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The experimental execution of chemical reactions is a context-dependent and time-consuming process, often solved using the experience collected over multiple decades of laboratory work or searching similar, already executed, experimental protocols. Although data-driven schemes, such as retrosynthetic models, are becoming established technologies in synthetic organic chemistry, the conversion of proposed synthetic routes to experimental procedures remains a burden on the shoulder of domain experts. In this work, we present data-driven models for predicting the entire sequence of synthesis steps starting from a textual representation of a chemical equation, for application in batch organic chemistry. We generated a data set of 693,517 chemical equations and associated action sequences by extracting and processing experimental procedure text from patents, using state-of-the-art natural language models. We used the attained data set to train three different models: a nearest-neighbor model based on recently-introduced reaction fingerprints, and two deep-learning sequence-to-sequence models based on the Transformer and BART architectures. An analysis by a trained chemist revealed that the predicted action sequences are adequate for execution without human intervention in more than 50% of the cases., In organic chemistry, synthetic routes for new molecules are often specified in terms of reacting molecules only. The current work reports an artificial intelligence model to predict the full sequence of experimental operations for an arbitrary chemical equation.

This comment is devoted to the paper "Exact solutions of nonlinear diffusion-convection-reaction equation: A Lie symmetry approach" (CNSNS, vol. 67 (2019), 253-263) in which several results are not new because were derived much earlier. Moreover, some results in the paper are incorrect or incomplete., Commun Nonlinear Sci Numer Simulat

Implementation of finite element method (FEM) needs special cares, particularly for essential boundary conditions that have an important effect on symmetry and number of unknowns in the linear systems. Moreover, avoiding numerical integration and using (off-line) calculated element integrals decrease the computational cost significantly. In this chapter we briefly present theoretical topics of FEM. Instead we focus on what is important (and how) to carefully implement FEM for equations that can be the core of a numerical simulator for a diffusion–advection-reaction problem. We consider general 2D and 3D domains, high contrast and heterogeneous diffusion coefficients and generalize the method to nonlinear parabolic equations. Although we use Matlab codes to simplify the explanation of the proposed method, we have implemented it in C++ to reveal the efficiency and examples are presented to admit it.

In this paper we investigate the numerical approximation of the fractional diffusion, advection, reaction equation on a bounded interval. Recently the explicit form of the solution to this equation was obtained. Using the explicit form of the boundary behavior of the solution and Jacobi polynomials, a Petrov-Galerkin approximation scheme is proposed and analyzed. Numerical experiments are presented which support the theoretical results, and demonstrate the accuracy and optimal convergence of the approximation method., 23 pages, 4 figures

This paper outlines the structure and usage of the GenChem system, which includes a chemical pre-processor GenChem.py) and a simple box model (boxChem). GenChem provides scripts and input files for converting chemical equations into differential form for use in atmospheric chemical transport models (CTMs) and/or the boxChem system. Although GenChem is primarily intended for users of the Meteorological Synthesizing Centre – West of the European Monitoring and Evaluation Programme (EMEP MSC-W) CTM and related systems, boxChem can be run as a stand-alone chemical solver, enabling for example easy testing of chemical mechanisms against each other. This paper presents an outline of the usage of the GenChem system, explaining input and output files, and presents some examples of usage. The code needed to run GenChem is released as open-source code under the GNU license.

In this paper we describe several finite element methods for solving the diffusion-convection-reaction equation. None of them is new, although the presentation is non-standard in an effort to emphasize the similarities and differences between them. In particular, it is shown that the classical SUPG method is very similar to an explicit version of the Characteristic-Galerkin method, whereas the Taylor-Galerkin method has a stabilization effect similar to a sub-grid scale model, which is in turn related to the introduction of bubble functions.

In this paper, the approximate solutions of the fractional diffusion equations described by the fractional derivative operator were investigated. The homotopy perturbation Laplace transform method of getting the approximate solution was proposed. The Caputo generalized fractional derivative was used. The effects of the orders &alpha, and &rho, in the diffusion processes was addressed. The graphical representations of the approximate solutions of the fractional diffusion equation and the fractional diffusion-reaction equation both described by the Caputo generalized fractional derivative were provided.

Both the chemical reaction mechanism and rate can largely explain the formation mechanism of the by-products of SF6. By understanding this mechanism, we can develop and improve models of the chemical kinetics of SF6 decomposition under discharge. Using quantum chemistry, this study compares the reaction mechanisms and rates of SF6−, SF5−, SF4−, and SF3− and H2O under discharge at 298 K–12 000 K and reveals the formation mechanism of their anionic by-products, i.e., SOF4−, SOF3−, SOF2−, and SOF−. The key parameters such as the reaction equation and reaction rate to improve the chemical kinetic models under partial and arc discharges were then obtained. In this study, the structural optimizations, vibrational frequencies, and zero-point energies of the reactants, products, complexes, intermediates, and transition states were calculated at the B3LYP/6-311G(d,p) level. The single-point energies of all species were calculated at the CCSD(T)/aug-cc-PVTZ level. The strengths and sites of weak interactions were determined from the electrostatic potential of the molecular surface, and the reaction rates were obtained using transition state theory. It has been found that SF6−, SF5−, SF4−, and SF3− combined with H2O to form weak-interaction complexes dominated by hydrogen bonding, thus providing the initial conditions for R1, R2, R3, and R4, respectively. All four reactions were composed of multiple elementary reactions with the first step being the rate-determining step. Moreover, compared to their corresponding reactions of SF5, SF4, SF3, and SF2 with H2O, they achieved lower potential energy barriers and higher reaction rates. Note that the reaction rates decreased in the following order: R3 > R2 > R1 > R4.

Realistic numerical simulations of pyrolysis and combustion require not only accurate modeling of fluid dynamics, but also a detailed characterization of chemical reactions. In recent years, increasing efforts have been devoted to the development of more complex reaction mechanisms, with high levels of detail and comprehensiveness. Clearly, thanks to new advancements in chemical kinetics, the size of mechanisms tends to grow with time. Recent trends suggest that in the near future mechanisms with more than ~ 20,000 species will be available. While such large mechanisms may provide very detailed information about the chemistry of combustion and pyrolysis, it is very expensive to accommodate them in numerical simulations. The computational cost can be prohibitive even for ideal systems (i.e., reacting systems in which no transport phenomena are included) or one-dimensional laminar flames. Even if the size (i.e., number of species and reactions) of detailed kinetic mechanisms is the most important challenge for the numerical simulations, also the stiffness (due to the wide range of chemicals times) of the nonlinear chemical equations governing the evolution of species plays a fundamental role in controlling the performance and the robustness of numerical algorithms. The points mentioned above suggest the need of using specifically conceived numerical techniques and tools for carrying out numerical simulations of reacting systems involving detailed kinetics (thousands of species and reactions). The purpose of this contribution is to present the equations of reacting systems typically adopted for developing and validating large kinetic mechanisms and discuss the basic numerical techniques to solve them efficiently. Moreover, because of their importance in the interpretation of results of simulations involving complex chemistry, numerical tools such as sensitivity analysis, rate of production, and reaction path analysis are introduced and described.

Education research results point to frequent misconceptions of basic chemical concepts among students at all levels of education. One reason for this is the abstract nature of the subject, which requires higher cognitive abilities of students in the learning process. Considering these insights, research was conducted with the aim to identify and analyze students’ misconceptions associated with the concept of chemical equation. The results served to design and implement a quasi-experiment aiming to enhance teaching of the mentioned concept, which is presently under way. The research, conducted in December 2013, included 357 eighth grade students from several primary schools. Multiple-choice questions were used in assessing the ability of associating two different ways of presenting a chemical reaction, namely chemical equation and particulate drawing. Analysis of the collected data revealed different misconceptions associated with insufficient understanding of the concepts of atom and molecule, stoichiometric number and index, reaction system and elementary reaction, and difficulties in applying the law of conservation of mass. The research results emphasize the value of particulate drawing as a teaching tool whose application in assessing knowledge enables an analysis of the conceptual understanding of the chemical equation., Rezultati edukacijskih istraživanja ukazuju na učestalost pogrešnih shvaćanja temeljnih kemijskih pojmova kod učenika svih obrazovnih razina. Uzrok tome je i apstraktna priroda nastavnog predmeta koja od učenika zahtijeva više kognitivne sposobnosti pri usvajanju sadržaja. Uzimajući u obzir te spoznaje, provedeno je istraživanje čiji je cilj bio prepoznati i analizirati pogrešna shvaćanja učenika povezana s konceptom jednadžbe kemijske reakcije. Rezultati ovog istraživanja poslužili su za osmišljavanje i provedbu kvazieksperimenta s ciljem unaprjeđenja poučavanja navedenog koncepta, koje je u tijeku. U istraživanju koje je provedeno u prosincu 2013. godine sudjelovalo je 357 učenika osmih razreda osnovnih škola. Zadatcima višestrukog izbora provjeravana je sposobnost povezivanja dvaju načina prikaza kemijske promjene, jednadžbom kemijske reakcije i čestičnim crtežom. Analizom prikupljenih podataka uočena su različita pogrešna shvaćanja povezana s nerazumijevanjem pojmova: atom i molekula, stehiometrijski broj i indeks, reakcijski sustav i elementarna pretvorba, kao i poteškoće primjene zakona o očuvanju mase. Rezultati ovog istraživanja ističu vrijednost čestičnog crteža kao nastavnog alata čija uporaba pri provjeri znanja omogućava analizu konceptualnog razumijevanja jednadžbe kemijske reakcije.

Physical chemistry, chemical physics 20(36), 23281 - 23293 (2018). doi:10.1039/C8CP01898G, Acid–base equilibria of carboxylic acids and alkyl amines in the aqueous surface region were studied using surface-sensitive X-ray photoelectron spectroscopy and molecular dynamics simulations. Solutions of these organic compounds were examined as a function of pH, concentration and chain length to investigate the distribution of acid and base form in the surface region as compared to the aqueous bulk. Results from these experiments show that the neutral forms of the studied acid–base pairs are strongly enriched in the aqueous surface region. Moreover, we show that for species with at least four carbon atoms in their alkyl-chain, their charged forms are also found to be abundant in the surface region. Using a combination of XPS and MD results, a model is proposed that effectively describes the surface composition. Resulting absolute surface concentration estimations show clearly that the total organic mole fractions in the surface region change drastically as a function of solution pH. The origin of the observed surface phenomena, hydronium/hydroxide concentrations in the aqueous surface region and why standard chemical equations, used to describe equilibria in dilute bulk solution are not valid in the aqueous surface region, are discussed in detail. The reported results are of considerable importance especially for the detailed understanding of properties of small aqueous droplets that can be found in the atmosphere., Published by RSC Publ., Cambridge

The aim of this paper is to present an explicit numerical algorithm based on improved spectral Galerkin method for solving the unsteady diffusion-convection-reaction equation. The principal characteristics of this approach give the explicit eigenvalues and eigenvectors based on the time-space separation method and boundary condition analysis. With the help of Fourier series and Galerkin truncation, we can obtain the finite-dimensional ordinary differential equations which facilitate the system analysis and controller design. By comparing with the finite element method, the numerical solutions are demonstrated via two examples. It is shown that the proposed method is effective.

Ongoing acidification of the global ocean necessitates a solid understanding of how biogeochemical processes are driving proton cycling and observed pH changes in natural waters. The standard way of calculating the pH evolution of an aquatic system is to specify first how biogeochemical processes affect total alkalinity, followed by the solution of a nonlinear acid-base equilibrium equation system. This approach, however, does not explicitly reveal how individual biogeochemical processes contribute to the overall proton cycling in the system. Here, we provide an extension of the classical acid-base theory that explicitly quantifies the proton production/ consumption by a given process, showing that it can be calculated as the proton-cycling sensitivity times the rate of the biogeochemical process at hand. The proton-cycling sensitivity emerges as a central concept in acid-base chemistry of natural waters and can be further decomposed as the ratio of a stoichiometric coefficient for the proton over the buffer factor. The stoichiometric coefficient for the proton expresses how many moles of protons would be produced per mole of reaction if buffering was absent, and is obtained by bringing the reaction equation of the process into a specific form: the fractional reaction equation at ambient pH. The buffer factor quantifies how acid-base systems attenuate the proton production/consumption by biogeochemical processes, and is identified as the negative of the partial derivative of the total alkalinity with respect to the proton concentration. Applying this new concept to an acidification scenario for the future surface ocean, we illustrate its potential to analyze proton cycling in natural waters. Thereby we show that a reduced buffer factor due to anthropogenic carbon input makes the ocean more vulnerable to any process influencing the pH.

A model in which the reaction parameters of catalyst layers from microscopic to macroscopic scales are the main focus was systematically developed for polymer electrolyte fuel cells (PEFCs). The effect of oxygen-containing species adsorbed on the Pt surface at the microscopic three-phase boundary (Qox), which suppresses the oxygen reduction reaction (ORR), was taken into account in the PEFC model. Qox was expressed as a reaction parameter in terms of the coverage factor, and was included in the Tafel reaction equation. The advantages of our model were verified by carrying out the model validation, in which the results of cell performance obtained by simulation agreed very well with experimented results. Because of this key improvement, the developed model was more realistic and accurate. The simulation results also demonstrates that the effect of adsorbed oxygen-containing species is a very important reaction parameter that influences cell performance and must be considered in order to achieve high-performance PEFCs.

International audience; In this paper, we propose a mathematical model to describe the functioning of a bioreactor landfill, that is a waste management facility in which biodegradable waste is used to generate methane. The simulation of a bioreactor landfill is a very complex multiphysics problem in which bacteria catalyze a chemical reaction that starting from organic carbon leads to the production of methane, carbon dioxide and water. The resulting model features a heat equation coupled with a non-linear reaction equation describing the chemical phenomena under analysis and several advection and advection-diffusion equations modeling multiphase flows inside a porous environment representing the biodegradable waste. A framework for the approximation of the model is implemented using Feel++, a C++ open-source library to solve Partial Differential Equations. Some heuristic considerations on the quantitative values of the parameters in the model are discussed and preliminary numerical simulations are presented.

We consider the optimal control problem governed by diffusion convection reaction equation without control constraints. The proper orthogonal decomposition(POD) method is used to reduce the dimension of the problem. The POD method may be lack of accuracy if the POD basis depending on a set of parameters is used to approximate the problem depending on a different set of parameters. We are interested in the perturbation of diffusion term. To increase the accuracy and robustness of the basis, we compute three bases additional to the baseline POD. The first two of them use the sensitivity information to extrapolate and expand the POD basis. The other one is based on the subspace angle interpolation method. We compare these different bases in terms of accuracy and complexity and investigate the advantages and main drawbacks of them., Comment: 19 pages, 5 figures, 2 tables

SBA-2 and STAC-1 are periodic mesoporous silicas with slightly different structures whose pore networks consist of spherical cavities interconnected by windows. This feature makes them attractive for adsorptive separation processes where the selectivity originates from molecular sieving. Recently, we were able to obtain realistic atomistic models for these materials by means of a kinetic Monte Carlo (kMC) method. In this paper, we evaluate the ability of the model to predict adsorption of both nonpolar (methane and ethane) and polar (carbon dioxide) adsorptives. Predictions are in good agreement with experimental data, demonstrating the potential of these kMC-based models for use in the design of adsorption processes and the materials used in them. In particular, we show that surface roughness is a key feature for predicting adsorption in SBA-2 materials at low pressures; this is especially relevant in prospective applications such as carbon dioxide capture. (Chemical Equation Presented).

International audience; Focus of this study is a numerical investigation of a reactive transport problem involving an irreversible bimolecular reaction. The advection dispersion reaction equation (ADRE) is typically adopted to model and interpret these types of reactive transport scenarios in porous media at the continuum (Darcy) scale. It is well documented that the theoretical derivation of the ADRE from pore scale system dynamics requires a set of assumptions which are not always met in the context of laboratory and/or field scale applications. We start by recounting the various upscaled formulations which can be obtained through the volume averaging method in the space defined by the Damkohler (Da) and Peclet (Pe) numbers characterizing the phenomenon. The transport problem is then simulated numerically at the pore scale considering different models of disaggregated porous media. Simulation results provide a framework to discuss the key features and appropriateness of continuum scale formulations which can be employed to describe the target geochemical systems for these settings. The impact of Pe, Da and pore space configuration on the parameters embedded in the upscaled formulations is analyzed. Conditions under which such formulations hold are discussed, with emphasis on the critical and widely studied advection dominated scenarios associated with fast reactions (i.e., large values of Pe and Da). Main results include the quantitative assessment of the ability of recently proposed (Porta et al., 2012 [1]) and standard (e.g., ADRE) continuum formulations to quantify the effect of pore scale incomplete solute mixing on upscaled reaction rates. (C) 2013 Elsevier Ltd. All rights reserved.

The operational feasibility temperature range of chemical looping combustion (CLC) and chemical looping reforming (CLR) of the fuels methane, propane, iso-octane and ethanol was explored using the common sulphates Na 2SO 4, CaSO 4 and MgSO 4 as oxygen carriers. The chemical reaction equations for each oxygen carrier and fuel were formulated and fed into the Chemical Equations module of HSC Chemistry 5.1 software to obtain the Gibbs free energy change ( Δ G) of the chemical reaction within the temperature range (200–1200∘C). The CLC and CLR process operational feasibility temperature range for the fuel-oxygen carrier combination was mapped excluding the undesirable formation of SO 2 in CLC and H 2S and S in CLR. Na 2SO 4 and CaSO 4 were found to be suitable for all CLC and CLR systems. MgSO 4 was not found to be a suitable candidate for CLC or CLR processes irrespective of the fuel considered. Methane was found to be better than the other fuels studied as the operational temperature range of CLC and CLR processes were wide for all the sulphates. This short cut methodology presented in this paper can be used to predict the operational feasibility of CLC and CLR processes of different fuels and oxygen carriers.

The concept of the limiting reactant in chemical reactions is one of the fundamental concepts learned by all students enrolled in introductory courses in chemistry at the high school and early undergraduate levels. Practicing chemists, and just about every technical professional involved in at least some aspect of chemistry, use the concepts of stoichiometry, and particularly the limiting reactant concept, in the workplace daily. In the practice of analytical chemistry, one makes use of an analytical method that relies upon a chemical reaction of an analyte (i.e., a chemical species present in a sample to be analyzed, for which concentration and other information is to be sought) with a suitable analytical reagent to generate a desired product, which yields a signal due to a change in a physical property (e.g., color, temperature, current, conductivity, etc.) of the product that is quantifiable and can be related to analyte concentration. The balanced chemical equation for the reaction must also be known. In a chemical analysis, the analyte is deliberately made the limiting reactant, with the analytical reagent added in excess, to ensure that all of the analyte reacts to yield the product and its signal, which in turn are proportional to the analyte concentration. The stoichiometric ratios among the analyte, analytical reagent, and product must be established for the analytical determination to yield reliable results.

International audience; The authors of the current chemistry curriculum -implemented in grade 12 in France- provided a criterion of change allowing predictions of direction of chemical changes and pointed out the difference to be made between experimental facts and models. A study analysing part of the curriculum content and the effects of teaching this content on students' reasoning was conducted. The content analysis presents the functioning of the thermodynamic model which highlights the links to be made between the experimental situation and the model when predicting the direction of a chemical change. This functioning specifies the role of the chemical equation and that of the criterion of change (comparing the reaction quotient to the equilibrium constant) and stresses the crucial points that may lead to misunderstandings. Written tests were administered to students after teaching to determine how they predicted the direction of a chemical change, whether they made a relevant choice between using the chemical equation and using the criterion of change and a clear distinction between the experimental situation and the thermodynamic model. Few students had a good understanding of the respective roles of criterion and chemical equation. A majority used the criterion to predict the direction of chemical changes relevantly but correct answers were not widespread. Two particular mistakes, the modification of the expression of the reaction quotient and the prediction of a change despite a missing reactant revealed that students do not properly understand the difference and the relationships between the experimental situation and the thermodynamic model.

Chemical equations are normally written in terms of specific ionic and elemental species and balance atoms of elements and electric charge. However, in a biochemical context it is usually better to write them with ionic reactants expressed as totals of species in equilibrium with each other. This implies that atoms of elements assumed to be at fixed concentrations, such as hydrogen at a specified pH, should not be balanced in a biochemical equation used for thermodynamic analysis. However, both kinds of equations are needed in biochemistry. The apparent equilibrium constant K' for a biochemical reaction is written in terms of such sums of species and can be used to calculate standard transformed Gibbs energies of reaction Δ(r)G'°. This property for a biochemical reaction can be calculated from the standard transformed Gibbs energies of formation Δ(f)G(i)'° of reactants, which can be calculated from the standard Gibbs energies of formation of species Δ(f)G(j)° and measured apparent equilibrium constants of enzyme-catalyzed reactions. Tables of Δ(r)G'° of reactions and Δ(f)G(i)'° of reactants as functions of pH and temperature are available on the web, as are functions for calculating these properties. Biochemical thermodynamics is also important in enzyme kinetics because apparent equilibrium constant K' can be calculated from experimentally determined kinetic parameters when initial velocities have been determined for both forward and reverse reactions. Specific recommendations are made for reporting experimental results in the literature.

Chemistry learning based on the intertextuality of chemistry demands relationship among chemistry representsation on three levels, which are macrocospic, microscopic, and symbol level with the daily experience of students and the social interaction developed by teachers. Through the research entitled “Pengembangan CD Pembelajaran Interaktif Kimia SMA Berbasis Intertekstualitas sebagai Alternatif Model Pembelajaran”, we will have a learning CD for teachers using competency standard and basic competency; concepts and indicators; the representation of chemistry materials on three levels macroscopic, microscopic, and symbol; learning description; and student work sheet. Before making the learning model, we will execute a standard analysis on the content of KTSP 2006, so we will get the concepts and indicators; potray the learning process of teachers in class and analyse the reference books both text books and hig school books. The data source for this research is the table of according to concept and indicators with competency standard and basic competency; the observation of learning process in class; the analysis table of three levels; hydrolysis materials from text books in high school and university; and the descriptive table of hydrolysis concept learning. The supporting instruments used are quistionaires to know the students expereince and essay texts to know the concept understanding the students have after the learning process. From the standard analysis of the content of KTSP 2006, we have three concepts and eight indicators. The first concept is classifying salt according to the forming compounds with the indicators (1) explain salt coming from strong acid and strong base; (2) explain salt coming from strong acid and weak base; (3) explain salt coming from weak acid and strong base; (4) explaining salt coming from weak acid and weak base. The second concept: hydrolysis is the ions reaction with water molecul producing ion H+ and or OH- with indicators (1) describe salt hydrolysis and (2) explain various kinds of salt hydrolysis. The third concept is salt undergoing hydrolysis can be acid base, or neutral with indicators (1) measure qualitatively the characteristic of acid, base, and neutral of salt by using some indicators and (2) count the ph of hydrolysied salt condensation. The observation result will be recorded on video then transcribed into text and smoothed to be a basic text. After that, we will do propotition degradation to gain global structure. Then, this data will be classified based on the intertextuality of chemistry. From the result of the data analysis we can conclude that the model teacher has not used the learning based on the intertextuality chemistry yet. The learning process of the model teacher is dominated by the symbol level and the social interaction developed by the teacher is not optimal; besides, there is no aspect of daily experience discussed in the learning process. The development of learning model based on intertextuality begins by making hydrolysis material representation in three levels. The compilation of hydrolysis material representation is executed in three steps: analysing high school books and university; making representation device; conducting validation to experts and practitioners; and final revision. In macroscopic level we do demonstration of the condensation and litmus test on soaps, alum, and salt and also the determination pH of the salt by using pH meter. In microscopic level we demonstrate pictures of species salt solutionbefore and after hydrolysis process. Then in symbolic level we demonstrate the formula of salt molecul, ionisation reaction equation and hydrolysis on salt, and mathematic formula in determining the concentration H+ and OH- to count pH and pOH. Later on, we make a description of the learning process equiped with learning media; clarify the demonstrations; present the model in front of experts and practitioners; record it in a learning CD. Meanwhile, the application of the learning CD in class will be conducted in the next research in the second year.Key words: interactive teaching media, intertextuality, teaching model.

Molecular programming aims to systematically engineer molecular and chemical systems of autonomous function and ever-increasing complexity. A key goal is to develop embedded control circuitry within a chemical system to direct molecular events. Here we show that systems of DNA molecules can be constructed that closely approximate the dynamic behavior of arbitrary systems of coupled chemical reactions. By using strand displacement reactions as a primitive, we construct reaction cascades with effectively unimolecular and bimolecular kinetics. Our construction allows individual reactions to be coupled in arbitrary ways such that reactants can participate in multiple reactions simultaneously, reproducing the desired dynamical properties. Thus arbitrary systems of chemical equations can be compiled into real chemical systems. We illustrate our method on the Lotka–Volterra oscillator, a limit-cycle oscillator, a chaotic system, and systems implementing feedback digital logic and algorithmic behavior.

Chemical equations that balance bond types and atom hybridization to different degrees are often used in computational thermochemistry, for example, to increase accuracy when lower levels of theory are employed. We expose the widespread confusion over such classes of equations and demonstrate that the two most widely used definitions of "homodesmotic" reactions are not equivalent. New definitions are introduced, and a consistent hierarchy of reaction classes (RC1-RC5) for hydrocarbons is constructed: isogyric (RC1) superset of isodesmic (RC2) superset of hypohomodesmotic (RC3) superset of homodesmotic (RC4) superset of hyperhomodesmotic (RC5). Each of these successively conserves larger molecular fragments. The concept of isodesmic bond separation reactions is generalized to all classes in this hierarchy, providing a unique sectioning of a given molecule for each reaction type. Several ab initio and density functional methods are applied to the bond separation reactions of 38 hydrocarbons containing five or six carbon atoms. RC4 and RC5 reactions provide bond separation enthalpies with errors consistently less than 0.4 kcal mol(-1) across a wide range of theoretical levels, performing significantly better than the other reaction types and far superior to atomization routes. Our recommended bond separation reactions are demonstrated by determining the enthalpies of formation (at 298 K) of 1,3,5-hexatriyne (163.7 +/- 0.4 kcal mol(-1)), 1,3,5,7-octatetrayne (217.5 +/- 0.6 kcal mol(-1)), the larger polyynes C(10)H(2) through C(26)H(2), and an infinite acetylenic carbon chain.

(Chemical Equation Presented) Hooks and feelers: The use of a bulky tripod-shaped stopper as a solid-phase attachment point proves essential for the controlled iterative solid-phase synthesis of an α-cyclodextrin [3]rotaxane (see picture). NMR spectroscopy confirms the structure and shows that both the cyclodextrin units have well-defined locations and orientations. © 2007 Wiley-VCH Verlag GmbH and Co. KGaA.

cited By 137; International audience; To and fro: Diphosphanylborane derivative 1 behaves as a tridentate, ambiphilic ligand towards rhodium(I) fragments (see picture). The presence of metal-→borane interactions in the resulting square-pyramidal complexes is highlighted by structural analyses and DFT calculations. (Chemical Equation Presented). © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.

We consider the problem of identification of a pollution source in a river. The mathematical model is a one-dimensional linear advection?dispersion?reaction equation with the right-hand side spatially supported at a point (the source) and a time-dependent intensity, both unknown. Assuming that the source becomes inactive after the time T*, we prove that it can be identified by recording the evolution of the concentration at two points, one of which is strategic.

We present a straightforward integration method to compute the abundance and temperature evolution in explosive scenarios. In this approach the thermal equation is implicitely coupled with chemical equations in order to avoid instabilities and ensure a gentle transition from the normal combustion regime to the quasi (QSE) and complete nuclear statistical equilibrium (NSE). Two nuclear networks, with 14 nuclei (alpha-network) and 86 nuclei (including protons and neutrons) respectively, have been considered. The scheme is suitable to cope with a variety of explosive burning regimes., 3 pages, 1 figure, proceedings "Nuclei in the Cosmos VII" 2002. To appear in Nucl. Phys. A

Summary The reaction of Cab-0-Sil, a highly dispersed vitreous quartz, with hexamethyldisilazane (HMDS) was studied in the temperature range 38O-50O0C, using ,’Si solid state NMR and other techniques. Such studies are of importance in view of deactivation procedures of fused silica at high temperatures in capillary GC. The commonly accepted reaction equation: predominates only below ca. 400“C.Above ca. 400°C the intermediate cleavage product Me,SiNH, reacts with surface silanot groups to form iSiOSiMe,NH, + CH,. At higher temperatures these groups may ultimately form (3iO),SiNH, groups (analogously to the formation of bi-and tridentate linkages starting from :SiOSiMe, groups), but (3SiO),SiNH, groups are also directly formed at lower temperatures, simultaneously with the iSiOSiMe3 groups, probably by reaction with siloxane bridges: The reactions of the trimethylsilylamine part of HMDS with Cab-0-Sil were confirmed by an independent series of silylations using N,N-dimethyltrimethylsilylamine instead of HDMS. The presence of amino groups as ESiNH, was confirmed by FT-IR. This may be one of the reasons why very high temperature silylation with disilazanes does not provide a satisfactory deactivation of fused silica GC columns: the active 5iOH groups are “replaced” by active ZSiNH, groups. However, such a material may be of interest in LC. Silylation with silazanes at ca. 350°C in humid atmosphere andlor after extensive hydroxylation (leaching) of the surface should yield sufficiently deactivated surfaces with a rather well defined surface structure. (Me,Si),NH + 2 iSiOH + 2 ESiOSiMe, + NH

A non-electronic lifetime indicator for packaging has been developed using a colour reaction and diffusion in microchannels. It uses oxygen uptake in polymers after an originality seal is broken by manipulation or regular opening. The oxygen triggers a colour change that is made visible after desired timing period at corresponding diffusion lengths. This timing sensor was realized within a microchannel suitable for packaging cap. Rigorous design was based on modeling the diffusion – reaction equation and experimentally determined diffusion coefficients. To achieve appropriate functionality, temperature effects must be compensated using nanofunctioned materials as well as substrate diffusion of other gases like water. The methodology and design rules are generalized for microfluidics packaging.

Iterative methods based on skew-symmetric splitting of initial matrix, arising from central finite-difference approximation of steady convection-diffusion-reaction (CDR) equation in 2-D domain are considered. The property of obtained large sparse nonsymmetric linear system Au = f is investigated. A new class of triangular and product triangular skew-symmetric iterative methods is presented. Sufficient conditions of convergence for iterative methods of solution CDR with variable coefficient of reaction and dominant convection are obtained. The results of numerical experiments for the solution of a two-dimensional CDR equation are presented. The uniform grid, central differences for the first derivatives, natural ordering of points, Peclet numbers Pe = 103, 104, 105, variable coefficient of reaction and different velocity coefficients have been used.

A procedure is suggested to identify key sample compositions in n-component alloys, providing the most crucial information with limited experimental effort. The proposed algorithm furthermore establishes the exact type of reaction equation of the (n + 1) phase invariant equilibria. Predictive calculations of phase equilibria in quaternary Mg-Al-Ca-Sr and quinary Mg-Al-Ca-Sr-Mn alloys are performed based on elaborate binary and ternary thermodynamic descriptions. Experimental work, using key samples and Mg-rich alloys, demonstrates the viability of the current approach in validating the predictions. Finally, the impact of using more or less elaborate ternary data is quantitatively exemplified for the quinary system Mg-Al-Ca-Sr-Mn by a comparison of different thermodynamic data sets.

A linear diffusion convection equation for the transport of a single chemical component through a membrane is coupled with a linear reaction equation describing interaction of the diffusing chemical with an immobile reactant. Expressions are derived for the flux responses in and out of the membrane to given concentrations applied at one side of the membrane while zero concentration is maintained at the other. These fluxes are compared with the corresponding fluxes for the complementary experiment in which the imposed boundary concentration at the two faces of the membrane are interchanged. Ussing-type flux-ratio relationships are obtained for the outflow fluxes, but the inflow fluxes respond in a different way, and it is found that the difference of the influxes for complementary experiments remain constant for all time.