Quasi-stable intermediate

Computer studies of the course of macroscopic ignition in shocked Hg-Og mixtures containing an excess of either reactant have revealed the occurrence under certain conditions of short-lived excursions of intermediate species concentrations significantly above the quasi-stable N = I partial equilibrium levels. These so-called spikes occur between the time of the peak chain reaction rate, which occurs when the lesser reactant is about 50% depleted, and the time of establishment of partial equilibrium proportions among the major intermediates. In an excess of H2, the partial equilibrium 0-atom concentration is quite... 

Studies of linear systems and systems without "intermediate interactions show that a positive steady state is unique and stable not only in the "thermodynamic case (closed systems). Horn and Jackson [50] suggested one more class of chemical kinetic equations possessing "quasi-ther-modynamic properties, implying that a positive steady state is unique and stable in a reaction polyhedron and there exist a global (throughout a given polyhedron) Lyapunov function. This class contains equations for closed systems, linear mechanisms, and intersects with a class of equations for "no intermediate interactions reactions, but does not exhaust it. Let us describe the Horn and Jackson approach. 

From the observation that N-(mcLhylcnc)morpholinium cations (17) induced the autocatalytic decomposition of tertiary amine W-oxides in combination with the proof that these intermediates were actually present in Lyocell dopes, the question arose why Lyocell solutions were stable at all. The answer is found in the fact that carbenium-iminium ions generated are immediately consumed by reaction with water and carbohydrate structures as the most abundant and most reactive nucleophiles present. Only when the local concentration of 17 increases to such a high level that the consumption by reaction with water or cellulose cannot compensate for its production, does the reaction with NMMO become uncontrollable and lead to an exothermic event. The pulp used in the Lyocell process acts as a quasi-stabilizer for the solvent NMMO, albeit with the drawback of increased chromophore generation. 

The quasi-stationary state approximation consists of setting Rj = 0 for very reactive and short-lived intermediates such as free radicals. The result is that molar enthalpies of these intermediates do not appear in the calculation of H. Therefore, it is necessary to know neither their standard heats of formation, nor their heat capacities, values of which are not as well known as those of stable species. 

Glasner et al. [43] suggested that, irrespective of which of the two salts is heated, during reaction a quasi-equilibrium involving both will occur. Decomposition involves the intermediate production of the relatively stable oxide, CfOj, and the third step (420 K), above, was identified as controlling the overall rate of reaction. 

The quasi-equilibrium approximation relies on the assumption that there is a single rate-determining step, the forward and reverse rate constants of which are at least 100 times smaller than those of all other reaction steps in the kinetic scheme. It is then assumed that all steps other than the rds are always at equilibrium and hence the forward and reverse reaction rates of each non-rds step may be equated. This gives simple potential relations describing the varying activity of reaction intermediates in terms of the stable solution species (of known and potential-independent activity) that are the initial reactants or final products of the reaction. The variation of the activities of reaction intermediates is, however, restricted by either the hypothetical solubility limit of these species or, in the case of surface-confined reactions and adsorbed intermediates, the availability of surface sites. In both these cases, saturation or complete coverage conditions would result in a loss of the expected... 

A quasi-stationary state is stable if a small excursion from it is self-correcting, but is unstable if the excursion escalates. Specifically, in a system as described here, stability is ensured if the net rates rx and rY decrease if the concentrations of X and Y increase. If this is true for one of the intermediates, but not for the other, the stabilizing and destabilizing tendencies counteract one another, and the (necessary and sufficient) condition for stability becomes... 

The new trinorborn-l-yl phosphite (7) has been prepared and shown to give quasi-phosphonium salts which are even more stable than those derived from trineopentyl phosphite thus the salt with methyl iodide had a half-life of more than 100 h in deuterochloroform at 150 C, compared to less than 2 h at 33 C for the neopentyl analogue. Although phenacyl chloride gave mainly the Arbuzov intermediate (8), with p-nitrophenacyl chloride a stable Perkow intermediate (9) could be isolated and its X-ray crystal structure studied. Trifluoroiodomethane with tris(dialkylamino)phosphines gave the normal trifluoromethylphosphonium salts (10) in the absence... 

Quasi-irreversible inhibition is observed when CYP metabohsm produces an intermediate that can form a stable metabolite-intermediate MI) complex. This is another example of mechanism-based inhibition. Erythromycin is one such quasi-irreversible CYP3A4 inhibitor. Upon didemethylation of its tertiary amine group and subsequent oxidation, the resulting nitroso species forms a tight complex with the Fe(II) atom of the CYP s heme unit. Unhke truly irreversible adducts, such complexes can be broken up, say by oxidation with potassium ferricyanide, but under normal physiological conditions this obviously doesn t happen. 

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