Results kinetically controlled responses

To justify these results, it may be assumed that the TS leading to C-alkylation will be more polar than that responsible for N-alkylation. This assumption, however, presumes the existence of kinetic control (which is not ensured in this case). 

In principle, there are two possible ways to measure this effect. First, there is the end-point measurement (steady-state mode), where the difference is calculated between the initial current of the endogenous respiration and the resulting current of the altered respiration, which is influenced by the tested substances. Second, by kinetic measurement the decrease or the acceleration, respectively, of the respiration with time is calculated from the first derivative of the currenttime curve. The first procedure has been most frequently used in microbial sensors. These biosensors with a relatively high concentration of biomass have a longer response time than that of enzyme sensors. Response times of comparable magnitude to those of enzyme sensors are reached only with kinetically controlled sensors. 

In response to a more fundamental question, a qualitative understanding of the origins of the treble diastereoselectivity exhibited in each cycloaddition has been developed. Arguments based on discussions in the literature have been combined with synthetic observations made in our own laboratories to assess the factors which control the reactivities and selectivities of exocyclic r-m-butadiene units, and endocyclic bisdienophilic units incorporating the (2.2. l]bicycloheptane skeleton. The syn/endo-H stereochemistry generated across each newly-formed cyclohexene ring in the cycloadditions between bisdiene and bisdienophilic building blocks is, we believe, a consequence of transition state considerations, and therefore a result of kinetic control. In spite of the sometimes tacitly assumed reversibility of the Diels-Alder reaction, evidence is provided in this review that confirms here at least the unidirectional nature of the cycloadditions under discussion. 

LSV is a powerful tool for the study of processes under purely kinetic control. Theoretical analyses of the response for various mechanisms have been carried out [13,15-26], and a series of papers [36,72,79,93] has been devoted to assimilating the theoretical results in a form useful to the experimentalist. For the general rate law, Eq. (33), the dependence of Ep on changes in log v>, log Ca, and log Cx, respectively, is linear with the slopes given by Eqs. (40)-(42), where a, b, and x are the reaction orders. 

Further reaction of diaziridines is responsible for the formation of bicyclic compounds 122a-c from aldehydes, chloramine and ammonia. The isomers 122a and 122b (R = various alkyl, aryl, and aralkyl groups) are obtained in a kinetically controlled reaction work-up in the presence of ammonium chloride yields an additional isomer 122c as a result of thermodynamic control. 

In most amperometric cytochrome b2 electrodes the reaction is followed by anodic oxidation of ferrocyanide at a potential of +0.25 V or above. The first of such sensors was assembled by Williams et al. (1970), who immobilized the enzyme (from baker s yeast) physically at the tip of a platinum electrode within a nylon net of 0.15 mm thickness. The large layer thickness resulted in a response time of 3-10 min. Owing to the low specific enzyme activity used, the sensor was kinetically controlled. Therefore the linear measuring range extended only up to 0.1 Km-A similar sensor has been applied by Durliat et al. (1979) to continuous lactate analysis. The enzyme was contained in a reaction chamber of 1 pi volume in front of the electrode. This principle has also been employed in the first commercial lactate analyzer using an enzyme electrode (Roche LA 640, see Section 5.2.3.3X With a sensor stability of 30 days and a C V below 5%, 20-30 samples/h can be processed with this device. 

An alternative process can be imagined where the Michael addition is the product-determining step (Case B, Scheme 7). In this circumstance, collapse of the dipolar intermediate is more rapid than retro-Michael addition and thus a kinetically controlled process is responsible for determining the relative configuration of the stereogenic centers that result from the conjugate addition.1... 

Although adsorption experiments with H-beta gave results in accordance with the preference for the linear product from 3-heptanone phenylhydrazone, with a bulky/linear adsorption ratio of 28/72 being found [23], and similar results were obtained for the indole isomers from 1-phenyl-2-butanone [22], total adsorption was small. Furthermore, when a sample of beta, the external surface of which had been passivated by silylation [24], was employed, no activity was found. This strongly suggests that it is the external surface, rather than the internal pore walls, which is responsible for the activity observed, thus variations in isomer distribution cannot be ascribed to pore-induced shape selectivity. Comparison of the calculated relative stabilities of the isomeric enehydrazines and indoles with the selectivity data of Table 1 indicate that the reactions are kinetically controlled, so that factors such as solvent effects are probably instrumental in establishing the observed product ratios. 

The relative stability of the tautomers of purine and pyrimidine bases is of fundamental importance to the structure and functioning of nucleic acids. The occurrence of rare tautomers was considered a factor responsible for the formation of mismatches leading to spontaneous mutations in the genetic code fl,2]. Cytosine, in particular, has been the subject of several studies, both experimental [3-5] and theoretical [5-15] which have provided a reliable picture of the relative stability of its tautomers, both in the gas phase and in solution. Tautomerization is generally the result of proton transfer (PT) reactions whose activation barriers may exert a kinetic control over the formation of some tautomers. As far as cytosine is concerned, a large majority of the studies available in the literature focus on the thermodynamic aspects of tautomerization and quite a few [16-19] are devoted to the elucidation of the kinetic aspects. The tautomerization of cytosine in the gas phase, with a special attention to the activation energy of the proton transfer reactions, has been afforded by this group in a previous paper [19]. By comparison with experimental data [4,5] it was... 

Kinetic methods call for no measurements of absolute values of the parameter typically used to monitor reactions (absorbance, fluorescence intensity, potential), but rather for their temporal variations as a result, kinetic measurements are free from the effects of factors that introduce errors in absolute values (e.g., turbidity, the liquid-liquid junction potential, and the presence of other absorbing or fluorescent substances provided they do not take part in the reaction of interest or alter the parameter response). However, strict timing and temperature control (to within 0.01-0.1°C) are essential to kinetic methods, which thus require modern, powerful instrumentation. 

Temperature control of a batch reactor In a particular batch reaction process, temperature control is critical to the safe operation of the process, as excess heat resulting from control failure could cause a mnaway reaction. The reaction kinetics are such that insufficient time is available for an operator to respond to a high-temperature alarm. It was also determined that any actions initiated by a high-temperature SIF would be inadequate to prevent an overpressure demand on the mpture disk due to the response time of the sensor. 

In 2001, Nagatani et al. reported a method to analyze ion adsorption-transfer kinetics using PMF [140]. The results just discussed were confirmed, and it was further shown that the PMF response for kinetically controlled adsorption is expressed as a semicircle in the complex plane in which the characteristic frequency of maximum imaginary component is proportional to the adsorption and desorption rate constants. Considering that the potential dependence of adsorption exhibits the opposite sign whether the process takes place from the aqueous or organic phase, the corresponding PMF responses appear in different quadrants of the complex plane. This work therefore confirms that the adsorption at an ITIES can take place on either side of the interface. 

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