Kinetically controlled phenomenon

The formation of glass is a kinetically controlled phenomenon and is directly associated with atomic or ioiric diffusion within the melt. This controls the formation and growth of microcrystals when the temperature of the liquid reaches the solidus line (i.e. the crystallization temperature). 

Summarizing, CoOi fN f oxynitride formation appears to be a kinetically controlled phenomenon [371], in accordance with the reported experimental observation of its thermal sensitivity [366]. The phases decompose into rock-salt CoO, metallic Co, and molecular nitrogen at higher temperatures. The broad phase width is also in accordance with theoretical calculations. Further entropy effects will additionally favor, but only very weakly, the formation of ternary phases due to substitutional entropy (O/N). In addition, there is no indication of anionic ordering in the calculations, and a random distribution of N and O is clearly preferred. 

In the previous sections it was shown that the formation of lamellae with folded chains was essentially a kinetically controlled phenomenon. This section treats the free energy of polymer crystallization and melting point depression. 

Shorter chain dienes have an increased propensity to form stable five-, six-, and seven-membered rings. This thermodynamically controlled phenomenon is known as the Thorpe-Ingold effect.15 Since ADMET polymerization is performed over extended time periods under equilibrium conditions, it is ultimately thermodynamics rather than kinetics that determine the choice between a selected diene monomer undergoing either polycondensation or cyclization. 

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

A more detailed picture of the temperature dependence of the growth is given in Figure 2.4, where the island density is plotted as a function of temperature. It can be seen that only in the temperature range from 207 to 288 K the growth is perfectly template controlled and the number of islands matches the number of available nucleation sites. This illustrates the importance of kinetic control for the creation of ordered model catalysts by a template-controlled process. Obviously, there has to be a subtle balance between the adatom mobility on the surface and the density of template sites (traps) to allow a template-controlled growth. We will show more examples of this phenomenon below. 

Langa et al. [26, 59, 60], while conducting the cycloaddition of N-methylazo-methine ylide with C70 fullerene, proposed a rather similar approach. Theoretical calculations predict an asynchronous mechanism, suggesting that this phenomenon can be explained by considering that, under kinetic control, microwave irradiation will favor the more polar path corresponding to the hardest transition state . 

Dimerization of lff-azepines is an extensively studied phenomenon and involves a temperature dependent cycloaddition process. At low (0°C for 1 R = Me) or moderate (130 °C for 1 R = C02R or CN) temperatures a kinetically controlled, thermally allowed [6 + 4] dimerization to the exo -adduct (73) takes place, accompanied by a small amount (<10%) of symmetrical dimer (74). The latter are thermodynamically favored and become the major products (83%) when the Iff-azepines are heated briefly at 200 °C. The symmetrical dimers probably arise by a non-concerted diradical pathway since their formation from the parent azepines by a concerted [6+6]tt cycloaddition, or from dimer (73) by a 1,3-sigmatropic C-2, C-10 shift are forbidden on orbital symmetry grounds. Dimerization is subject to steric restraint and is inhibited by 2-, 4- and 7-substituents. In such cases thermolysis of the lif-azepine brings about aromatization to the correspondingly substituted JV-arylurethane (69JA3616). 

To explain this phenomenon, there are two seperate processes to consider. The first and most important one for this reason is the formation of the oxaphosphetane. The addition of the ylide to the aldehyde can, in principle, produce two isomers with either a Z or E substituted double bond. The following elimination step is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state. With unstabilized ylides the syn diastereomer of the oxaphosphetane similar to 61 is formed preferentially. This step is kinetically controlled and therefore irreversible, and predominantly the Z-alkene 62 that results reflects this. 

In this publication the author describes the phenomenon that most times the thermodynamically less stable product (see 29) of the two possible rings (e.g. 5-exo and 6-endo) is formed. Today looking at the 5-exo cyclization it is known that, although the generated primary radical is less stable than a secondary one, stereoelectronic effects favor reaction to the kinetically controlled product. According to MO-calculations, for a successful cyclization, an angle of 70° of the incoming radical to the plane of the alkene-/alkyne-bond is necessary.11... 

This result does not coincide with the IET value (3.721) that is, MRE does not hold the geminate limit. MRE equations can be justified in the kinetic control limit only. Moreover, as was shown in Section V.D at uA / uc, MRE loses the phenomenon of the delayed fluorescence through the particle with a shorter lifetime (see Fig. 3.28). This also put it out of comparison. 

The kinetic model reproduces satisfectorily experimental results. Deactivation experiments seems to indicate that the mechanism of deactivation changes with the nature of the contaminant used. When a strong poison for active acidic sites like pyridine is used, the catalyst gets totally deactivated when its concentration is over 250 ppm. In this case, the deactivation is fester than with CS2, hut not as fest as an acid base reaction should be. The behavior can be explained assiuning that the pyridine reaction with acidic sites is a diffesion controlled phenomenon enhanced by its molecular size, which is very near to the zeolite pore size. The presence of a mixed mechanism of deactivation and inhibition is also evident. 

The oxidation reactions of carbon and sulfur on hydroprocessing catalysts seem to be kinetically controlled by oxygen diffusion inside the catalyst porosity. Figure 3 shows the carbon and sulfur removal for Cat C which contains a very high amount of nickel and molybdenum, and an appreciable load of carbon. It is clear that the sulfur elimination occurs at higher temperatures than for the other catalysts and is simultaneous to carbon combustion. A tentative explanation of this phenomenon would be that the diffusion of oxygen in the microporosity is limited by coke deposit which needs to be at least partly removed to allow complete sulfur oxidation. 

In order to understand the phenomenon of double asymmetric induction, we need to have a clear picture of the inherent selectivities of each of the chiral partners in closely related single asymmetric induction processes. Consider for example the kinetically controlled aldol addition reactions shown in Scheme 1.5... 

The enhancement of E selectivity by benzoic acid is especially striking. This phenomenon was first reported in 1974 (100b), but little is known regarding its origin. In the examples mentioned above, benzoic acid preferentially catalyzes the E-selective Wittig pathway and does not cause E/Z equilibration of the products (99). Thus, the enhanced ( )-enoate selectivity is the result of kinetic control. 

In all of these examples, the reaction products are under kinetic control. That is, the product mixture is determined by which product is made fastest rather than which is thermodynamically most stable. The Diels-Alder cycloaddition reaction gives another example of this phenomenon. The reaction of methyl acrylate with cyclopentadiene gives a mixture of two endo and exo products (see Scheme 2.5). 

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