Reaction kinetics and dynamic

C2.18.3.1 EXPERIMENTAL STUDIES OF THERMAL ETCHING REACTION KINETICS AND DYNAMICS... 

For a reaction to produce a new phase, the new phase must first form (nucleate) from an existing phase or existing phases. Nudeation theory deals with how the new phase nucleates and how to predict nudeation rates. The best characterization of the present status of our understanding on nudeation is that we do not have a quantitative understanding of nudeation. The theories provide a qualitative picture, but fail in quantitative aspects. We have to rely on experiments to estimate nudeation rates, but nudeation experiments are not numerous and often not well controlled. In discussion of heterogeneous reaction kinetics and dynamics, the inability to predict nudeation rate is often the main obstacle to a quantitative understanding and prediction. The nudeation theories are... 

Recent advances in experimental techniques, particularly photoionization methods, have made it relatively easy to prepare reactant ions in well-defined states of internal excitation (electronic, vibrational, and even rotational). This has made possible extensive studies of the effects of internal energy on the cross sections of ion-neutral interactions, which have contributed significantly to our understanding of the general areas of reaction kinetics and dynamics. Other important theoretical implications derive from investigations of the role of internally excited states in ion-neutral processes, such as the effect of electronically excited states in nonadiabatic transitions between two potential-energy surfaces for the simplest ion-molecule interaction, H+(H2,H)H2+, which has been discussed by Preston and Tully.2 This role has no counterpart in analogous neutral-neutral interactions. 

Hydrocarbon polymers are particularly susceptible to attack by atomic oxygen in LEO. The reactions of atomic oxygen with hydrocarbon molecules in the gas phase serve as models for the relatively unstudied reactions of atomic oxygen with a hydrocarbon surface. A wealth of knowledge of gas-phase reactions is available, largely because these reactions are important in combustion and in atmospheric chemistry. Studies of both reaction kinetics and dynamics have revealed many of the mechanisms by which atomic oxygen reacts with gaseous alkanes and alkenes. A summary of probable reaction pathways is presented in Fig. 4. 

To this end, significant effort has been devoted over the last 15 years to the development of theories of chemical reaction kinetics and dynamics based on ideas gleaned from nonlinear dynamics. It has been found that techniques similar to those used to analyze instabilities in weather patterns and the formation of galaxies can be employed to visualize pre- and postreactive phase space. This makes it possible to determine what kinds of motions a molecule must execute to react. In turn, this knowledge can be used to make a prediction reaction rate which is significantly more accurate than RRKM theory. 

Influence on reaction kinetics and dynamics. React. Kinet. Catal. Lett., 65, 322-329, 1998. 

One of the tasks undertaken by the Computational Chemistry Branch at NASA Ames Research Center is to provide critically needed chemical and physical data for NASA hypersonics projects such as the aeroassisted orbital transfer vehicle and the scramjet propulsion system in the National Aerospace Plane. In order to meet this goal, we have embarked on theoretical studies of the reaction kinetics and dynamics of high temperature air (T 5000 50000 K) and moderately high temperature hydrogen-air mixtures T < 3000 K). 

Koda S., M. M. Kotani, K. Someno, and K. Awaga, Eds., Graduate l ecture. Physical Chemistry II, Reaction Kinetics and Dynamics, 285 pp, Tokyo Kagaku Dojin, Tokyo 2011 (in Japanese),... 

Franklin J L (ed) 1979 Ion-Molecule Reactions, Part I, Kinetics and Dynamics (Stroudsburg, PA Dowden, Hutchinson and Ross)... 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

J. I. Steiafeld, J. S. Francisco, and W. L. Hase, Chemical Kinetics and Dynamics, Prentice Hall, Englewood Chffs, N.J., 1989. Oriented more toward gas-phase reactions and iacludes more advanced microscopic iaterpretations from the perspective called chemical physics. 

The examples given above represent only a few of the many demonstrated photochemical appHcations of lasers. To summarize the situation regarding laser photochemistry as of the early 1990s, it is an extremely versatile tool for research and diagnosis, providing information about reaction kinetics and the dynamics of chemical reactions. It remains difficult, however, to identify specific processes of practical economic importance in which lasers have been appHed in chemical processing. The widespread use of laser technology for chemical synthesis and the selective control of chemical reactions remains to be realized in the future. 

Chemical vapor deposition processes are complex. Chemical thermodynamics, mass transfer, reaction kinetics and crystal growth all play important roles. Equilibrium thermodynamic analysis is the first step in understanding any CVD process. Thermodynamic calculations are useful in predicting limiting deposition rates and condensed phases in the systems which can deposit under the limiting equilibrium state. These calculations are made for CVD of titanium - - and tantalum diborides, but in dynamic CVD systems equilibrium is rarely achieved and kinetic factors often govern the deposition rate behavior. 

This review deals largely with work from my own laboratory. It attempts to show the reader some of the recent developments in the field and the breadth of the scientific questions which are being addressed through investigations of the kinetics and dynamics of ion-molecule reactions as mediated through the presence of bound solvent molecules. 

It is important to propose molecular and theoretical models to describe the forces, energy, structure and dynamics of water near mineral surfaces. Our understanding of experimental results concerning hydration forces, the hydrophobic effect, swelling, reaction kinetics and adsorption mechanisms in aqueous colloidal systems is rapidly advancing as a result of recent Monte Carlo (MC) and molecular dynamics (MO) models for water properties near model surfaces. This paper reviews the basic MC and MD simulation techniques, compares and contrasts the merits and limitations of various models for water-water interactions and surface-water interactions, and proposes an interaction potential model which would be useful in simulating water near hydrophilic surfaces. In addition, results from selected MC and MD simulations of water near hydrophobic surfaces are discussed in relation to experimental results, to theories of the double layer, and to structural forces in interfacial systems. 

C. K. Ingold and E. D. Hughes, "Dynamics and Mechanism of Aliphatic Substitutions," Nature 132 (1933) 933934 C. K. Ingold, E. D. Hughes, and S. Masterman, "Reaction Kinetics and the Walden Inversion. Pt. I. Homogeneous Hydrolysis and Alcoholysis of beta-n-Octyl Halides," JCS 140 (1937) 11961201 and subsequent articles. 

In what I regard as the world of change (essentially chemical kinetics and dynamics), there are three central equations. One is the form of a rate law, v = /[A],[B]...), and all its implications for the prediction of the outcome of reactions, their mechanisms, and, increasingly, nonlinear phenomena, and the other closely related, augmenting expression, is the Arrhenius relation, k = Aexp(-EJRT), and its implications for the temperature-dependence of reaction rates. Lurking behind discussions of this kind is the diffusion equation, in its various flavors starting from the vanilla dP/dt = -d2P/dl2 (which elsewhere I have referred to as summarizing the fact that Nature abhors a wrinkle ). 

This review has attempted to highlight recent progress in the development and use of time-resolved FTIR emission in three areas of chemical kinetics and dynamics, namely, product state distributions in photodissociation processes, product state distributions in chemical reactions, and rate processes involving the formation and loss of the internally excited species... 

Therefore, the simplest procedure to get the stochastic description of the reaction leads to the rather complicated set of equations containing phenomenological parameters / (equation (2.2.17)) with non-transparent physical meaning. Fluctuations are still considered as a result of the external perturbation. An advantage of this approach is a useful analogy of reaction kinetics and the physics of equilibrium critical phenomena. As is well known, because of their nonlinearity, equations (2.1.40) reveal non-equilibrium bifurcations [78, 113]. A description of diffusion-controlled reactions in terms of continuous Markov process - equation (2.2.15) - makes our problem very similar to the static and dynamic theory of critical phenomena [63, 87]. When approaching the bifurcation points, the systems with reactions become very sensitive to the environment fluctuations, which can even produce new nonequilibrium transitions [18, 67, 68, 90, 108]. The language developed in the physics of critical phenomena can be directly applied to the processes in spatially extended systems. 

The thermal characteristics of a reaction, including its heat production rate, the necessary cooling power, and the reactant accumulation, are fundamental for safe reactor operation and process design. A successful scale-up is achieved, only when the different characteristic time constants of the process, such as reaction kinetics, thermal dynamics of the reactor, and its mixing characteristics are in good agreement [9]. If we focus on the reaction kinetics and thermal dynamics, that is, we consider that the reaction rate is slow compared to the mixing rate, in principle, there are two ways to predict the behavior of the industrial reactors ... 

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