Elementary Chemical Processes

Lee Y T 1987 Molecular beam studies of elementary chemical processes Science 236 793-8... 

Tree J 1975 Shock wave studies of elementary chemical processes Modern Deveiopments in Shock Tube Research ed G Kamimoto (Japan Shock Tube Research Society) pp 29-54... 

Degradation is appreciable only if K > l/tr this occurs whenever the coefficient a M e offsets the dissociation energy term — U0. It is physically unrealistic that an elementary chemical process can have negative activation energy, therefore the permissible maximum for K is controlled by the pre-exponential factor A (i — 1). 

Development of the quantum mechanical theory of charge transfer processes in polar media began more than 20 years ago. The theory led to a rather profound understanding of the physical mechanisms of elementary chemical processes in solutions. At present, it is a good tool for semiquantitative and, in some cases, quantitative description of chemical reactions in solids and solutions. Interest in these problems remains strong, and many new results have been obtained in recent years which have led to the development of new areas in the theory. The aim of this paper is to describe the most important results of the fundamental character of the results obtained during approximately the past nine years. For earlier work, we refer the reader to several review articles.1 4... 

The brief review of the newest results in the theory of elementary chemical processes in the condensed phase given in this chapter shows that great progress has been achieved in this field during recent years, concerning the description of both the interaction of electrons with the polar medium and with the intramolecular vibrations and the interaction of the intramolecular vibrations and other reactive modes with each other and with the dissipative subsystem (thermal bath). The rapid development of the theory of the adiabatic reactions of the transfer of heavy particles with due account of the fluctuational character of the motion of the medium in the framework of both dynamic and stochastic approaches should be mentioned. The stochastic approach is described only briefly in this chapter. The number of papers in this field is so great that their detailed review would require a separate article. 

Any elementary chemical process (also referred to as a primary photoreaction) in which an electronically excited molecular entity yields a primary photoproduct. See Primary Photoproduct... 

V.V. Voevodskii, Physics and Chemistry of Elementary Chemical Processes (in Russian) (Khimiya, Moscow, 1970). 

During the past decade, the study of photoinitiated reactive and inelastic processes within weakly bound gaseous complexes has evolved into an active area of research in the field of chemical physics. Such specialized microscopic environments offer a number of unique opportunities which enable scientists to examine regiospecific interactions at a level of detail and precision that invites rigorous comparisons between experiment and theory. Specifically, many issues that lie at the heart of physical chemistry, such as reaction probabilities, chemical branching ratios, rates and dynamics of elementary chemical processes, curve crossings, caging, recombination, vibrational redistribution and predissociation, etc., can be studied at the state-to-state level and in real time. 

In the photochemistry of larger molecules the same physical principles apply and the sequence of events is essentially the same, although descriptions are more complex and less precise. Opposite to diatomic, the polyatomic molecules can yield a multitude of different sets of products. To establish the photodissociation mechanism, the nature of the elementary chemical process undergone by an electronically excited molecular entity (primary photoreaction) yielding primary photoproducts should be known, eg in the case of alkanes both the radical fission... 

For a single, irreversible step in a chemical reaction, i.e., an elementary chemical process, the rate of the reaction is proportional to the concentrations of the reactants involved in the process. The constant of proportionality is called the rate constant, or the unitary rate constant to highlight the fact that it applies to an elementary process. A subtlety that may be introduced into rate expressions is to use chemical activities (see Chap. 10) and not simply concentrations, but activity coefficients in biological systems are generally taken to be near 1. 

Nov. 21, 1931, Tbilisi, Georgia, USSR - May 13, 1985) Dogonadze was one of the founders of the new science - electrochemical physics [i]. The main scientific interests of Dogonadze were focused on condensed-phase reactions. His pioneering works of 1958-59 have laid the foundations of the modern quantum-mechanical theory of elementary chemical processes in electrolyte solutions. He developed a comprehensive quantum-mechanical theory of the elementary act of electrochemical reactions of -> electron and -> proton transfer at metal and - semiconductor electrodes [ii—v]. He was the first to obtain, by a quantum-mechanical calculation, the expression for the electron transfer probability, which was published in 1959 in his work with -> Levich. He conducted a number of studies on the theory of low-velocity electrons in disordered systems, theory of solvated electrons, and theory of photochemical processes in solutions. He made an impressive contribution to the theory of elementary biochemical processes [vi]. His work in this area has led to the foundation of the theory of low-temperature -> charge-transfer processes cov-... 

Primary photochemical process (primary photoreaction) Any elementary chemical process undergone by an electronically excited molecular entity and yielding a primary photoproduct. 

We have already demonstrated that the behavior of an arbitrary combina tion of elementary chemical processes cannot be described in terms of only one thermodynamic force. Nevertheless, sometimes it is possible describe a stationary stepwise stoichiometric process using only one thermodynamic force. 

Near thermodynamic equilibrium, similar linear relationships are also valid for elementary chemical processes, as well as for stepwise processes where the rates are proportional to the difference between the thermodynamic mshes of the initial and final reaction groups (see Section 1.4.2). Here, the criterion of proximity to thermodynamic equilibrium is relationship jA jl < RT, where Arij is the affinity for the transformation of reaction group i to reaction group j. In fact, while... 

The values of the primary quantum yields, found in the photolysis of ketones with y-H atoms, were explained by Brunet and Noyes on the basis of steric effects that diminish the probability of the formation of the cyclic complex. Following the original suggestion of Whiteway and Masson, Martin and Pitts proposed the internal conversion of the cyclic structure to be responsible for the low primary quantum yields observed in the photolysis of ketones capable of forming such a structure. In contrast to other interpretations. Wagner and Hammond explained the low quantum yields by an elementary chemical process, suggesting that the y-H atom transfer is reversible, i.e. that the biradical, after vibrational relaxation, may convert back into the ground state ketone molecule, viz. 

E.E.Nikitin, Dynamics of elementary chemical processes, Khim. Fiz. 6, 1603 (1987)... 

Lee, Y.T. Reactive scattering I Nonoptical methods. In Atomic and Molecular Beam Methods. Scoles, G., Ed., Oxford University Press, New York, 1987, Vol. 1, 553-568. Lee, Y.T. Molecular beam studies of elementary chemical processes. In Nobel Lectures in Chemistry 1981-1990. Fraegsmyr, T., Malstrom, B.G., Eds., World Scientific, Singapore, 1992, 320-357. 

Tunon I, Martins-Costa MTC, Millot C, Ruiz-Lopez MF. Molecular dynamics simulations of elementary chemical processes in liquid water using combined density functional and molecular mechanics potential I. Proton transfer in strongly H-bonded complexes. J Chem Phys 1997 106 3633-3642. 

Figure 1. Time dependence of elementary chemical processes in solutions. In polar solutions, most of these primary events (electron detachment, ion-ion pair inter-conversion, concerted electron-proton transfer, and electron solvation) occur in less than 2 X 1Q s and are controlled by vibrational or electronic responses of the reaction medium.

See a recent Special Issue Elementary Chemical Processes in Liquids and Solutions of J. Chim. Phys. 1996, 93, 1577-1938. 

The scope of this article is to account for some aspects regarding the possibihty of overcoming steric hindrances in chemical reactivity. This is one of the main themes of research in all fields of chemistry we will present here a progress report on our understanding of elementary chemical processes and especially some recent achievements in experimental techniques for controlling the spatial orientation of molecules. For the time being, these achievements represent tools for investigations of the basic mechanisms, but perspectives are open to also exploit them for applied purposes. 

In the slow reaction hmit, information about the elementary chemical process is explicitly presented in the formula for the rate coefficient (Levitsky, Macheret, Fridman, 1983a,b). 

The possibility to use laser radiations to achieve the so-called "coherent control" of molecular dissociation or of atomic photoionization has been predicted since the advent of laser sources in the early sixties. It was expected that, thanks to the coherence and monochromaticity properties of the laser light, one could selectively choose a dissociation channel and the spatial orientation of ejection of the fragments (either ions or electrons or even neutrals) in an elementary chemical process. However, earlier attempts, based on simple photoabsorption processes, have been unsuccessful and it is only recentiy that experiments have been shown to enable one to achieve such a goal in some selected systems. Amongst the various scenarios which have been explored, one of the most promising is based on the realization of quantum interferences in so-called "two-colour" photodissociation or... 

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