Oxygen exchange from

Acid catalysis of ester hydrolysis is also very effective. Oxygen exchange from water is observed under most cases, supporting addition-elimination. Specific-acid catalysis is the most common mode of hydrolysis, although general-acid catalysis is observed with more electrophilic esters. 

Kinetic studies on racemization, oxygen exchange reaction using H2lsO, and theoretical studies clarified the mechanism for racemization of optically active chalcogenic acids.34,37 The mechanism of racemization of tellurinic acids is different from that of the corresponding seleninic acids. In fact, the mechanism for racemization of optically active tellurinic acids was found to involve an... 

FIGURE 3.4 Measured effective surface oxygen exchange coefficient of the LSM/YSZ composites as a function of the LSM content. (From Ji, Y. et al., Solid State Ionics, 176 937-943, 2005. With permission.)... 

The proton transfer processes described above induce interesting effects on the geometry of these metal complexes upon protonation (see also Section II). If it is assumed that the equatorial cyano ligands form a reference plane and are stationary for any of these distorted octahedral cyano oxo complexes, the protonation/deprotonation process as illustrated in Scheme 3 is responsible for the oxygen exchange at the oxo sites. This process effectively induces a dynamic oscillation of the metal center along the O-M-O axis at a rate defined by kmv, illustrated in Fig. 15. This rate of inversion is determined by the rate at which the proton is transferred via the bulk water from the one... 

Fig. 17. Temperature and pH dependence of oxygen exchange on the Re(V) center fx = 1.2-1.5 m (KN03). The insert shows a line drawn through the three points at high pH at 34.2°C which indicates a negligible k0 (7). (Adapted with permission from Roodt, A. Leipoldt, J. G. Helm, L. Abou-Hamdan, A. Merbach, A. E. Inorg. Chem. 1995, 34, 560-568. Copyright 1995 American Chemical Society.)...

This reasoning also holds for the reactivity on the Mo(IV) center as illustrated in Fig. 19b and confirmed experimentally (7). A deviation of the experimental points for the inversion of the coordination polyhedron was obtained from the oxygen exchange on the oxo site in the [MoO(OH2)(CN)4]2 complex at pH <6 (Fig. 19b was observed and was interpreted in Section VI,B). 

In summary, it is clear from the above-discussed aspects that it was possible by multinuclear NMR (oxygen-17, nitrogen-15, carbon-13, and technetium-99) to successfully study the very slow cyanide exchange and the slow intermolecular oxygen exchange in these oxocy-ano complexes and correlate them both with the proton-transfer kinetics. Furthermore, the interdependence between the proton transfer and the actual dynamic inversion of the metal center was clearly demonstrated. 

H2O. The plot of In (1 — F) is linear with time for at least four half-lives and it can be shown that all four oxygens in V04 exchange, from the distribution of 0 between VO and HjO at equilibrium. They are thus equivalent. 

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