Weakly activated dissociation

The dissociation of H2 on Pt(l 11) was studied quite early in the evolution of surface dynamics because of the importance of Pt as a catalyst for hydrogenation reactions. It was first suggested that the H2 and D2 exchange reaction implied relatively weak H2 (D2) dissociation that occurs only at steps on Pt(lll) [289], but later reanalysis of these experiments [290] and more extensive measurements [291] implied that H2 dissociation occurs on the terraces, with only a modest enhancement of dissociation at the steps. 

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Figure 3.11. Typical experimental behaviors for dissociative adsorption probabilities S with respect to incident energy Et in (a) and with respect to Ts in (b) for limiting dissociation behaviors. The solid lines are for direct (weakly activated) dissociative adsorption and the dashed lines are for a precursor-mediated dissociation.

It is striking that only silver has been found to be a good catalyst. Why is silver unique The mechanism described here may provide an answer. The key factor might be that silver can dissociate O2, but the oxygen atoms are so weakly chemisorbed that epoxidation is possible. Moreover, silver only weakly activates the C-H bonds in ethylene. Definitive answers on the potential and the explanation of promoters and modifiers are waiting for more information. 

The chemisorption of water (dissociative adsorption) involving the breaking of the siloxane bridges and the formation of new silanol groups is a rapid nonactivated (or weakly activated) process. 

In this scheme, the initial dissociative adsorption is followed by fast formation of a triatomic 03 intermediate (with equal probability from 1602 or 1802 of the isotopically non-equilibrated 1602 + 1802 gas phase) which serves as the propagator of step (c). The latter has the character of a chain reaction, scrambling the gas phase into isotopic equilibration with reversible second-order kinetics as observed experimentally. Another aspect of experimental observations at 295 K readily accounted for by eqns. (35) was the diminution of activity as successive doses of the isotopically non-equilibrated (1602 4- 1802) were introduced, since step (a) shows destruction of ZnCU8. The alternative weak-activation case is... 

Binding thermodynamics defines the average number of active cross-links at any point in time, while the binding kinetics defines how quickly the active cross-links dissociate. This study identifies an important and striking influence on bulk properties as it would seem intuitive that the number of active cross-links (thermodynamics) would dominate over the timescale each cross-link remains active (dissociation kinetics) in determining material properties. However, this study clearly identifies that the dynamic nature of supramoiecuiar interactions is directly responsible for observed properties and for the comparative differences with covalently aoss-linked systems, rather than the relative weakness of the interactions. 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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