Kinetical criterion

For the determination of stabilizations of carbonium ions the equilibrium constants of carbonylation-decarbonylation have been used in previous Sections. For the ions discussed in this Seetion, however, the rate constants of decarbonylation are not known and, therefore, the rate constants of carbonylation will be used as a criterion for such stabilizations. This kinetic criterion is a useful indicator if there are no significant steric factors in the carbonylation step and if this step is indeed rate-determining in the overall process (Hogeveen and Gaasbeek, 1970). The following rate constants in Table 2 are of particular importance. 

Secondly, the stabilization of alkylcarbonium ions can be conveniently determined by measuring the equilibrium constants of the carbonylation-decarbonylation reactions. For some cases the rates of carbonylation are used as a kinetic criterion for stabilization. 

Law C.K. and Egolfopoulos F.N., A kinetic criterion of flammability limits The C-H-O-inert system, Proc. Combust. Inst., 23 413 21,1990. 

In a practical sense, stability of a dispersion ofttimes is accompanied by a retarded separation of the phases. Unfortunately, a quantitative definition cannot be based on this rate of separation because of the overwhelming influence of density, viscosity, and thermal effects. In short, a kinetic criterion, such as sedimentation rate, is not as likely to portray stability as one based on thermodynamic considerations. In this latter category are sediment volumes, turbidity, consistency, and electrical behavior. 

The rate of disappearance of the diazo compound, e.g. the quantum yield of the photolysis, should be independent of the nature and concentration of the reactants. This is the kinetic criterion for the intermediacy of a free carbene. The quantum yield data of diazo compound photolyses are shown in Table 1. Carbenes are named in the usual way by using... 

The application of a rotating ring-disc electrode in ASV resulted in a kj for the Cd-EDTA complex of 12.4 s-l the kf determination was less reliable however (Shuman and Michael, 1975). With a rotating disc electrode applied at different rotation speeds, the same authors (Shuman and Michael, 1978) determined a first order dissociation rate constant for copper chelates in coastal Atlantic waters (in the order of 2 s l). Based on a kinetic criterion the chelation of copper in these samples was estimated. 

A simple kinetic criterion is applicable in those a-eliminations which lead to the formation of dimeric olefins (see Fig. 2). In basic media, a number of organic halides and onium salts, characterized by the absence of a /J-hydrogen, undergo this reaction. It can be formulated as... 

The only other published application of this kinetic criterion is to... 

The best way to prove the existence of the hydroxyl radical is to perform kinetic competition experiments with hydroxyl-radical scavengers [125]. Using the kinetic criterion, we can also exclude the intermediacy of the ferryl oxidant. However, such experiments in isolated heart models are, in practice, very difficult. 

The transformation of initiator into living centres, referred to monomer conversion, is an important kinetic criterion of the type of reaction discussed. Litt has shown that, at the moment of complete monomer transformation to polymer, the fraction of consumed initiator, xl = ([l]0 — [l])/[l]0, is given by [24]... 

A comparison of the rate constants given in Scheme 21 with the rate constant of the disrotatory ( forbidden ) ring-opening reaction in the related Wittig system 329" - 330", however, allows the evaluation of a kinetic criterion for the thermal conrotation in the cases of 331" and 335 ". At 20°C the following ratios of rate constants are calculated ... 

To describe the thermodynamics and kinetics of clusters with due regard for the multiplicity of the structure states (SS) of these nuclei, it is necessary to introduce the SS sets. The mutual transformations of clusters by atom attach ment or detachment, or by atom rearrangements are possible. It means that the SS set may be ordered to form a space. Then, it is possible to consider the nucleation of noncrystalline nuclei (clusters) as a diffusion process in an extended space of cluster structure states (CSS), which is, in fact, the join of hierarchy of ultrametric subspaces. The kinetic criterion of glass transition can be formulated as a condition of the overwhelming transformation of a liquid into a noncrystalline amorphous solid. 

The Rate of Cluster Nucleation and the Kinetic Criterion of the Glass Transition... 

There is one important fact worth mentioning. The thermodynamic criterion as well as the kinetic criterion may be used to judge the purity of a surfactant solution only for time scales only up to the chosen adsorption/desorption time t j. If the measurements for checking the purity are performed in a time interval 0 < t < t, the result is valid only in a time interval t < lOt. After longer times, the situation at an interface can change and none of the conditions hold. 

Kinetics criterion K is used as an antioxidant activity criterion of the investigated substances. It reCbcts quantity of oxygen and their radicals which have reacted with AO (or their mixes) in a minute ... 

Table 2 shows results of measurements of the total phenol type antioxidant content in units of GA concentration (C, mg/1 GA), obtained by ammetric, and kinetic criterion (K, pmol/lxmin), characterizing the total antioxidant activity with respect to oxygen and its radicals, for more than 100 samples of alcoholic drinks. 

TABLE 2 Results of measurements of the total phenol type antioxidant content (C, mg/1 GA), and kinetic criterion (K, mmol/lxmin). 

If superoxide anion (O ) reacts in reaction (7) with an antioxidant (R-OH), CO decreases, the potential increases according to (9) and the maximnm of EV O cnrrent haves shift in positive area. The valnes of kinetic criterion K arise with increasing of the total phenol AO content, reaching valnes of 1.3-1.5 pmol/lxmln for samples of red wines (Table 2, ISq 66, 101, 108). Fignre 4 shows the correlation between K and C for 40 samples of diy and semi-diy red wines from Table 2. The correlation coefficient was r = 0.8212. 

FIGURE 4 Correlation dependence between the kinetic criterion K and the total AO content C for dry and semi-dry red wines (r = 0.8212). 

This method of research has good sensitivity. It is simple and cheap. However, as in any electrochemical method of this type, the scatter of instrument readings at given measurement conditions is rather high (up to a factor of 1.5-2.0). Therefore, each sample was tested 3-5 times, and results were averaged. The maximmn standard deviation of kinetic criterion K for all investigated samples was within 30%. 

TABLE 2 The total content of AOs in the juice samples and their kinetic criterion K. 

For all samples of extracts, except an extract of the lemon cmsts, dominating character of interaction of extract components with oxygen and its radicals has not phenolic, but the catalytic nature. This interaction proceeds on the mechanism (12), characteristic for substances of 4th group in Table 1. Lemon cmsts extract works on the mechanism of classical AO (8). Unlike values of the phenolic AO content measured by an ammetric method, the kinetic criterion has appeared maximum not only for extract of green tea, but also for mint extract, minimum-for extract of black tea. In spite of the fact that the phenol AO content in extracts of tea and mint has appeared much more, than in lemon cmsts. Dual total activity of tea and mint extracts is de Qied not by phenol type substances, but, apparently, various metal complexes, present in them, and catalyze of proceeding chemical processes. Considerable shift of the ER 0 current maximum potential in positive area attests in favor of enough high content of phenol substances in teas and mint extracts. 

Haddon and Fukunaga showed that a direct relationship exists between the resonance energies and the HOMO—LUMO gaps in [4n 2] annulenes and thereby, as one of the first, demonstrated the connection between the thermodynamic and kinetic criterion of the aromatic character. Indeed, they showed that... 

Efficiency of methods has allowed studying dyrramics of AO content and activity change in same objects drrring time. Good correlation between the total phenol antioxidant content in the studied samples and values of the kinetic criterion defirring activity with respect to oxygen and its radicals is observed. 

Correlation dependence between the kinetic criterion K and the total AOs content is presented on Figure 3. The results of measurements spent for juices of medicinal plants, show high correlation (r = 0, 96) between these methods. The explanation of received results is resulted [8],... 

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