Comments on current aspects of chemical kinetics

An aged kineticist was awakened in 1989 from a 50-year slumber. After arduous study he sized-up the present situation, with occasional reflections on the state of knowledge in 1939.     

The development of a chemical engineering data base

The organization of a scientific data-base system and a management system is outlined. The chemical engineering data base includes seven separate data bases. Examples of applications of the inorganic thermochemistry data base are given for the production of phase diagrams and simulation of the QSL method for lead smelting. Interfacing of the various units is discussed.     

An electronic database with a compilation of k0 and related data for NAA

An electronic data base (DB) containing recently evaluated k ₀ and related data has been developed. The tables composing the DB are relationally linked to support data integrity. The purpose of the DB development is to make an official source of data used for electronic synchronization of the input parameters needed for the k ₀ methodology, which is developing in numerous laboratories. Such solution saves time when updating, ensures the quality of the primary data and hence of the analysis results, and due to the recording of the updating history preserves traceability of the data in time.     

Spin-selective recombination kinetics of a model chemical magnetoreceptor

We determine the spin-selective kinetics of a carotenoid-porphyrin-fullerene triad that has previously been used to establish the principle that a photochemical reaction could form the basis of the magnetic compass sensor of migratory birds and show that its magnetic sensitivity can be understood without invoking quantum Zeno effects.     

Mechanism and kinetics of dithiobenzoate‐mediated RAFT polymerization. I. The current situation

Investigations into the kinetics and mechanism of dithiobenzoate‐mediated Reversible Addition–Fragmentation Chain Transfer (RAFT) polymerizations, which exhibit nonideal kinetic behavior, such as induction periods and rate retardation, are comprehensively reviewed. The appreciable uncertainty in the rate coefficients associated with the RAFT equilibrium is discussed and methods for obtaining RAFT‐specific rate coefficients are detailed. In addition, mechanistic studies are presented, which target the elucidation of the fundamental cause of rate retarding effects. The experimental and theoretical data existing in the literature are critically evaluated and apparent discrepancies between the results of different studies into the kinetics of RAFT polymerizations are discussed. Finally, recommendations for further work are given. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5809–5831, 2006     

Consecutive reactions in chemical kinetics: A method for the treatment of experimental data

A method is proposed for the treatment of data from a kinetic system of first order consecutive reactions by using an optimization procedure which converges to the correct solution of the two mathematically possible solutions and which does not need initial estimates of the parameters close to those to be determined.

---

, , , .

     

An automated vapor-generation and data collection instrument for the evaluation of chemical microsensors

An instrument has been built which can generate up to twelve different vapors from bubbler or permeation tube sources. These vapors can be individually selected, diluted to concentrations typically in the range of 1 to 10 000 mg/m³ volume and delivered at a programmable flow rate to the system output. Mixtures of two vapors can also be easily generated. The system is fully automated using a small personal computer and permits completely unattended operation during elaborate vapor-exposure sequences. Parameters describing the vapor test conditions are automatically stored in the computer disk memory, along with the sensor responses to the test. This instrument has proved to be extremely valuable for rapidly testing prototype chemical sensors.     

A combined linear and nonlinear factor analysis program package for chemical data evaluation

An underlying variable factor analysis program (UVFA) is described. The theories of principal component analysis and nonlinear least-squares projection techniques are outlined and compared. Several applications from various chemical fields are presented which show that a complete analysis of the underlying structure and dimensionality of a chemical data set should always include these nonlinear projection techniques.     

Artificial neural network model for the evaluation of chemical kinetics in thermally induced solid-state reaction

The artificial intelligence technique is utilized to improve evaluation of thermally induced solid-state reaction kinetics. A general regression neural network (GRNN) model was applied to directly determine the kinetic triplets, i.e., activation energy, pre-exponential factor, and mechanism model. The effect of number of heating rate on prediction performance of the GRNN model was assessed based on the estimation indictors. The obtained kinetic triplets based on the triple heating rates were considered to be accepted. The prediction ability of the GRNN model was very robust at more than three heating rates. The relative errors for kinetic parameters derived from five heating rates were within ±?4%, and the cognition rates for mechanism models were up to 99.6%. The developed GRNN model was successfully applied in the high-temperature synthesis of Li₄Ti₅O₁₂/C composites. It is expected that the model also could be extended to estimate the kinetics of other solid-state reactions.

     

Reactant subspaces and kinetics of chemical reaction networks

This paper studies a chemical reaction network’s (CRN) reactant subspace, i.e. the linear subspace generated by its reactant complexes, to elucidate its role in the system’s kinetic behaviour. We introduce concepts such as reactant rank and reactant deficiency and compare them with their analogues currently used in chemical reaction network theory. We construct a classification of CRNs based on the type of intersection between the reactant subspace R and the stoichiometric subspace S and identify the subnetwork of S-complexes, i.e. complexes which, when viewed as vectors, are contained in S, as a tool to study the network classes, which play a key role in the kinetic behaviour. Our main results on new connections between reactant subspaces and kinetic properties are (1) determination of kinetic characteristics of CRNs with zero reactant deficiency by considering the difference between (network) deficiency and reactant deficiency, (2) resolution of the coincidence problem between the reactant and kinetic subspaces for complex factorizable kinetics via an analogue of the generalized Feinberg–Horn theorem, and (3) construction of an appropriate subspace for the parametrization and uniqueness of positive equilibria for complex factorizable power law kinetics, extending the work of Müller and Regensburger.     

The association reaction between C2H and 1-butyne: a computational chemical kinetics study

The potential energy surfaces (PES) for the reaction of the C(2)H radical with 1-butyne (C(4)H(6)) have been studied using the CBS-QB3 method. Density functional B3LYP/cc-pVTZ and M06-2X/6-311++G(d,p) calculations have also been performed to analyze the reaction energetics. For detailed theoretical calculation on the total reaction mechanism, the initial association reactions on more and less substituted C atoms of 1-butyne are treated separately followed by a variational transition state theory (VTST) calculation to obtain reaction rates. The successive unimolecular reactions from the association reaction complexes are subjected to Rice-Ramsperger-Kassel-Marcus (RRKM) calculations for reaction rate constants and product branching ratios. The calculated rate constants in the temperature range 70-295 K for both the association reactions are found to be highly temperature dependent at low temperatures, which is contrary to the experimental findings of temperature independent association rates. We have explained this observation with the help of variational nature of the transition states, and we found a "loose" transition state at low temperatures. The calculated product branching ratios for the unimolecular reactions generally agree with the available experimental data, although some channels show a significant method dependency and therefore the correlation with experiment is lost to some extent. Our detailed reaction energetics calculations confirm that the C(2)H + C(4)H(6) reaction proceeds without an entrance barrier and leads to the important products ethynylallene + CH(3), 1,3-hexadiyne + H, 3,4-hexadiene-1-yne + H, 2-ethynyl-1,3-butadiene + H, 3,4-dimethylenecyclobut-1-ene + H and fulvene + H exothermic by 25-75 kcal mol(-1), with strong dependence of the product distribution on the association mode of C(2)H with C(4)H(6), making these reactions fast under low temperature conditions of Titan's atmosphere. Therefore this study can provide a detailed picture of the complex hydrocarbon formation mechanism in the upper atmosphere.     

Tackling the chemical diversity of microbial nonulosonic acids – a universal large-scale survey approach

Nonulosonic acids, commonly referred to as sialic acids, are a highly important group of nine-carbon sugars common to all domains of life. They all share biosynthetic and structural features, but otherwise display a remarkable chemical diversity. In humans, sialic acids cover all cells which makes them important for processes such as cellular protection, immunity and brain development. On the other hand, sialic acids and other nonulosonic acids have been associated with pathological processes including cancer and viral infections. In prokaryotes, nonulosonic acids are commonly associated with pathogens, which developed through molecular mimicry a strategy to circumvent the host''s immune response. However, the remarkably large chemical diversity of prokaryotic nonulosonic acids challenges their discovery, and research on molecular characteristics essential for medical applications are often not feasible. Here, we demonstrate a novel, universal large-scale discovery approach that tackles the unmapped diversity of prokaryotic nonulosonic acids. Thereby, we utilize selective chemical labelling combined with a newly established mass spectrometric all-ion-reaction scanning approach to identify nonulosonic acids and other ulosonic acid-like sugars. In doing so, we provide a first molecular-level comparative study on the frequency and diversity across different phyla. We not only illustrate their surprisingly wide-spread occurrence in non-pathogenic species, but also provide evidence of potential higher carbon variants. Many biomedical studies rely on synthetic routes for sialic acids, which are highly demanding and often of low product yields. Our approach enables large-scale exploration for alternative sources of these highly important compounds.

A novel large-scale survey approach for microbial nonulosonic acids (sialic acids) including a first molecular level comparative study is presented.     

The kinetics of solid‐state reactions toward consensus,Part 2: Fitting kinetics data in dynamic conventional thermal analysis

This paper is the second part of a review of some of the controversial kinetic aspects of conventional thermal analysis (TA). In this part the physico‐geometric kinetics for the solid‐state reactions were examined. The main problems discussed are (1) problems in fitting kinetics data: the identification of the f (α) function, (2) the suitability of conventional dynamic methods of TA, and (3) the complex reactions. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 193–208, 2002; DOI 10.1002/kin.10028