Deceleration effect

The same situation is observed in the series of alkyl-substituted derivatives. Electron-donating alkyl substituents induce an activating effect on the basicity and the nucleophilicity of the nitrogen lone pair that can be counterbalanced by a deactivating and decelerating effect resulting from the steric interaction of ortho substituents. This aspect of the reactivity of thiazole derivatives has been well investigated (198, 215, 446, 452-456) and is discussed in Chapter HI. 

However, the CO tolerance at Pt-Co degraded at 70 °C. As seen in Fig. 10.9, the HOR activity of Pt-Co at a given dco is close to that of pure Pt, although the deceleration effect on the CO adsorption rate was still observed to some extent at 70 °C. Such a deactivated electrode cannot recover the original CO tolerance. This can certainly be ascribed to a severe dealloying of the nonprecious metal component (Co) in hot acid solution. We will discuss this in Section 10.3.2. 

With the realization that the cycloamyloses form stable monomolecular inclusion complexes in solution came the idea that the inclusion process might affect the reactivity of an organic substrate. This idea was initially pursued by Cramer and Dietsche (1959b) who discovered that the rates of hydrolysis of several mandelic acid esters are enhanced by the cycloamyloses. More recently, the inclusion process has been shown to exert both accelerating and decelerating effects on the rates of a variety of organic reactions. The remainder of this article will be devoted to a discussion of these reactions in an attempt to review, compare, and unify the many intriguing facets of cycloamylose catalysis. 

The reaction of cyclohexaamylose with a series of p-carboxyphenyl esters is an example of a decelerating effect which may be clearly attributed to nonproductive binding. Rate effects imposed by cyclohexaamylose on the hydrolyses of three such esters are summarized in Table IX. As the hydrophobicity of the ester function is increased by alkyl substitution, the hydrolysis is inhibited the stability of the inclusion complex, on the other... 

The spreading behavior of droplets on a non-flat surface is not only dependent on inertia and viscous effects, but also significantly influenced by an additional normal stress introduced by the curved surface. This stress leads to the acceleration-deceleration effect, or the hindering effect depending on the dimensionless roughness spacing, and causes the breakup and ejection of liquid. Increasing impact velocity, droplet diameter, liquid density, and/or... 

If the polyelectrolyte can coordinate strongly to a metal ion, marked deceleration effects can be noted, as, for example, in the reactions of Ni + and Co + with pad in the presence of polyphosphates. Modifications of equilibria constants in these micelles must also be recognized as contributing to rate change, e. g., ligand pK or keto-enol equilibria may be altered. 

Prokunin AN23,24). Flow deceleration effect. Two maxima in dependencies of force upon time undercoditions of x = const and respective flow deceleration (which... 

All of these decelerating effects are easily overridden by appropriate alkene substitution. Indeed, any terminal alkene substituent will decelerate 6-endo cyclization. To accelerate S-exo cyclization, one requires only an electronegative alkene substituent at C-6. 

Activity and stability are often comparable to values in aqueous media. Many substrates which cannot be made to react in water or in pure organic solvents such as hexane owing to lack of solubility can be brought to reaction in microemulsions. Whereas enzyme structure and mechanism do not seem to change upon transition from water to the microemulsion phase (Bommarius, 1995), partitioning effects often are very important. Besides an enhanced or diminished concentration of substrates in the vidnity of microemulsion droplets and thus of enzyme molecules, the effective pH values in the water pool of the droplets can be shifted in the presence of charged surfactants. Frequently, observed acceleration or deceleration effects on enzyme reactions can be explained with such partitioning effects (Jobe, 1989). 

Allen, A. D. Kanagasabapathy, V. M. Tidwell, T. T. Doubly destabilized carbocations. Strong aryl delocalization and the attenuation of rate decelerating effects of CFj and CN groups. 

Mobility — The (ionic or electric) mobility u of an ion is given by the drift velocity v (the velocity of an ion at equilibrium between the accelerating effect of the electric field and the decelerating effect of the viscous medium (Stokes friction)) of an ion and the effective electric field E v... 

The following examples further illustrate the selectivity that can emerge from this analysis (Scheme 6). Divinylcyclopropane (30) underwent exclusively the cleavage of the siloxy-substituted ring with activation enhancement of ca. 5 kcal mol", whereas the silyl-substituted counterpart (31) preferred the cleavage of the carbon-substituted ring due to the decelerating effect of silicon. ... 

Among the experimental findings were the inhibiting effect of the polymer, the yield restriction in the course of the reaction, and the decelerating effect of salts dissociating during dissolution. These results can be explained by the displacement of equilibrium toward the formation of mixed ion pairs. 

Table 2.1 must inevitably raise some queries, e.g. how does continuous exposure to 25°C 75% RH equate with actual fluctuating conditions, i.e. is it an accelerating or decelerating effect How does 25°C 75% RH equate with 48 months under... 

Such solvent-assisted elimination could result by the SN2-intermedi-ate mechanism (6), but the high degree of carbocation character indicated by the large decelerating effect of CF3 in XI and the noncumulative isotope effect, which was quantitatively accounted for on the basis of the ion-pair scheme in equation 4 (27), argues against the former interpretation. 

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