Quantifying competition between reaction

From SCRP spectra one can always identify the sign of the exchange or dipolar interaction by direct exammation of the phase of the polarization. Often it is possible to quantify the absolute magnitude of D or J by computer simulation. The shape of SCRP spectra are very sensitive to dynamics, so temperature and viscosity dependencies are infonnative when knowledge of relaxation rates of competition between RPM and SCRP mechanisms is desired. Much use of SCRP theory has been made in the field of photosynthesis, where stnicture/fiinction relationships in reaction centres have been connected to their spin physics in considerable detail [, Mj. 

The most straightforward way to quantify the competition between the transferase reaction and hydrolysis is to measure the initial ratio of these two reactions. Intuitively, one would assume the transferase/hydrolysis ratio to decrease with increasing water activity because of the effect of water as a reactant This is often the case when lipases are used as catalysts [34—36]. However, in reactions catalyzed by glycosidases and proteases the transferase/hydrolysis ratio can either increase or decrease with increasing water activity [37, 38]. 

The competition between transferase and hydrolysis reactions can be described in terms of nucleophile (acceptor) selectivities of the enzymes, and selectivity constants can be defined. These constants are meant to quantify the intrinsic selectivity of the enzymes. Selectivity constants in combination with the concentrations (or thermodynamic activities) of the competing nucleophiles give the transferase/hydrolysis ratio. The selectivity constants are defined as follows [38, 39] ... 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5] ... 

Nucleophilicities relative to a standard solvent can be quantified by the Swain-Scott equation (12)66, in which k and k0 are the second-order rate constants for reactions of the nucleophile and solvent respectively, and s is a measure of the sensitivity of the substrate to nucleophilicity n. By this definition, the nucleophilicity of the solvent is zero. For all reactions examined, there will be competition between attack by solvent (present in large excess) and reaction with added anionic nucleophiles. Hence, only n values well above zero can be obtained with satisfactory reliability. In the original work66, the solvent was water and all but one of the substrates were neutral s was defined as 1.0 for methyl bromide and was calculated to be 0.66 for ethyl tosylate the lowest reliable n value reported was 1.9 for picrate anion, but a value of < 1 for p-tosylate anion was reported66 in a footnote. 

This notion of faradic yield is most often used for quantifying the competition between several reactions occurring simultaneously at a given interface. It represents the fraction of the actual current used for the half-reaction in question. The faradic yield (either anodic or cathodic) of a particular half-reaction is therefore inferior to 100%. 

We note from this expression a clean separation into various rate-limiting processes. The first term on the rhs of Eqn. 55 quantifies the process of electron percolation through the layer. The second term describes competition between the surface reaction represented by the rate constant k and the layer reaction represented by the second-order rate constant k. The larger of these two terms dominates. Equation 55 therefore quantifies the transport and kinetics of mediation in the W 1 limit provided we can neglect the direct electrode reaction. The reciprocal form of this expression for k ME nwans that the slower term, whether transport or kinetic, determines the modified electrode rate constant k E-The LIXe ratio determines the location of the reaction zone in the layer. If Xe L, corresponding to a thick film, then tanh (LIXe) = 1, and substrate S penetrates only a distance Xe into the layer. In this case the layer term for the mediation kinetics reduces to kboxXE. On the other hand when L Xe, tanh (LIXe) J-, Xe, and the mediation kinetics are so slow that the layer term reduces to kboKL and the entire layer is used in the mediation reaction. 

This approach was shown to yield block copolymers but its efficiency was not quantified, although it was apparent that it was subject to side reactions which left a significant quantity of the starting polymeric material as the homopolymer. The reaction could possibly involve disproportionation between two molecules of the transient silver adduct (reaction 6) in competition with the uni-molecular decomposition into radicals as shown above. It is difficult to distinguish between the last two alternatives kinetically since both are bimolecular, and both would be reduced if the rate of reaction between the silver salt and the polystyrene lead adduct were retarded, with a consequent increase in copolymerization efficiency. 

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