Kinetically controlled responses

No steady-state theory for kinetically controlled heterogeneous IT has been developed for micropipettes. However, for a thin-wall pipette (e.g., RG < 2) the micro-ITIES is essentially uniformly accessible. When CT occurs via a one-step first-order heterogeneous reaction governed by Butler-Volmer equation, the steady-state voltammetric response can be calculated as [8a]... 

Kinetically controlled deprotonation of a,p-unsaturated ketones usually occurs preferentially at the a -carbon adjacent to the carbonyl group. The polar effect of the carbonyl group is probably responsible for the faster deprotonation at this position. 

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). 

The qualitative analysis consists e.g. in stimulating pure sorption steps by using a step inlet disturbance in the fluid concentration of the corresponding species. The responses are different depending on whether the step is kinetically controlled or not (see Figure 2, 3). 

The electrochemical response, here the plateau current, first increases with the substrate concentration before reaching a limit as the kinetic control passes from reaction (1) to reaction (2). The variation with substrate concentration is never linear over the entire concentration range. 

If the electrode reaction (1.1) is kinetically controlled, the response depends on both the parameter p and the kinetic parameter k [26,27]. If the electrode size is constant and the frequency is varied, both parameters p and k ate changed. Also, if a certain reaction is measured at constant frequency, with a range of microelectrodes having various diameters, the apparent reversibility of the reaction decreases with the decreasing diameter because of radial diffusion. So, the relationship between... 

If tlie 1,2-addition is reversible (the nucleophile is a good leaving group), then we get thermodynamic control and the conjugate addition product predominates. When the 1,2-addition is not reversible (the nucleophile is a poor leaving group), we get kinetic control and simple addition. Stereochemical considerations are also partly responsible, since it will be easier for larger nucleophiles, especially enolate... 

The complex that RNA polymerase forms at the promoter site just prior to initiation. Some bacterial promoters require high NTP concentrations to initiate efficient transcription, because this represents a status report on the stores of ATP, UTP, GTP, and CTP needed for RNA synthesis. Nature has evolved a kinetic control device high initiating ATP and GTP concentrations must be present to stabilize an otherwise short-lived polymerase-promoter complex. The reader may also recall that bacterial translation is also tightly controlled, and amino acid starvation leads to ppGpp synthesis, the so-called stringent-response agent that also potently inhibits RNA polymerase. Such kinetic control ensures that NTP and amino acid concentrations are adequate before transcription and translation occur. 

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. 

We have established that the volume change kinetics of responsive gels are usually diffusion-controlled processes. Even when the diffusion analysis failed, the rates were comparable to or slower than a classical diffusive process. The implications of this for practical applications are quite negative, since diffusive processes are quite slow. A gel slab 1 mm thick with a diffusion coefficient of 10-7 cm2/s will take over an hour to reach 50% of equilibrium and more than six hours to reach 90% of equilibrium in response to a stimulus. This is far too slow for almost all potential applications of these materials. Since diffusion times scale with the square of dimension, decreasing the characteristic dimension of a sample will increase the rates dramatically. Thus if an application can make use of submillimeter size gels, millisecond response times become possible. Unfortunately, it may not always practical to use gels of such small dimension. 

Nevertheless, the authors do not comment on the different endo/exo selectivity observed in reactions with furan (the endo adduct is predominant, Scheme 55) and cyclopentadiene (only exo adducts are obtained, Scheme 54) under kinetic control conditions. They explain the changes in the facial diastereoselec-tivity, observed in the absence of ZnCl2, as a consequence of the increase in population of conformation B (Fig. 9), attributed to dipole-dipole repulsion. However they pay no attention to the role of the diene, which could be mainly responsible for the observed differences. With furan, the de decreased, but the major adduct remains the same, whereas with cyclopentadiene, the 7r-facial... 

Expression (10.34) shows that a conventional Cottrell response for an oxidation is obtained, superimposed on the continuation of the reduction reaction profile. Expressions for kinetic control in one of the two steps and in both steps have been derived6. 

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