Elementary reaction, elucidation

The study of the rates of chemical reactions is called kinetics. Chemists study reaction rates for many reasons. To give just one example, Rowland and Molina used kinetic studies to show the destructive potential of CFCs. Kinetic studies are essential to the explorations of reaction mechanisms, because a mechanism can never be determined by calculations alone. Kinetic studies are important in many areas of science, including biochemistry, synthetic chemistry, biology, environmental science, engineering, and geology. The usefulness of chemical kinetics in elucidating mechanisms can be understood by examining the differences in rate behavior of unimolecular and bimolecular elementary reactions. 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

Although reaction rate expressions and reaction stoichiometry are the experimental data most often used as a basis for the postulation of reaction mechanisms, there are many other experimental techniques that can contribute to the elucidation of these molecular processes. The conscientious investigator of reaction mechanisms will draw on a wide variety of experimental and theoretical methods in his or her research program in an attempt to obtain information about the elementary reactions taking... 

For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. 

Through modeling of global experiments it is possible to elucidate the mechanism and identify a number of rate coefficients that must be determined accurately. In this procedure sensitivity and reaction path analyses are essential tools. The sensitivity analysis identifies the bottlenecks in the chemical conversion process, that is the rate-controlling elementary reactions. Reaction path analysis provides information about the major reaction pathways responsible for the production and consumption of each species. 

Although RDX strains the limits of accurate applicability of quantum chemistry methods, they have provided valuable information about the decomposition mechanism. Ultimately, it is expected that quantum chemistry will play a vital role in elucidating the elementary reaction mechanisms for energetic materials. In fact, given the experimental difficulties for molecules such as RDX we must look to quantum chemistry. While the quantum chemistry description of the fundamental properties and elementary reactions of RDX is still a work in progress, much has been... 

Unfortunately, the unequivocal elucidation of these mechanisms requires real time measurements of the nascent products and an analysis of the kinetics of the propagation steps. Some studies on individual elementary reactions are discussed below. 

Rate laws are employed to evaluate reaction mechanisms in soil-water systems. To accomplish this, kinetics are used to elucidate the various individual reaction steps or elementary reactions. Identifying and quantifying the elementary steps of a complex process allow one to understand the mechanism(s) of the process. For example, unimolecular reactions are generally described by first-order reactions bimolecular reactions are described by second-order reactions,... 

Which experimental approach can best reveal the chemical dynamics of carbon-centered radicals Recall that since the macroscopic alteration of combustion flames, atmospheres of planets and their moons, as well as of the interstellar medium consists of multiple elementary reactions that are a series of bimolecular encounters, a detailed understanding of the mechanisms involved at the most fundamental microscopic level is crucial. These are experiments under single collision conditions, in which particles of one supersonic beam are made to collide only with particles of a second beam. The crossed molecular beam technique represents the most versatile approach in the elucidation of the energetics and... 

One-plus rate equations play a key role in network elucidation. Perhaps the most difficult step in that endeavor is the translation of a mathematical description of experimental results into a correct network of elementary reaction steps. The observed behavior can usually be fitted quite well by a traditional power law with empirical, fractional exponents, at least within a limited range of conditions. This has indeed been standard procedure in times past. However, such equations are highly unlikely to result from a combinations of elementary steps. Their acceptance may be expedient, but as far as network elucidation is concerned they are a dead... 

Although, as already indicated, direct studies of hydrocarbon oxidation have not been conducive to the determination of kinetic data for elementary reactions, in specific cases, under carefully selected conditions, particular key processes can be isolated, mechanisms elucidated and kinetic data obtained. The approach will be illustrated by studies on the oxidation of C3H6 over the range 650-800 K at pressures 60Torr. The complexity of the system is limited under these conditions by three factors ... 

Both industrial demand and academic interest stimulated detailed research of the synthesis of lactam polymers and elucidation of the polymerization mechanisms. Until recently, nylon 6 has been the only industrially produced lactam polymer and, therefore, most of the extensive investigations are dealing with the polymerization of caprolactam [2]. The mechanisms of the main elementary reactions derived for caprolactam are applicable to other lactams as well, except to the / /-substituted ones. 

Q.24.12 What does the elucidation of an elementary reaction include ... 

Obviously, linear Gibbs energy relationships are quite useful because of their predictive power. However, it must be remembered that they are based on fundamental concepts which originate in the theory of elementary reactions. Thus, the experimental study of these relationships has helped elucidate the energy profiles involved in the basic processes which occur in solutions. 

The Michaelis-Menten Formalism has been remarkably successful in elucidating the mechanisms of isolated reactions in the test tube. There are numerous treatments of this use of kinetics, and many of these provide a thoughtful critique of the potential pit falls. In short, reliable results can be obtained with steady-state methods if one is careful to follow the canons and if one remembers that several mechanisms may yield the same kinetic behavior. Isotope exchange, pre-steady state, and other transient or relaxation kinetic techniques, as well as various chemical and physical methods, also have been applied in conjunction with steady-state kinetic methods to dissect the elementary reactions within an enzyme-catalyzed reaction and to distinguish between various models (e.g., see Cleland, 1970 Kirschner, 1971 Segel, 1975 Hammes, 1982 Fersht, 1985). 

Simple reaction kinetics will never suffice, but one can try to elucidate the various elementary reactions and then set up all the rate equations (differential equations) and solve the whole set numerically. In fact, this often is the only way to determine the reaction scheme with any confidence, since only quantitative agreement between calculated and observed product concentrations as a function of time guarantees its correctness. It is far beyond the scope of this book to discuss even a simple example. 

Composite reactions consist of multiple elementary reaction steps that occur in series, in parallel, or both. Many geochemical reactions are composites of several elementary reaction steps. This makes elucidating their reaction mechanisms very challenging because their reaction order and molecularity are not related in a simple way. Marin and Yablonsky (2011) offer extensive guidance about dealing with composite reactions. 

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