Solution-phase reactants

Excesses of solution-phase reactants and reagents can be used to drive reactions to completion these reagents can then be washed away from polymer-bound intermediates. 

Direct sequestration of a reactant by an insoluble resin is impractical if the kinetics is sluggish and impossible if the solution-phase reactant does not contain a functionality to enable direct sequestration. These limitations led several research groups to use bifunctional solution-phase linking reagents, also referred to as sequestration-enabling-reagents. 33... 

A solution-phase reactant will be depleted during the operation of a three-dimensional electrode (electrocatalysts), and diffusion of this species from a reservoir at the face of the electrode represents a loss by concentration difference. 

While experiments involving solution-phase reactants have provided deep insights into the dynamics of heterogeneous electron transfer, the magnitude of the diffusion-controlled currents over short timescales ultimately limits the maximum rate constant that can be measured. For diffusive species, the thickness of the diffusion layer, S, is defined as S = (nDt)1/2, where D is the solution-phase diffusion coefficient and t is the polarization time. Therefore, the depletion layer thickness is proportional to the square root of the polarization time. One can estimate that the diffusion layer thickness is approximately 50 A if the diffusion coefficient is 1 x 10-5 cm2 s-1 and the polarization time is 10 ns. Given a typical bulk concentration of the electroactive species of 1 mM, this analysis reveals that only 10 000 molecules or so would be oxidized or reduced at a 1 pm radius microdisk under these conditions The average current for this experiment is only 170 nA, which is too small to be detected with high temporal resolution. 

Given that the reaction kinetics of the forward and backward reactions are first order in Ox and Red, respectively, measurements of ks, kc, or ka, and a, AHf and/or AHf provide a detailed phenomenological description of the electrochemical kinetics for solution-phase reactants at a given electrode-electrolyte interface. It is also of fundamental interest, however, to evaluate rate parameters for adsorbed (or "surface attached ) reactants or reaction intermediates (Sect. 2.3). 

In contrast to this method, another PASP strategy, known as the resin-capture approach, makes use of resins that transiently sequester solution-phase products, allowing solution-phase reactants, reagents, and by-products to be filtered from the resin-bound products. The products are subsequently released from the sequestering resin to afford the desired purified solution-phase products (Scheme 18). 

One may at first find it hard to believe that underlying gas phase dynamics can be observed in solution phase reactions. However, a close look at the simulations of A + BC reactions in rare gas solutions shows a time period when the gas phase and solution phase reactant dynamics appear to be quite simi-lar.21 This type of behavior can also be seen in the studies of an Sn2 reaaion in... 

Microelectrodes open up the possibility of probing redox processes in small sample volumes or physically small spaces. For example, the redox properties of sample volumes as small as a few picoliters have been interrogated. Experiments of this kind are possible because, as described by Eq. (10), for solution phase reactants, the depletion layer thickness, S, depends on the experimental timescale. 

Despite the many elegant investigations that have been conducted into the heterogeneous electron transfer dynamics of solution phase reactants, the magnitude of the diffusion-controlled current at short times ultimately places a lower limit on the accessible timescale. As described by Eq. (10), the thickness of the diffusion layer... 

The rate of reactions where one reactant is bound to a membrane are often reduced compared to those in solution due to steric factors, yet greatly accelerated if both partners are restricted to the membrane. Menger and Azov assayed the cleavage of cholesterol-linked nitrophenol ester 26 by soluble hydroxamate 27 to quantify this effect (Figure 12a). The reaction of 26 in POPC vesicles (10mol%) was 22 times slower than for the comparable solution-phase reaction of 27 with nitrophenylacetate 28. Similarly, using membrane-bound hydroxamate nucleophile 29, with nitrophenylacetate 28 resulted in a approximately twofold reduction in reaction rate relative to solution-phase reactants. 

Capacitive currents are generated by events occnrring within very small distances from an electrode surface and their magnitude is controlled by the microscopic area of the electrode. In most electrochemical experiments designed to probe the redox properties of solution phase reactants, the timescale of the measurement is often snch that the diffnsion layer is several times larger than the critical dimension (e.g., radins) of the microelectrode. For example, the depletion layer thickness, (5, can be estimated as ... 

Irrespective of the sample volume, the amount of sample probed in an electrochemical experiment depends on the timescale. This sensitivity arises because, for solution phase reactants, diffusion is typically the dominant mode of mass transport. When the response is under semi-infinite linear diffusion control, the thickness of the diffusion layer, 3, is given by equation (6.1.5.1). Taking a typical diffusion coefficient of 1 X 10 cm sec in aqueous solution, equation (6.1.5.1) indicates that for a conventional electrochemical experiment employing an electrode of 3 mm diameter and an electrolysis time of 1 sec, a volume of approximately 10 pL will be electrolyzed. In contrast, for a 5-pm radius microelectrode and a 50-psec electrolysis time, the volume will be less than 30 fL ... 

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