Oxygen exchange kinetics

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

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... 

The effect of temperature and [H+] on the oxygen exchange was studied for the W(IV) system by simple pH manipulation as was the case for the Re(V) mentioned above. However, here the pH had to be varied around high values (12-14) to attain the slow isotopic exchange conditions. The kinetic results were treated similarly to those of the Re(V) and are listed, together with the activation parameters, in Table V. 

It has previously been concluded that even in strong acidic solution, the dioxotetracyanoosmate(VI) complex cannot be protonated to form the oxo aqua complex or even the corresponding hydroxo oxo complex. The pA i and pKa2 values have been estimated to be substantially less than -1, which is also supported by the relationship between pKa values and 170 and 13C chemical shifts (Table II). Extreme slow kinetic behavior, as expected in the case of a +6 charged metal center for a dissociative activation exchange process, has been observed, with only an upper limit for the oxygen exchange determined (Table II). 

As in the case of the oxygen exchange described above in Section IV (see also Fig. 16), well-behaved first-order kinetics were observed for all the cyanide exchange reactions for which the modified exponential form of the McKay equation (8) was used to determine the observed rate constants. 

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. 

Kinetic Data for Aqua Substitution and Oxygen Exchange in Oxo Cyano and Related Complexes of Re(V) and Tc(V)... 

The intermediate has a finite lifetime, but it is not free the less stable secondary carbonium ion is stabilized by specific interaction with two molecules of water. The same kinetic study on primary alcohols made by Dostrovsky and Klein (79) shows that oxygen exchange in dilute acid solution does not proceed by way of an ion, but by a concerted mechanism. For the same reason the elimination reaction has to be of a concerted nature and cannot proceed via an unsolvated carbonium ion. 

Carbonyl oxygen exchange was found during the cupric ion-catalyzed hydrolysis of DL-phenylalanine ethyl ester-carbonyl-O18 at pH 7.3 (11). This indicates that an additional intermediate is formed in this reaction. A mechanism (II) consistent with both the kinetic evidence and the oxygen-exchange evidence is given below. 

However, detection of the tetrahedral intermediate in the addition of a nucleophile to an ester, acid halide, amide or anhydride must be adduced from kinetic evidence, in particular the evidence of oxygen exchange in such an intermediate. Such tracer work has established the presence of symmetrical addition compounds in the hydrolysis of esters23, amides and acid chlorides24. Since the attempts to detect such intermediates have played a considerable part in the development of hydrolysis studies, it is worthwhile considering this point in some detail. 

This follows, since, if step C is slow, step B must also be slow and thus the addition intermediate would accumulate. Satchell found no spectrophoto-metric or kinetic evidence for this with either chloracetyl or /3-chloropropionyl chlorides. This conclusion is obviously of wide potential significance in the hydrolyses of acyl derivatives and even the observation of oxygen exchange during solvolysis (one of the main supports of the tetrahedral intermediate postulate) does not rule reaction (26) out, since the acylation could remain synchronous in part. 

Most of the kinetic results so far obtained upon oxygen exchange are summarized in Figs. 3 to 6, which also show the kinetics of the equilibration reaction plotted to the same scale. Before attempting an interpretation of these observations, it is desirable to consider first the nature and extent of the oxide surface and the adsorption of oxygen thereon and also to review briefly other experimental evidence regarding the interaction between gas and surface. 

Table 43 Kinetic Parameters for Oxygen-exchange Processes at 25 °C...

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