Fragmentation activation volume

For all investigated reactions, the photosubstitution quantum yield decreased significantly with increasing pressure. Under the assumption that nonradiative deactivation is relatively independent of pressure, the pressure dependence of /(l — < ) represents that of the photochemical reaction [Eq. (27)]. The positive volumes of activation fit well into the picture of a dissociative mechanism, that is, release of CO. This model cannot account for the observed trends in AF ( /(1 — < )) especially as a function of solvent. For this reason, a second way to account for the observed data was presented [100] according to which CO dissociation leads to a trigonal bipyramidal M(CO)5 fragment with dissociated CO within the solvent cage. The latter species can either recombine with CO, be trapped by solvent, or bind to the nucleophile L, which results in a competition between these reaction paths. The difference in the pressure dependence for the recombination with CO or combination with L can be used to account for the observed activation volumes. 

A reasonable estimate of the number of flaws needed to sustain an equilibrium value of A can be made based on the concepts introduced earlier in this section. To achieve a nominal fragment size of d, the number of activated flaws per unit volume should be of order d From the equilibrium... 

Much of the chemistry of vinylidene complexes has been developed with catalytic applications in mind, as detailed later in this volume. Early examples had low activity for alkene metathesis, although complexes containing imidazolylidene ligands showed improved efficiencies [35]. However, in many cases, reactions of the vinylidene ligand have resulted in transformation to other carbon-based ligands which have not been released from the metal fragment. 

Table IV. The total pore volume is only a little changed, and the surface area is diminished a good deal. Nevertheless, activity greatly improves because large diameter pores are created in place of the small ones. This undoubtedly makes the gel more fragmentable during polymerization, and the more open pore structure permits an easier escape of the polymer chains from the fragments. Note (Table IV) that most of the pore volume of the active ID hydrogel is in pores with diameters in the range 100-1000 A.

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