Isotope exchanges of

There is ample evidence that the reductive elimination of alkanes (and the reverse) is a not single-step process, but involves a o-alkane complex as the intermediate. Thus, looking at the kinetics, reductive elimination and oxidative addition do not correspond to the elementary steps. These terms were introduced at a point in time when o-alkane complexes were unknown, and therefore new terms have been introduced by Jones to describe the mechanism and the kinetics of the reaction [5], The reaction of the o-alkane complex to the hydride-alkyl metal complex is called reductive cleavage and its reverse is called oxidative coupling. The second part of the scheme involves the association of alkane and metal and the dissociation of the o-alkane complex to unsaturated metal and free alkane. The intermediacy of o-alkane complexes can be seen for instance from the intramolecular exchange of isotopes in D-M-CH3 to the more stable H-M-CH2D prior to loss of CH3D. 

Two theories exist for the lack of isotope fractionation in chondmles. Both rely on the concept of quasi-statical (i.e., almost equilibrium) exchange of isotopes between liquid and gas followed by permanent loss of rock-forming elements to the escaping gas phase. One model (Alexander 2003) implies that the chondmles formed in such close proximity to one... 

Diffusive exchange of isotopes, affecting age determination (closure temperature)... 

In the following analysis, adsorption models for solid diffusion control are applicable for isotopic exchange, i.e. exchange of isotopes, while in the case of liquid diffusion control and the intermediate case, only adsorption models for linear equilibrium can be used for isotopic exchange. 

So far, the analysis has been restricted to adsorption and exchange of isotopes (isotopic exchange). However, in most cases, the ion-exchange process involves the exchange of... 

Exchange of trace components The equations for adsorption (diffusion) can be equally applied in the case of isotopic exchange (exchange of isotopes) with minor changes. The same equations can be also be used in the case of the exchange of trace components of different valences (Helfferich, 1962). This is the case where the uptake or release of an ion takes place in the presence of a large amount of another ion in both the solid and liquid phase. In such systems, the amounts removed ate so small that the concentrations in both phases are practically constant, and thus in turn the individual diffusion coefficients also remain unaffected. Moreover, the rate-controlling step is the diffusion of the trace ion. 

The N02 decomposition is considerably more complicated and is discussed in more detail in Section IV-B-3. However, it will be shown that the important N03 species, especially at higher temperatures (200-400°C), is most probably the symmetric one, as Ashmore and co-workers8,9 contend. Still, there have also been arguments in favor of the pernitrite species in this system. Ogg337 observed the exchange of isotopic oxygen between 02 and N02 in the presence of NO, which he argued could only be explained by the mechanism... 

Two other reactions have been examined and will be discussed briefly. One is the exchange of isotopic nitrogen,... 

Figure 2. Arrhenius plots for the heterogeneous exchange of isotopic oxygen on NgO and Au/MgO catalysts of varying gold dispersion. See Table I for catalyst code numbers.

The Zr and Hf complexes may be photochemically more labile than the corresponding Ti dicarbonyl , even though CH is formed from its photolysis in the presence of Hj. It is now apparent that irradiation can induce exchange of isotopically labeled CO, and simple photosubstitution products result from irradiation of the complex in hexane in the presence of a trifluorophosphine sparge ... 

In exchange reactions chemical reaction gives appreciable exchange of isotopes between the reactants. These reactions can be used for the separation of isotopes. Obviously, only partial separations can be achieved through exchange reactions, because these are equilibrium reactions. Some examples of application of exchange reactions are given below. 

Table 1329 is a partial list of separation factors for exchange of isotopes of carbon, nitrogen, oxygen, and sulfur between an aqueous solution and a gas phase containing compounds of these elements. 

Isotope effects result from changes in the stiffness of bonding of isotopic atoms. Kinetic isotope effects reflect differences between reactant and transition state, and equilibrium isotope effects come from differences between reactant and product. Equilibrium isotope effects are easily measured experimentally, and they can also be calculated from the force fields derived for small molecules from vibrational frequencies. They are commonly expressed as fractionation factors, which are equilibrium isotope effects for exchange of isotope with a reference molecule such as water or ammonia. Tables of fractionation factors are in Refs. (96) and (98) and further values are in Ref. (99). 

Diffusion is a significant and predictable process of intercrystalline (and intracrystalline) isotope exchange. Discussion will concentrate on self-diffusion involving exchange of isotopes of major elements such as oxygen, which is distinct from chemical diffusion or tracer diffusion (Lasaga 1998). The Arrhenius equation -E/RT... 

The experiment involved allowing the enzyme to stand in the presence of G—O—PO3H and H2 P04 ions, but in the absence of fructose. There was an exchange of isotopic phosphorus, which went into the G—O—POsH". This does not occur in the absence of the enzyme, and it is concluded that an equilibrium such as... 

Qq does not take into account those microstates resulting from differing orientations and the exchange of isotopes of atomic nuclei. The numbers of such microstates do not change in chemical reactions. It is, therfore, justifiable to ignore them in thermochemical calculations. 

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