Initiation and termination mechanisms in kinetic gelation simulations

A kinetic gelation simulation is presented which includes a distinct initiator species decaying exponentially with time such that the rate of initiation varies as in an actual polymerization. The effects of the initiator quantity and the initiator decay constant on the polymerization are shown along with the effect of varying the probability that two radicals on adjacent sites will terminate. Varying the initiation rate and termination probability are examined in order to determine their influence on the trapping of radicals, the relative reactivity of pendant functional groups, and the overall structure of the polymer. In general it appears that the incorporation of an initiator enables the simulation to more adequartely describe the polymerization reaction, particularly time-dependent phenomena such as rate of reaction and radical concentrations.     

Thermochemical and kinetic analysis of the thermal decomposition of monomethylhydrazine: an elementary reaction mechanism

The reaction kinetics for the thermal decomposition of monomethylhydrazine (MMH) was studied with quantum Rice-Ramsperger-Kassel (QRRK) theory and a master equation analysis for pressure falloff. Thermochemical properties were determined by ab initio and density functional calculations. The entropies, S degrees (298.15 K), and heat capacities, Cp degrees (T) (0 < or = T/K < or = 1500), from vibrational, translational, and external rotational contributions were calculated using statistical mechanics based on the vibrational frequencies and structures obtained from the density functional study. Potential barriers for internal rotations were calculated at the B3LYP/6-311G(d,p) level, and hindered rotational contributions to S degrees (298.15 K) and Cp degrees (T) were calculated by solving the Schr?dinger equation with free rotor wave functions, and the partition coefficients were treated by direct integration over energy levels of the internal rotation potentials. Enthalpies of formation, DeltafH degrees (298.15 K), for the parent MMH (CH3NHNH2) and its corresponding radicals CH3N*NH2, CH3NHN*H, and C*H2NHNH2 were determined to be 21.6, 48.5, 51.1, and 62.8 kcal mol(-1) by use of isodesmic reaction analysis and various ab initio methods. The kinetic analysis of the thermal decomposition, abstraction, and substitution reactions of MMH was performed at the CBS-QB3 level, with those of N-N and C-N bond scissions determined by high level CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) calculations. Rate constants of thermally activated MMH to dissociation products were calculated as functions of pressure and temperature. An elementary reaction mechanism based on the calculated rate constants, thermochemical properties, and literature data was developed to model the experimental data on the overall MMH thermal decomposition rate. The reactions of N-N and C-N bond scission were found to be the major reaction paths for the modeling of MMH homogeneous decomposition at atmospheric conditions.     

Advanced kinetic tools for the evaluation of decomposition reactions

Summary An advanced kinetic study on the thermal behaviour of pyrotechnic ignition mixtures has been carried out by differential scanning calorimetry using different B/KNO3 mixtures (50:50, 30:70, 20:80) as a model reaction. The experimental conditions applied (isochoric conditions/closed crucibles and isobaric conditions/open crucibles) as well as the composition of the mixtures noticeably influences the relative thermal stabilities of the energetic materials. The kinetic study focused on the prediction of the thermal stability of the different mixtures both in extended temperature ranges and under temperature conditions at which ordinary investigation would be very difficult. Using advanced numerical tools [1], thermal ageing and influence of the complex thermal environment on the heat accumulation conditions were computed. This can be done for any surrounding temperature profile such as isothermal, non-isothermal, stepwise, modulated, shock, adiabatic conditions and additionally for temperature profiles reflecting real atmospheric temperature changes (yearly temperature profiles of different climates with daily minimal and maximal fluctuations). Applications of accurate decomposition kinetics enabled the determination of the time to maximum rate under adiabatic conditions (TMR_ad) with a precision given by the confidence interval of the predictions. This analysis can then be applied for the examination of the effects of the surrounding temperature for safe storage or transportation conditions (e.g. determination of the safe transport or storage temperatures).     

Thermal decomposition reactions and kinetic parameters of nitritobismuthates(III)

Thermal studies have been carried out on nitritobismuthates(III) of the silver group having the general formula M₂Ag[Bi(NO₂)₆] where M = Cs, Rb, K, and the sodium group having the formula M₂Na[Bi(NO₂)₆] where M = Cs, Rb.Typical thermal curves, temperatures of the endothermic peaks and percentage weight loss, calculated from the TG curve, are shown. On the basis of the results of thermal and X-ray analysis the mechanisms of the thermal decomposition reaction were determined for nitritobismuthates(III).The thermal curves were used to calculate kinetic parameters, activation energy E_a and order of reaction n by Zsako's and Coats and Redfern's methods. A comparison of the thermal stabilities of the salts under study was made. Within the same group of nitritobismuthates(III), the activation energy and the decomposition temperatures of the salts increases when the difference between the radii of outer sphere cations increases.     

Hyperspherical analysis of kinetic paths for elementary chemical reactions and their angular momentum dependence

Exploiting the hyperspherical mapping of potential energy surfaces, the evolutions of minima (valley bottoms) and saddles (ridges) as a function of the kinetic radius are identified as kinetic paths for studying reaction mechanisms and dynamics. The effect of angular momentum is investigated quantum-mechanically in a hyperspherical harmonic basis and a lower bound to adiabatic evolution is established. By a classical stability analysis, the dependence of angular momentum effects on configuration is studied. Some features of kinetic paths (such as the occurrence of symmetry degeneracies and bifurcations related to elementary catastrophes) are briefly illustrated for kinetic paths on surfaces for H₃ and H₃⁺.     

Comparative method to evaluate reliable kinetic triplets of thermal decomposition reactions

Reliable kinetic information for thermal analysis kinetic triplets can be determined by the comparative method: (1) An iterative procedure or the KAS method had been established to obtain the reliable value of activation energy E _a of a reaction. (2) A combined method including Coats-Redfern integral equation and Achar differential equation was put forward to confirm the most probable mechanism of the reaction and calculate the pre-exponential factor A. By applying the comparative method above, the thermal analysis kinetic triplets of the dehydration of CaC₂O₄·H₂O were determined, which apparent activation energy: 81±3 kJ mol^-1, pre-exponential factor: 4.51·106-1.78·10⁸ s^-1, the most probable mechanism function: f(α)=1 or g(α)=α, which the kinetic equation of dehydration is dα/dt=Ae^-E _a ^/RT. This revised version was published online in August 2006 with corrections to the Cover Date.     

The measurement of meaningful kinetic parameters for solid state decomposition reactions

Following previous work on the measurement of meaningful activation energies and the application of Constant Rate Thermal Analysis (CRTA) to the determination of kinetic parameters [1, 2], here we further examine sources of error in determining activation energies and go on to consider the form of the alpha function and the value ofA. Using theoretical arguments based on transition state theory, we conclude that allowing significant pressures of product gas to appear in the reaction environment will lead to very high values for apparent activation energies. We note that, although this is observed in practice for calcium carbonate, it in no way invalidates the application of the Arrhenius equation to solid state decomposition reactions, provided care is taken to avoid this type of distortion of experimental results. We attempt to determine the alpha function for the decomposition of calcium carbonate using data gathered from a variety of different types of temperature programme and reaction conditions. We find that the apparent alpha function depends on the method adopted and the experimental conditions used. We propose an explanation of why this occurs and tentatively introduce a new way of looking at the development of a reaction interface for this type of reaction. We review the literature and conclude that, while significant variations for the activation energy for the decomposition of calcium carbonate exist, a critical appraisal leads to good agreement amongst values that follow good experimental practice and reliable methods of data reduction. The apparent divergence of results can be explained in the light of the theoretical arguments advanced and the easily understood sources of experimental error.

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Zusammenfassung Im Anschlu\ an vorangehende Arbeiten zur Messung sinnvoller Aktivierungsenergien und zur Anwendung von CRTA bei der Bestimmung kinetischer Parameter untersuchen wir hier weitere Fehlerquellen bei der Bestimmung der Aktivierungsenergien und stellen überlegungen zur Form der Alpha-Funktion bzw. zum Wert von A an. Ausgehend von theoretischen Argumenten, die auf der Theorie des übergangszustandes basieren, schlossen wir darauf, da\ es unter Berücksichtigung bedeutender Drücke der in der Reaktionsumgebung auftretenden gasförmigen Produkte zu sehr hohen Werten für die scheinbaren Aktivierungsenergien kommt. Es sei bemerkt, da\, obwohl dies in der Praxis für Calciumkarbonat beobachtet werden kann, es in keiner Weise die Anwendung der Arrheniusschen Gleichung bei Feststoff-Zersetzungsreaktionen in Frage stellt, vorausgesetzt, man vermeidet diese Art von Beeinflussung der Versuchsergebnisse. Für die Zersetzung von Calciumkarbonat versuchten wir, die Alpha-Funktion unter Anwendung von Daten zu bestimmen, die einer Reihe verschiedener Temperaturprogramme und Reaktionsbedingungen entstammen. Man fand, da\ die scheinbare Alpha-Funktion von der angewendeten Methode und den Versuchsbedingungen abhÄngt. Es wird eine ErklÄrung dafür vorgeschlagen und vorlÄufig eine neue Betrachtungsweise für derartige Reaktionen eingeführt. Bei einer Durchsicht der Literatur konnte darauf geschlossen werden, da\ zwar sehr viele Varianten für die Aktivierungsenergie von Calciumkarbonat existieren, da\ aber eine kritische EinschÄtzung zu einer guten übereinstimmung derjenigen Werte führt, denen gute Versuchspraktiken und eine zuverlÄssige Datenverdichtung zugrunde liegen. Die scheinbaren Unterschiede der Ergebnisse können unter Berücksichtigung der vorgebrachten theoretischen Argumente und leicht verstÄndlicher experimenteller Fehlerquellen erklÄrt werden.

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One of the authors (M. Reading) would like to thank the French goverment for a scholarship that enabled him to carry out part of the work in France and J. Rouquerol and F. Rouquerol for allowing him to use their specialised thermobalance.     

Determination of kinetic mechanisms for reactions measured with thermoanalytical instruments

The work considers the methods and techniques, allowing the assignment of the kinetic mechanisms to the chemical reactions evaluated from signals of thermoanalytical measurements. It describes which information about the kinetic mechanisms can be found from either model-free or model-based methods. The work considers the applicability of both methods and compares their results. The multiple-step reactions with well-separated peaks can be equally analyzed by both methods, but for overlapping peaks or for simultaneously running parallel reactions the model-free methods provide irrelevant results.     

The kinetics and mechanisms of elementary NF2 reactions with O and N atoms

A combined EPR–LMR spectrometer with a fast-flow system has been used to investigate the kinetics and mechanisms of NF₂ reactions with O and N atoms at 298 K. The overall rate constants of these reactions are: k₀ = (2.8 ± 0.4) × 10^?11 cm³/s and k_N = (5.7 ± 0.8) × 10^?11 cm³/s. The stoichiometry of the reactions with respect to O, N, NF₂, F, and NO has been determined. The statistical theory of bimolecular reactions has been used for interpretation of the results obtained.     

Evaluation of kinetic parameters and mechanisms of complex reactions from thermal analysis data

A method is for TA data-processing: the method of exponential multipliers (MEM). The application of this method permits determination of the steps of the process, the type of the kinetic function, the type of the reaction and the kinetic parameters of the reaction steps. The method is simple and is easily carried out with the use of a mini-computer. The efficiency of the method is illustrated on the example of the thermal decomposition of ammonia copper chromate. The mechanism of the process and the kinetic constants were determined. They agree with available literature data.

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Zusammenfassung Ein Verfahren zur Verarbeitung von TA-Daten wurde entwickelt: das Verfahren exponentieller Multiplizierglieder (MEM). Die Anwendung dieses Verfahrens erlaubt die Bestimmung der Schritte eines Vorganges, des Typs der kinetischen Function, des Reaktionstypes und der kinetischen Parameter der Reaktionsschritte. Es handelt sich um eine einfache Methode, die auf einem Mini-Computer leicht verwirklicht werden kann. Die Wirksamkeit dieses Verfahrens wird am Beispiel der thermischen Zersetzung von Ammoniumkupferchromat gezeigt. Der Mechanismus und die kinetischen Konstanten des Prozesses wurden bestimmt, sie stimmen mit erreichbaren Literaturdaten überinen.

     

Extracting the mechanisms and kinetic models of complex reactions from atomistic simulation data

Determining reaction mechanisms and kinetic models, which can be used for chemical reaction engineering and design, from atomistic simulation is highly challenging. In this study, we develop a novel methodology to solve this problem. Our approach has three components: (1) a procedure for precisely identifying chemical species and elementary reactions and statistically calculating the reaction rate constants; (2) a reduction method to simplify the complex reaction network into a skeletal network which can be used directly for kinetic modeling; and (3) a deterministic method for validating the derived full and skeletal kinetic models. The methodology is demonstrated by analyzing simulation data of hydrogen combustion. The full reaction network comprises 69 species and 256 reactions, which is reduced into a skeletal network of 9 species and 30 reactions. The kinetic models of both the full and skeletal networks represent the simulation data well. In addition, the essential elementary reactions and their rate constants agree favorably with those obtained experimentally. © 2019 Wiley Periodicals, Inc.     

Thermal decomposition mechanisms of the energetic benzotrifuroxan:1,3,3-trinitroazetidine cocrystal using ab initio molecular dynamics simulations

Ab initio molecular dynamics simulations were performed to investigate the thermal decomposition mechanisms of the energetic benzotrifuroxan (BTF):1,3,3-trinitroazetidine (TNAZ) cocrystal at high temperature. It is found that there are four initial reaction mechanisms involved in the decomposition of the cocrystal. Subsequent decomposition channels can be divided into three types: BTF-chain isomerization, C─NO₂ bond homolysis, and ring opening. After that, one main path is that long chains decomposed into small radicals gradually after the ring opening. The other is that a new ring was formed after the ring opening and then it will break by degrees. Releasing of the H radicals and oxygen-containing groups plays an important role in the whole decomposition process. We also studied the release mechanisms of nitrogen gas and carbon dioxide in the later decomposition stage. Our study may provide new insights into the initiation mechanisms and subsequent decomposition of cocrystal explosives at high temperature.     

Calculation of vibronic couplings for phenoxyl/phenol and benzyl/toluene self-exchange reactions: implications for proton-coupled electron transfer mechanisms

The vibronic couplings for the phenoxyl/phenol and the benzyl/toluene self-exchange reactions are calculated with a semiclassical approach, in which all electrons and the transferring hydrogen nucleus are treated quantum mechanically. In this formulation, the vibronic coupling is the Hamiltonian matrix element between the reactant and product mixed electronic-proton vibrational wavefunctions. The magnitude of the vibronic coupling and its dependence on the proton donor-acceptor distance can significantly impact the rates and kinetic isotope effects, as well as the temperature dependences, of proton-coupled electron transfer reactions. Both of these self-exchange reactions are vibronically nonadiabatic with respect to a solvent environment at room temperature, but the proton tunneling is electronically nonadiabatic for the phenoxyl/phenol reaction and electronically adiabatic for the benzyl/toluene reaction. For the phenoxyl/phenol system, the electrons are unable to rearrange fast enough to follow the proton motion on the electronically adiabatic ground state, and the excited electronic state is involved in the reaction. For the benzyl/toluene system, the electrons can respond virtually instantaneously to the proton motion, and the proton moves on the electronically adiabatic ground state. For both systems, the vibronic coupling decreases exponentially with the proton donor-acceptor distance for the range of distances studied. When the transferring hydrogen is replaced with deuterium, the magnitude of the vibronic coupling decreases and the exponential decay with distance becomes faster. Previous studies designated the phenoxyl/phenol reaction as proton-coupled electron transfer and the benzyl/toluene reaction as hydrogen atom transfer. In addition to providing insights into the fundamental physical differences between these two types of reactions, the present analysis provides a new diagnostic for differentiating between the conventionally defined hydrogen atom transfer and proton-coupled electron transfer reactions.     

Use of kinetic data obtained by the decomposition of single crystals for studies on endothermic decomposition reactions of ultra-fine particles

The kinetics of endothermic decomposition reactions of ultra-fine powders is described in terms of a spherical particle model which assumes that the kinetic data obtained for decomposition of single crystals of the same composition and under the same experimental conditions still hold for the powders.Rate equations are derived and compared with data obtained in vacuo and at constant temperature for the thermal decomposition of ultra-fine particles of calcium carbonate. The agreement is satisfactory.     

Fast racemization and dynamic kinetic resolution of primary benzyl amines

We prepared a Pd nanocatalyst (average diameter of Pd nanoparticles = 1.73 nm) displaying a remarkable activity for the racemization and dynamic kinetic resolution (DKR) of 1-methylbenzylamine. It was eight times more active than the previous best. The DKR of 1-methylbenzylamine with the Pd nanocatalyst (2 mol %) in the presence of a thermostable lipase (Novozym 435) was complete in 6 h at 70 °C. The DKRs of other benzyl amines also proceeded to completion in 6 h under similar conditions except the amount of Pd nanocatalyst.     

Carbonyl-forming elimination reactions. A kinetic study of the decomposition of β-nitroalcohols in water

β-Nitroalcohols $\text{1}$a–g undergo, in aqueous buffered solution, a specific base catalyzed carbonyl-forming elimination. The transition state for the rate-limiting CC bond breakage step has been estimated to be rather reactant-like.     

Relative rates of nucleophilic reactions of benzyl carbocation formed in photolysis of benzyl chloride and benzyl acetate in aqueous solution

Benzyl chloride and benzyl acetate were photolyzed in 30% methanol–water mixtures (V/V) at 0°C. The photolysis produces benzyl carbocations that react with nucleophiles. The reaction products were analyzed by gas chromatography or liquid chromatography. From the amounts of products the relative values of rate constants of reactions of benzyl carbocation with nucleophiles N and water k(N)/k(H₂O) were calculated. Benzyl carbocation reacts with I^?, Br^?, Cl^?, and Ac^? ions with approximately diffusion-controlled rate. A value of 2.4 × 10⁷ dm³ mol^?1 s^?1 for the rate constant k(H₂O) and a lifetime of 0.7 ns were estimated for benzyl carbocation in the aqueous solution.     

Nickel-catalyzed Heck-type reactions of benzyl chlorides and simple olefins

Nickel-catalyzed intermolecular benzylation and heterobenzylation of unactivated alkenes to provide functionalized allylbenzene derivatives are described. A wide range of both the benzyl chloride and alkene coupling partners are tolerated. In contrast to analogous palladium-catalyzed variants of this process, all reactions described herein employ electronically unbiased aliphatic olefins (including ethylene), proceed at room temperature, and provide 1,1-disubstituted olefins over the more commonly observed 1,2-disubstituted olefins with very high selectivity.     

High-temperature decomposition of amorphous and crystalline cellulose: reactive molecular simulations

We study the thermal decomposition of cellulose using molecular simulations based on the ReaxFF reactive force field. Our analysis focuses on the mechanism and kinetics of chain scission, and their sensitivity on the condensed phase environment. For this purpose, we simulate the thermal decomposition of amorphous and partially crystalline cellulose at various heating rates. We find that thermal degradation begins with depolymerization via glycosidic bond cleavage, and that the order of events corresponds to a randomly initiated chain reaction. Depolymerization is followed by ring fragmentation reactions that lead to the formation of a number of light oxygenates. Water is formed mainly in intermolecular dehydration reactions at a later stage. The reaction rate of glycosidic bond cleavage follows a sigmoidal reaction model, with an apparent activation energy of 166?±?4 kJ/mol. Neither the condensed phase environment nor the heating programme have appreciable effects on the reactions. We make several observations that are compatible with mechanisms proposed for cellulose fast pyrolysis. However, due to the absence of anhydrosugar forming reactions, the simulations offer limited insight for conditions of industrial interest. It remains unclear whether this is a natural consequence of the reaction conditions, or a shortcoming of the force field or its parameter set.

Graphic abstract