Exchange Rate Mechanism

Blenzen, A., Foglia, F., Furet, E., Helm, L., Merbach, A. E., and Weber, J. (1997). Second coordination shell water exchange rate mechanism Experiments and modelling on hexaaquochromium(III). J. Amer. Chem. Soc. 118, 12777-87. 

Neither pyrazoles nor pyrazolium salts react by this mechanism which has been described for imidazoles and imidazolium salts (Section 4.01.1.7.4). As exchange rates show (Section 4.04.2.1.7(iii)), it is considerably more difficult to generate an ylide from a pyrazolium salt than from an imidazolium salt (at C-2). 

The occurrence of energy transfer requires electronic interactions and therefore its rate decreases with increasing distance. Depending on the interaction mechanism, the distance dependence may follow a 1/r (resonance (Forster) mechanism) or e (exchange (Dexter) mechanisms) [ 1 ]. In both cases, energy transfer is favored by overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

For such a mechanism, the overall second-order formation rate constant is given by the product of the first-order constant ktx and the equilibrium constant Kos. The characteristic solvent exchange rates are thus often useful for estimating the rates of formation of complexes of simple monodentate ligands but, as mentioned already, in some cases the situation for macrocyclic and other polydentate ligands is not so straightforward. 

Figures 19.6 through 19.11 detail the isotopic exchange rates during water-gas shift for the formate, C02, and Pt-CO bands, in switching from the 12C to 13C label. In all cases, the reactive exchange rates of formate and C02 were virtually identical, implicating the formate species as the likely intermediate to the water-gas shift catalytic mechanism over Pt/Zr02 and PtNa/Zr02 catalysts. The DRIFTS spectra at the top of each figure show the switching of these species from...

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