Intermediates, compared with transition

Catalysis by micelles of the hydroxide-ion catalysed hydrolysis of substrates appears to be qualitatively understood on the basis of a concentration effect of reactant on, or around, the micelle surface and need not necessarily involve a dilference in the free energies of activation in the micelle and bulk phase. That is not to say that the cationic micelles could not and do not cause electrostatic stabilisation of the transition state. The cationic micelle surface can act as an electrostatic sink for the anionic intermediate leading to its stabilisation, but a rate enhancement requires preferential stabilisation of this intermediate compared with the reactant. The small rate enhancement of the micelle catalysed reaction, about 50-fold, is equally well explained by considering that the increased concentration of reactants at the micelle surface leads to a higher observed rate. Incorporation of the reactants into a limited volume decreases the entropy loss that is associated with bringing reactants together in the transition state and this leads to an increase in the pseudo first-order rate constants in the presence of surfactant micelles. Cationic micelles of CTAB have also been shown to facilitate the alkaline hydrolysis of the cephalosporin, cephalexin (Yatsuhara el al., 1977). 

Two reasons may be offered for the enhanced /3-deuterium isotope effect in vinyl cations as compared with carbonium ions (193). As pointed out by Noyce and Schiavelli (21), in the transition state of a vinyl cation, the isotopically substituted C—H bond is ideally suited for overlap with the developing vacant p orbital, as the dihedral angle between the empty p orbital and C—H bonds is zero in the intermediate, as shown in structure 239. Shiner and co-workers (195)... 

We hope that the preceding discussions have developed the concept of a conical intersection as being as real as many other reactive intermediates. The major difference compared with other types of reactive intermediate is that a conical intersection is really a family of structures, rather than an individual structure. However, the molecular structures corresponding to conical intersections are completely amenable to computation, even if their existence can only be inferred from experimental information. They have a well-defined geometry. Like the transition state, the crucial directions governing dynamics can be determined andX2) even if there are now two such directions rather than one. As for a transition structure, the nature of optimized geometries on the conical intersection hyperline can be determined from second derivative analysis. 

We prefer our state charts like this, because they are easier to reconcile with the actuality of the software compared with instantaneous event models and they tie in better with the actions. In some notations, they would have an event representing the start of an action and another for the end of it, and a state in between representing the transitional period. But at a given level of abstraction, we do not know enough to characterize the intermediate state, because that is defined only in the more-detailed layers of the model. 

Semiempirical calculations have been used to study the mechanism of the ring opening of cyclopropanone and substituted analogues in a range of solvents of varying polarity. Transition states and oxyallyl intermediates have been characterized, as have the effects of solvents on their stability. The results are also compared with kinetic data in the literature. 

For alkyl esters the reaction is close to symmetrical, and it is likely that the breakdown of the tetrahedral intermediate is partially rate-determining, and will therefore be general acid-catalyzed. For aryl esters breakdown to products will be faster, and the formation of the tetrahedral intermediate should determine the rate. This might account for the more favourable entropies of activation found by Moffat and Hunt199 for the hydrolysis of aryl trifluoroacetates if the single transition state (of the addition step) occurs early, the loss of degrees of freedom compared with the initial state will also be less complete. 

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