Elementary act

At the present time the concept of catalytic (or ionic-coordination ) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

In multistep reactions, the number of particles of any intermediate produced in unit time in one of the steps is equal to the number of particles reacting in the next step (in the steady state the concentrations of the intermediates remain nnchanged). Hence, the rates of all intermediate steps are interrelated. Writing the rate v. of an individual step as the number of elementary acts of this step that occur in nnit time, and the rate v of the overall reaction as the number of elementary acts of the overall reaction that occur within the same time, we evidently have... 

Studies of photoelectrochemical phenomena are of great theoretical value. With light as an additional energy factor, in particular, studies of the elementary act of electrochemical reactions are expedited. Photoelectrochemical phenomena are of great practical value as well. One of the most important research activities nowadays is development of electrochemical devices for a direct conversion of luminous (solar) into electrical energy and photoelectrochemical production of hydrogen. 

The elementary act of an electrochemical redox reaction is the transition of an electron from the electrode to the electrolyte or conversely. Snch transitions obey the Franck-Condon principle, which says that the electron transition probability is highest when the energies of the electron in the initial and final states are identical. 

Many papers have been published regarding HTSCs used as inert, nonconsumable electrodes for kinetic and mechanistic studies of various electrode reactions occurring at them. Most of these studies were performed at room temperature when the materials were not in their state of superconductivity. Unfortunately, to date a given reaction has rarely been studied at similar temperatures just above and below r , that is, at temperatures where the same material is once in its normal state and once in its superconducting state. The electronic stracture of materials differs sharply between these two states, and quantitative studies under these conditions might provide valuable information as to the mechanism of the elementary act of charge transfer from the electrode to a reacting species, and vice versa. 

ELECTRIC DOUBLE-LAYER EFFECTS ON THE ELEMENTARY ACT OF ELECTRON TRANSFER... 

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

Krishtalik, L. I., The mechanism of the elementary act of proton transfer in homogeneous and electrode reactions, J. Electroanal. Chem., 100, 547 (1979). 

However, in many papers on polymerization and copolymerization of organotin monomers, the role of complex formation in elementary acts of polymer formation has been either ignored or considered inadequately. 

This ensures the necessary ordering of reacting side groups for an elementary act to take place although the copolymer macromolecules exhibit a low mobility 109). 

To show more clearly the difference between this new approach and that used earlier, we will briefly summarize the model which was widely used for the calculation of the probability of the elementary act of charge transfer processes in polar media. 

The configurational model was used for the calculation of the elementary act in the reactions of solvated electrons21 and in the electrochemical generation of solvated electrons.22 The results for the activation free energy of the process of electrochemical generation of solvated electrons as a function of the reaction free energy... 

Further development of the basic model and the detailed analysis of the dependence of the symmetry factor on the potential and the temperature54 have shown that there are additional factors which can affect the elementary act of this reaction. These investigations led to the formulation of the charge variation model (CVM)55 which will be discussed in the next section. 

A model for the diffusion of light ions in structured liquids has been suggested recently by Schmidt.61 The elementary act of diffusion is considered in this model to be the transfer of the ion from one cage formed by solvent molecules to a neighboring one. 

Each step includes elementary acts that require different properties of the metal, for example, sufficiently low ionization potential to favor oxidative addition, sufficiently weak metal-carbon bonds, tendency to form square-planar complexes and to reach pentacoordination to allow insertion, a sufficiently high electron affinity to allow reductive elimination, and so on. Some properties are conflicting and a compromise has to be reached. 

One more reason for which chain reactions have an advantage over molecular reactions is the restrictions that are imposed on the elementary act by the quantum-chemical rule of conservation of symmetry of orbits of bonds, which undergo rearrangement in the reaction [4]. If this rule is applied, the reaction, even if it is exothermic, requires very high activation energy to occur. For example, the reaction... 

The rate constants of this reaction per reacting bond are close for subsequent hydrocarbon and polymer in solution, however, much different in the liquid and solid phases. Two factors are important for this difference the rigid polymer cage (see Chapter 19) and the additional activation of adjacent segments to change the C—C bond angles in the P02 + PH elementary act. The absolute values of kp per reacting C—H bond for solid polymers are collected in Table 13.6. 

The above kinetic scheme of the bimolecular reaction simplifies physical processes that proceed via the elementary bimolecular act. To react, two reactants should (a) meet, (b) be oriented by the way convenient for the elementary act, and (c) be activated to form the TS and then react. Hence, not only translational but also rotational diffusion of particles in the solution and polymer are important for the reaction to be performed. So, the more detailed kinetic scheme of a bimolecular reaction includes the following stages diffusion and encounter the reactants in the cage, orientation of reactants in the cage due to rotational diffusion, and activation of reactants followed by reaction [5,13]. 

Reaction dynamics deals with the intra- and intermolecular motions that characterize the elementary act of a chemical reaction. It also deals with the quantum states of the reactants and product. Since the dynamic study is concerned with the microscopic level and dynamic behaviour of reacting molecules, therefore, the term molecular dynamics is employed. 

Here we 11 consider a more general case assuming tne possibility of the cross-link formation between any two sites of the molecule raeeroaching one to another to some critical distance /we ll call such pairs "contacts"/ and assuming that the rate constant of the elementary act does not depend on the chain conformation as a whole and the nearest environment. Besides we ll assume that the reaction is a kinetically-controlled one, i.e. the system, reaches the state of the conformational equilibrium, between two consequent cross-links formations but the elementary act is irreversible and so fast that the chain conforma.tion remains constant during it Fs-sl. 

Sensitivity to the magnetic held strength has also been observed in the process of photoisomerization of tran -stilbene (tS) into c -stilbene (cSt) in the presence of pyrene (P). The reaction was run in a solution with acetonitrile, DMSO, or hexafluorobenzene being a solvent. During the reaction, spin polarization was established (Lyoshina et al. 1980). The spin polarization was explained by the following sequence of the elementary acts ... 

If a solution, being in contact with an electrode, contains photosensitive atoms or molecules, irradiation of such a system may lead to photoelectro-chemical reactions or, to be more exact, electrochemical reactions with excited particles involved. In such reactions the electrons pass either from an excited particle to the electrode (the anodic process) or from the electrode to an excited particle (the cathodic process). In this case, an elementary act of charge transfer has much in common with ordinary (dark) electrochemical redox reactions, which opens a possibility of interpreting certain aspects of photochemical processes under consideration with the use of concepts developed for general quantum mechanical description of electrode processes. 

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