogen ignition in the diffusion-controlled chain-termination mode detected the dependence of the rate “constant” for heterogeneous chain initiation on the model of the reaction in the gas. (3) For the first time, the change in the specific rate of heterogeneous chain initiation during a single ignition was determined. (4) method for controlling the low-pressure flame mode by affecting tubes far from the reactor was proposed and applied. Original Russian Text ? E.N. Aleksandrov, S.N. Kozlov, N.M. Kuznetsov, 2006, published in Doklady Akademii Nauk, 2006, Vol. 407, No. 5, pp. 630–633. Presented by Academician A.E. Shilov October 5, 2005     

A new method for the determination of the rate constants for the disproportionation of semiquinone radicals from nonstationary kinetics of Chain reactions between quinoneimines and hydroquinones

A new method is suggested for the determination of the rate constants for the disproportionation of semiquinone radicals k ₆ from the kinetics of chain reactions between quinoneimines and hydroquinones under nonstationary conditions. The k ₆ values were determined in solvents of various natures, including comparatively low-polarity solvents in which hydroquinones are poorly soluble. The k ₆ values were found from the dependence of the degree of quinoneimine concentration lowering during a certain time (∼20 s) after reaction beginning (when the reaction occurred under nonstationary conditions) on the rate of initiation. Techniques for overcoming difficulties caused by the absence of the limiting chain propagation stage in reactions between quinoneimines and hydroquinones were suggested. The developed method of the nonstationary kinetics of chain reactions was applied to selectively determine the rate constant for the disproportionation of 2,5-dichloroseniquinone radicals, which is one of parallel chain termination stages in the chain reaction between N,N’-diphenyl-1,4-benzoquinonediimine and 2,5-dichlorohydroquinone, k ₆ = (5.8−7.1) × 10⁶ l/(mol s) in chlorobenzene at 298 K.     

Kinetics of the liquid-phase oxidation of cyclooctene in the presence of MnO2

The kinetics of the liquid-phase oxidation of cyclooctene by molecular oxygen in the presence of a MnC₂ catalyst is studied. It is found that MnC₂ is an initiator of this reaction and has no effect on the steps of chain propagation and termination. The oxidation occurs by a radical chain mechanism with the escape of radicals to the bulk of the reaction mixture. Radicals are formed by the interaction of the olefin with the solid catalyst surface. The kinetic parameters of the reaction are calculated.     

Modeling of the kinetics of the heterogeneous-homogeneous conversion of methane into ethane and ethylene in the absence of oxygen in the gas phase

Computer calculations were carried out on the kinetics of the gas phase chain process for the conversion of methyl radicals into higher hydrocarbons in an oxygen-free atmosphere based on a scheme of reactions consisting of 23 homogeneous elementary steps and the heterogeneous stage of methyl radical formation. The results of the calculations are in good agreement with experimental kinetic results obtained for the interaction of methane with the oxidized surface of the perovskite catalyst KNaSrCoO₃. L. V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31 Prospekt Nauki, Kiev 03039, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 4, pp. 222–225, July–August, 2000.     

The isokinetic effect as related to the activation energy for the gases diffusion in coal at ambient temperatures

The present paper contains theoretical analysis and a thorough discussion of the applicability of Fick’s approach to modelling CO₂ and CH₄ diffusion in heterogeneous coal, especially with regard to the estimation of a single diffusion coefficient from the dependence defined by a number series. The computations were performed for high rank coal depending on the grain size of coal samples and within a narrow range of temperature − 293 and 303 K. The results of the model’s application to the experimental data show that the method of estimation of the diffusion coefficient is very important. Our results also show that the diffusion coefficient changes are closely related with the one-parameter analysis of coal grain size distribution.     

Strong effect of a solid surface on a liquid-phase radical chain chemiluminescence reaction. Oxidation of UIV by dioxygen

The reaction of uranium(IV) with O₂ in aqueous solutions of HClO₄ is accompanied by chemiluminescence (CL). The CL intensity and the reaction kinetics are appreciably affected by introduction of the glass beads with a radius of 1 mm. In the presence of 1600 beads (the surface area of the glass is 200 cm²), the yield of CL dereases 32-fold. The effect of the solid surface on the kinetic and chemiluminescence parameters of the liquid-phase radical chain reaction is due to the increased role of chain termination upon heterogeneous decay of the HO ₂ ^. and^.OH radicals.     

Estimation of the critical temperature of thermal explosion for azido-acetic-acid-2-(2-azido-acetoxy)-ethylester using non-isothermal DSC

A method for estimating the critical temperatures (T _b) of thermal explosion for energetic materials is derived from Semenov’s thermal explosion theory and the non-isothermal kinetic equation dα/dt=A ₀ T ^B f(α)e^−E/RT using reasonable hypotheses. The final formula of calculating the value of T _b is $ \left( {\frac{B} {{T_b }} + \frac{E} {{RT_b^2 }}} \right) $ \left( {\frac{B} {{T_b }} + \frac{E} {{RT_b^2 }}} \right) (T _b−T _e0=1. The data needed for the method, E and T _e0, can be obtained from analyses of the non-isothermal DSC curves. When B=0.5 the critical temperature (T _b) of thermal explosion of azido-acetic-acid-2-(2-azido-acetoxy)-ethylester (EGBAA) is determined as 475.65 K.     

Investigation of autoacceleration effects during the solution polymerization of styrene

The possibility of obtaining increases in the rate and degree of polymerization through a decrease in the termination rate in nonviscous, homogeneous solution polymerizations of styrene has been investigated. Decreases in the termination rate were achieved through decreasing segmental diffusion of the propagating macroradical by greater occlusion, on the average, of the radical in the coiled polymeric chain. Coiling of the polymeric chain was effected by polymerizing styrene in thermodynamically poor (θ) solvents near the θ temperature for polystyrene. Examples of such systems are diethyl oxalate at 51.5°C. and cyclohexane at 34.6°C. Polymerization under these conditions did lead to a decrease in the k_t/k_p² kinetic ratio; this decrease resulted in increases in the degree of polymerization, but changes in the rate of polymerization, in contrast to the marked increases noted in viscous solution or heterogeneous polymerizations, were not observed. Possible explanations for the latter observations are discussed.     

Studies on the reactivity of cis-RuCl2 fragment in Ru(PPh3)2(TaiMe)Cl2 with N,N-chelators (TaiMe = 1-methyl-2-(p-tolylazo)imidazole). Spectral and electrochemical characterisation of the products

Dechlorination of Ru(PPh₃)₂(TaiMe)Cl₂ (TaiMe = p-Me-C₆H₄-N=N-C₃H₂NN(1)-Me (1), 1-methyl-2-(p-tolylazo)imidazole) has been carried out in acetone solution by Ag⁺ and reacted with N,N’-chelators to synthesise [Ru(PPh₃)₂ (TaiMe)(N,N’)]²⁺. The complexes have been isolated as their perchlorate salts. The N,N’ chelators are 1-alkyl-2-(phenylazo)imidazoles (PaiX, X = Me, Et, CH₂Ph); 2-(arylazo)pyridines, (Raap,p-R-C₆H₄-N=N-C₅H₄N; R = H, Me, Cl); 2-(arylazo)pyrimidines (Raapm,p-R-C₆H₄-N=N-C₃N₂H₂; R = H, Me, Cl); 2,2’-bipyridine (bpy) and 1,10-phenanthroline (o-phen). Unsymmetrical N,N’ chelators may give two isomers and this is indeed observed. The¹H NMR spectral data refer to the presence of two isomers in the mixture in different proportions. With consideration of coordination pairs in the order of PPh₃, PPh₃; N,N (N refers to N(immidazole)) and N’,N (N’ refers to N(azo)), the complexes have been characterised astrans-cis-cis andtrans-trans-trans configuration; the former predominates in the mixture. Electrochemical studies exhibit high potential Ru(III)/Ru(II) couple and quasireversible N=N reduction. Electronic spectra show high intensity (ε ∼ 10⁴) MLCT transition in the visible region (520 ±10) nm along with a shoulder (ε ∼ 10³) in the longer wavelength region.     

The Influence of Chain Length‐Dependent Propagation on the Evaluation of the Chain Length Dependence of the Rate Coefficient of Bimolecular Termination, 1. PLP–SEC Methods

Chain length distributions have been calculated for polymers prepared by pulsed laser polymerization (PLP) under the condition that not only chain termination but also chain propagation is subject to chain length dependence. The interplay between these two features is analyzed with the chain length dependence of the rate coefficient of termination k_t introduced in the form of a power law and that of propagation k_p modeled by a Langmuir‐type decrease from an initial value for zero chain length to a constant value for infinite chain lengths. The rather complex situation is governed by two important factors: the first is the extent of the decay of radical concentration [R] during one period under pseudostationary conditions, while the second is that termination events are governed by [R]² while the propagation goes directly with [R]. As a consequence there is no general recommendation possible as to which experimental value of k_p is best taken as a substitute for the correct average of k_p characterizing a specific experiment. The second point, however, is apparently responsible for the pleasant effect that the methods used so far for the determination of k_t and its chain length dependence (i.e., plotting some average of k_t versus the mean chain‐length of terminating radicals on a double‐logarithmic scale) are only subtly wrong with regard to a realistic chain length dependence. This is especially so for the quantity k_t* (the average rate coefficient of termination derived from the rate of polymerization in a PLP system) and its chain length dependence.     

Autoacceleration of the free-radical chain luminescent oxidation of U(IV) with xenon trioxide in aqueous HClO<Subscript>4</Subscript>

Autoacceleration is observed in the chemiluminescent radical chain oxidation of U(IV) with xenon trioxide in aqueous perchloric acid in a Teflon (not conventional glass) reactor. The decay of chemiluminescence accompanying the reaction between U(IV) and XeO₃ in the case of excess oxidizer obeys a first-order kinetic equation only at low U(IV) concentrations of 10^?5–10^?6 mol/l. Autoacceleration takes place at comparatively high reactant concentrations, when the contribution from the heterogeneous loss of radicals on the reactor walls into chain termination is comparatively small and the role of degenerate chain branching reactions is significant. It is inferred that a critical phenomenon rare for liquid-phase radical chain processes takes place in U(IV) oxidation with xenon trioxide: a comparative small increase in the reactant concentrations causes this chemiluminescent redox process to pass from the quasi-steady-state regime to autoacceleration.     

Postpolymerization effect on molecular weight distribution generated by pulsed laser radical polymerization

The postpolymerization effect on molecular weight distribution (MWDs) and on the Pulsed Laser Polymerization (PLP) technique for evaluation of kinetic constants is investigated. General expressions for moments are derived for a polymerization scheme that contains the reactions of chain initiation, propagation, and termination by recombination or disproportionation, under polymerization initiation by an arbitrary sequence of radiation pulses. The results of calculation of MWDs and of the weight-average degree of polymerization (P _w) for methyl methacrylate are presented. It is shown that the P _w value strongly depends on postpolymerization. A new method for determining the rate constants of chain propagation and chain termination from a single experiment by polymerization with packets of laser pulses is presented.     

Kinetics of hydrogen oxidation near the lower explosion limit

Kinetics of hydrogen oxidation near the lower explosion limit in the kinetic region of chain termination has been studied. Major effects, causing deviations of the reaction kinetics from calculations by the linear theory of branched chain processes, are shown to be (1) the inhibition of the reacting mixture by the products of interaction of active centers with vacuum grease or with impurities contained in it and (2) the heterogeneous negative interaction of reaction active centers. The kinetics of hydrogen oxidation in this region has been calculated with consideration of the heterogeneous negative chain interaction. A set of parameters has been obtained that make it possible to determine by the shape of the kinetic curves the sign and the value of nonlinear interaction of chains near the lower explosion limit. It has been shown that the experimental data are in good agreement with the calculations, provided the heterogeneous negative chain interaction is taken into consideration and the inhibiting effect of impurities is eliminated. The rate of heterogeneous generation of chains on a quartz surface treated with hydrofluoric or boric acid has been determined.     

Radical polymerization of N-vinylcaprolactam in benzene solutions in a wide conversion range

The kinetics of radical polymerization of N-vinylcaprolactam initiated by the thermal decomposition of AIBN at 60°C in monomer solutions in benzene has been studied in a wide range of conversions. The heat of polymerization of N-vinylcaprolactam is 76.0 ± 0.9 kJ/mol; at initial conversions, the polymerization of N-vinylcaprolactam is of the first order with respect to the monomer and of the 0.5th order with respect to the initiator. The ratio of chain propagation and chain termination rate constants k _p/k _ter ^0.5 is 0.578 l^0.5/(mol s)^0.5, thus suggesting a high propagation rate constant k _p > 10³ l/(mol s). At a high initial concentration of the monomer, the kinetic curves demonstrate a weakly pronounced gel effect, and, in the gel permeation chromatography curves of the polymers, the second high-molecular-mass mode emerges, whose intensity grows with conversion. The observed kinetic features are interpreted in terms of the diffusion control of the gel effect.     

Interparticle diffusion of aromatic hydrocarbons on aerosil surface

It has been demonstrated by the photobleaching method that the photostability of pyrene molecules on the surface of pyrogenic nonporous SiO₂ (aerosil) is independent of the surface concentration of the fluophor. Through a kinetic analysis of the recovery of fluorescence of the luminophore and solution of the two-dimensional diffusion equation, coefficients of interparticle diffusion were estimated for the molecules of pyrene and N,N-dimethylaniline. Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 31 Nauki Prospect, Kiev, Ukraine 252039. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 1, pp. 23–26, January–February, 1998.     

Gas permeation properties of poly(silamine) membrane

The gas permeation characteristics of poly(silamine) membrane, which consists of alternating 3,3-dimethyl-3-silapentane and N,N′-diethylethylenediamine units in the main chain, were investigated. Though poly(silamine) shows high flexibility (glass transition temperature of −88°C), the gas permeabilities were much lower than those of other rubbery polymers such as poly(dimethylsiloxane) and natural rubber. The activation energies of diffusion in poly(silamine) were much higher than that of natural rubber. On the basis of these results, we propose a model such that the interaction between the Si atom and gas molecules (O₂ and N₂) prevents the free diffusion of the gas molecule in the poly(silamine) membrane. © 1997 John Wiley & Sons, Ltd.     

A comparative study of in- and post-source decays of peptide and preformed ions in matrix-assisted laser desorption ionization time-of-flight mass spectrometry: Effective temperature and matrix effect

In-source decay (ISD) and post-source decay (PSD) of a peptide ion ([Y₆ + H]⁺) and a preformed ion (benzyltriphenylphosphonium, BTPP) generated by matrix-assisted laser desorption ionization (MALDI) were investigated with time-of-flight mass spectrometry. α-Cyano-4-hydroxycinammic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) were used as matrices. For both ions, ISD yield was unaffected by delay time, indicating rapid termination of ISD. This was taken as evidence for rapid expansion cooling of hot “early” plume formed in MALDI. CHCA was hotter than DHB for [Y₆ + H]⁺ while the matrix effect was insignificant for BTPP. The “early” plume temperature estimated utilizing previous kinetic results was 800–900 K, versus 400–500 K for “late” plume. The results support our previous finding that the temperature of peptide ions interrogated by tandem mass spectrometry was lower than most rough estimates of MALDI temperature.