Database of atomistic reaction mechanisms with application to kinetic Monte Carlo

Kinetic Monte Carlo is a method used to model the state-to-state kinetics of atomic systems when all reaction mechanisms and rates are known a priori. Adaptive versions of this algorithm use saddle searches from each visited state so that unexpected and complex reaction mechanisms can also be included. Here, we describe how calculated reaction mechanisms can be stored concisely in a kinetic database and subsequently reused to reduce the computational cost of such simulations. As all accessible reaction mechanisms available in a system are contained in the database, the cost of the adaptive algorithm is reduced towards that of standard kinetic Monte Carlo.     

A comparison of direct methods to determine n-th order kinetic parameters of solid thermal decomposition for use in fire models

Pyrolysis models for burning solids in fire simulations are sensitive to the values of the activation energy, frequency factor, and reaction order that characterize the thermal decomposition of the solid to gaseous fuel, so direct measurement of these kinetic parameters is recommended, and simple methods are preferred. Three direct methods were evaluated with regard to the ability of their kinetic parameters to reproduce the thermal decomposition of five polymers measured by differential thermogravimetric analysis using the reaction order model. It was found that the two multiple heating rate methods produced identical, physically based kinetic parameters, while the peak property method produced non-physical kinetic parameters. However, all of these kinetic parameters in a single-step reaction order model gave reasonably good conversion histories for non-charring and moderately charring polymers. For a highly charring polymer, the conversion histories were poorly described without a multiple step reaction. The temperature at the maximum rate of conversion was found to be essentially independent of the reaction order, which decouples the frequency factor from the reaction order in the direct kinetic methods. Any of the direct methods are sufficiently accurate to obtain kinetic parameters for pyrolysis models because of the inherent spatial and temporal averaging of reaction rates at the burning surface of a thick solid and the uncertainty in the heat transfer mechanisms and thermo-physical parameters used in the models.     

Combined kinetic analysis of solid-state reactions: a powerful tool for the simultaneous determination of kinetic parameters and the kinetic model without previous assumptions on the reaction mechanism

The combined kinetic analysis implies a simultaneous analysis of experimental data representative of the forward solid-state reaction obtained under any experimental conditions. The analysis is based on the fact that when a solid-state reaction is described by a single activation energy, preexponetial factor and kinetic model, every experimental T-alpha-dalpha/dt triplet should fit the general differential equation independently of the experimental conditions used for recording such a triplet. Thus, only the correct kinetic model would fit all of the experimental data yielding a unique activation energy and preexponential factor. Nevertheless, a limitation of the method should be considered; thus, the proposed solid-state kinetic models have been derived by supposing ideal conditions, such as unique particle size and morphology. In real systems, deviations from such ideal conditions are expected, and therefore, experimental data might deviate from ideal equations. In this paper, we propose a modification in the combined kinetic analysis by using an empirical equation that fits every f(alpha) of the ideal kinetic models most extensively used in the literature and even their deviations produced by particle size distributions or heterogeneities in particle morphologies. The procedure here proposed allows the combined kinetic analysis of data obtained under any experimental conditions without any previous assumption about the kinetic model followed by the reaction. The procedure has been verified with simulated and experimental data.     

铝酸钠溶液中析出一水软铝石过程的宏观动力学

对铝酸钠溶液中析出一水软铝石过程的宏观动力学进行了研究.在考虑主要影响因素的前提下,提出了动力学模型,并通过线性回归计算出方程的参数,给出了相应的动力学方程.研究结果表明:分解反应对A l2O3过饱和度和游离NaOH浓度的反应级数分别为2.24和-2.24,前者与大多数研究者的结论一致,而后者则说明溶液中游离NaOH是对反应影响较大的动力学制约因素;反应的表观活化能为62.248 kJ/mol.     

Effect of water addition on the cure kinetics of an epoxy‐amine thermoset

In this study, the effect of water addition on cure kinetics in an epoxy‐amine thermoset was investigated. Near FTIR spectra demonstrated that a small amount of water addition significantly accelerated the cure reaction in terms of epoxide conversion, with water acting as a catalyst for the reaction. Use of a modified mechanistic model allowed direct comparison of the effect of hydroxyl groups generated from water addition to those generated from the polymer chain. The comparison of those kinetic parameters shows that the two effects are very close, in which difference in the logarithmic value of the reaction constant is less than one order of magnitude over all the reaction conditions. The kinetic study also confirmed a strong negative substitution effect for this system. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Reactions of antioxidants with molecular oxygen. Part III: Influence of phenolic stabiliser structures on their oxidation in an inert matrix

The oxidation of five different phenolic antioxidants in inert substrates has been investigated varying the temperature and oxygen pressure conditions. The stabiliser consumption in this reaction has been quantified by steric exclusion chromatography. The tendency of the variation of the residual antioxidant concentration with exposure time in the presence of oxygen shows a simple apparent first order process in all cases. The kinetic parameters were determined applying this simple kinetic model. The influence of structural characteristics of stabilisers under study on kinetic parameters of the direct reaction with the oxygen has been analysed. On the other hand, the information obtained about reaction products generated for each antioxidant helped us to explain the results obtained.     

Determination of Polyurethane Foam Growth Kinetics by a Simple Column Height Test

In the present work, the height of foam column has been used to monitor the reaction of the isocyanate group. Kinetic modeling describes the conversion versus time curves. The global reaction can be divided into two different paths: the blowing reaction and the polymerization. The kinetics of the reaction of toluene diisocyanate with a polypropylene glycol has been studied with auxiliary blowing agents; the concentration of NCO and OH groups could be considered equivalent during all the experiments as a model for similar polyurethane systems. Despite the complexity of the system in which different kinetic orders are possible, as well a physically controlled diffusion process, data were fitted to a simple kinetic model of an apparent order. The results obtained have enabled us to propose a reaction path and catalyzed second‐order kinetic model.     

Mechanistic study of the Soai autocatalytic reaction informed by kinetic analysis

Detailed kinetic studies of the Soai reaction affirm the basic mechanistic picture previously developed in kinetic, spectroscopic and molecular modelling studies, and also suggest that refinements to the proposed elementary reaction steps must be considered. The alkanol reaction product is driven strongly and without bias towards the formation of homochiral and heterochiral dimers. The reaction is catalyzed by only the homochiral dimers. Consideration of the kinetic profiles from reactions carried out with different initial dialkylzinc concentrations support a proposal for a tetrameric transition state, but further experimental work is required to delineate the nature of this species. The power of reaction calorimetry as a kinetic tool for discerning subtle effects of the shape of the kinetic profile is highlighted. The importance of combining kinetic evidence with spectroscopic and other characterization tools is emphasized.     

Kinetics of the thiazolium ion-catalyzed benzoin condensation.

The formation of benzoin (Ph-CHOH-CO-Ph) from two molecules of benzaldehyde, catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium bromide in methanol buffered with Et(3)N/Et(3)NH(+)Cl(-) has been studied. Initial-rate studies at various concentrations of PhCHO (0.1-1.7 M) showed that the reaction is close to being first order in PhCHO. Following the reaction in deuteriomethanol, (1)H NMR spectroscopy allowed rate constants for all three kinetically significant steps to be determined. These show that all three steps are partially rate-determining. A normal deuterium kinetic isotope effect for the overall reaction (k(H)/k(D) approximately 3.4) is observed using PhCDO, and a large inverse solvent isotope effect (k(D)/k(H) approximately 5.9) is observed using deuteriomethanol, consistent with the kinetic scheme presented here.     

Investigations of Pd-catalyzed ArX coupling reactions informed by reaction progress kinetic analysis

This Perspective highlights how the methodology of reaction progress kinetic analysis can provide a rapid and comprehensive kinetic profile of complex catalytic reaction networks under synthetically relevant conditions in a fraction of the number of experiments required by classical kinetic analysis. This approach relies on graphical manipulation of the extensive data sets available from accurate in situ monitoring of reaction progress under conditions where two concentration variables are changing simultaneously. A series of examples from Pd-catalyzed coupling reactions of aryl halides demonstrates how a wealth of kinetic information may be extracted from just three experiments in each case. Even before proposing a reaction mechanism, we can determine reaction orders in substrates, propose a resting state for the catalyst, and probe catalyst stability. Carrying out this kinetic analysis at the outset of a mechanistic investigation provides a framework for further work aimed at seeking a molecular-level understanding of the nature of the species within the catalytic cycle. To be considered plausible, any independent mechanistic proposal must be shown to be consistent with this global kinetic analysis.     

Prediction of thermal stability of fresh and aged parchment

The hyphenated thermal analysis-mass spectrometry technique (TA-MS) was applied for the investigation of the thermal behavior of reference and aged parchment samples. The kinetic parameters of the process were calculated independently from all recorded TA and MS signals. The kinetic analysis showed the distinct dependence of the activation energy on the reaction progress. Such behavior is characteristic for the multistage mechanism of the reaction. The comparison of the kinetic parameters calculated from the different signals i.e. TG, DSC, MS for H₂O, NO and CO₂, however, indicated that they were differently dependent on the aging of the sample. For the parchment samples, the aging almost does not change the kinetics of the decomposition calculated from the DSC data: the influence of aging seems to be too negligible to be detected by these techniques. On the other hand, the much more sensitive mass spectrometric technique applied to the kinetic analysis allowed monitoring of visible changes in the thermal behavior of the parchment samples due to the aging process. The influence of aging was especially visible when the MS signals of water and nitric oxide were applied for the determination of the kinetic parameters. The applied method of the kinetic analysis allowed also the prediction of the thermal behaviour of reference and aged parchment samples under isothermal and modulated temperature conditions. Presented results have confirmed the usefulness of thermoanalytical methods for investigating behaviour of such complicated systems as leather or parchment.     

Pitfall of an initial rate study: on the kinetics and mechanism of the reaction of periodate with iodide ions in a slightly acidic medium

The kinetics of the periodate-iodide reaction has a contradictory history dating back to almost a century. This reaction has been reinvestigated spectrophotometrically in the pH range 3.13-5.55 in both buffered (acetic acid/acetate) and unbuffered solution at T=25.0+/-0.1 degrees C with an I=0.5 M ionic strength. The spectra between 290 and 500 nm were recorded and the reaction was followed until at least 95% of one of the reactants was consumed. The stoichiometry has been found to be strongly dependent on pH, but the rate of the initial step is independent of pH within the pH range studied. An eight-step kinetic model is proposed with four fitted kinetic parameters to take all the important characteristics of the experimental curves into account. On the basis of the model, a perfect reconciliation of the previous contradictory results is presented. It is shown that the kinetic parameters obtained from the initial rate of formation of a product unavoidably leads to misinterpretation of the results in the case of a branching mechanism (and stoichiometry).     

High performance liquid chromatography study on the kinetics and mechanism of chlorite-thiosulfate reaction in slightly alkaline medium

Chlorite-thiosulfate reaction was studied by high performance liquid chromatography under slightly alkaline solution by monitoring the concentration of thiosulfate and tetrathionate simultaneously during the course of reaction. It is demonstrated that various polythionates are formed and the composition of polythionates mainly depends on pH. Initial rate studies have revealed that the formal kinetic order of hydrogen ion is unambiguously unity but that of chlorite ion is significantly larger, while that of thiosulfate is lower than one. A 10-step kinetic model is proposed with seven fitted and three fixed parameters by simultaneous evaluation of kinetic data taking all the important characteristics of the measured kinetic curves into account. It is also enlightened that higher formal kinetic order of chlorite and lower kinetic order of thiosulfate than unity is an inherent feature of the system and may explain the systematic deviation between the measured and the calculated data encountered in previous studies.     

Extraction of the contribution of kinetic profile for reaction of a chemical species in unknown matrices by standard addition method

A new and simple strategy is applied to resolve kinetic profile for the reaction of an analyte in unknown matrices, using standard addition method (SAM). The proposed method uses kinetic spectrophotometric data obtained by standard addition of analyte into unknown mixtures followed by the reaction of analyte with a proper reagent. The proposed method extracts kinetic profile for the reaction of an analyte by averaging the kinetic profiles obtained by subtraction of kinetic profiles after and before standard addition. The rate constant can be obtained using computational curve fitting. The performance of method was evaluated by using synthetic data as well as several experimental data sets. The proposed method can be applied to obtain kinetic profiles of the reactions in the presence of additive interference as well as multiplicative interferences. Hydroxylation reaction of diphenylcarbazide (DPCI) in the presence of diphenylcarbazone (DPCO) as a real system at various pHs was also studied by the present method. The rate constant and the order of the hydroxylation reaction were determined from extracted kinetic profiles.     

Synthesis of polyketides via diastereoselective acetalization

[reaction: see text] Diastereoselective acetalization of pseudo-C(2)-symmetric 1,3,5-triol systems is a general strategy for the rapid generation of polyketides. The oxidative acetalization reaction shown above was studied under both kinetic and thermodynamic conditions, using synthetic 1,3,5-triol units. In addition, all possible stereochemical variants of a 1,3,5-triol system were prepared from the corresponding acetal to expand the synthetic versatility of this method.     

杯[6]芳烃-双锰卟啉催化苯乙烯环氧化反应的动力学

研究了杯[6]芳烃－双锰卟啉仿P450酶模型化合物对苯乙烯环氧化反应的催化性能，考察了催化剂浓度、底物浓度、氧源浓度及加入方式等对催化反应的影响，结果表明，由于具有较大疏水孔穴的杯[6]芳烃的引入，使金属卟啉的催化性能得以较大的提高，且反应遵从Michaetis-menten规律，在实验基础上，本文提出了一种可能的反应机理。     

On the complexity of kinetics and the mechanism of the thiosulfate-periodate reaction

The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.     

Die bestimmung kinetischer parameter endothermer zersetzungsreaktionen unter nicht-isothermen bedingungen

By using the kinetic parameters calculated from non-isothermal measurements according to Freeman and Carroll a computer programm is given, which permits the determination of the more probable reaction mechanism by use of 17 different kinetic equations. The computer programm includes the kinetic equation for chemical reaction, nucleation, phase boundary reaction and diffusion and can easily be extended to other equations. The evaluation of the experimental values by a distinct kinetic equation is quantitatively characterized by means of the correlation coefficient.The kinetic parameters are calculated on the basis of a regression analysis. By dividing the thermogravimetric curve in different reaction intervals a differential calculation is possible.Decomposition of CaCO₃, MgCO₃ and CaSO₄·2H₂O is discussed from the results of the computer calculation. It is seen that the kinetic parameters and the reaction mechanism are not constant during the reaction. Further experiments for a physical interpretation are being carried out.     

Kinetic and catalytic aspects of dimethylterephtalate transesterification also through the use of model molecules

Kinetic and catalytic aspects of DMT transesterification reaction, key step in the industrial production of polyethylenterephtalate (PET), have been deepened also through the use of model molecules such as CH₃COO–Ph–X. These molecules give the same reactions with ethylene glycol occurring in the DMT transesterification too. The use of model molecules in the study of this reaction seems therefore to be promising in perspective. As we observed, however, the nature of the para-substituent X has a very strong influence on the reaction rate, when salts of bivalent metals such as Zn, Cd, Co, Mg, Mn, are used as catalysts, while this influence is much smaller when a tetravalent metal, as Ti, is used. This fact suggests that the transesterification mechanism operating in the two cases is different and this suggestion has been confirmed by applying Hammett's approach to the available kinetic data. Experimental kinetic runs were all performed at 193°C, by withdrawing small samples of the reaction mixture at different reaction times. These samples were gaschromatographically analyzed. We found volcano shaped curves of the reaction rates as a function of the metal ion acidities and we obtained different trends with a maximum of activity shifted from a metal to another for different substrates. A comparison of the kinetic results obtained, respectively, with the model molecules and DMT will be reported too.