Chemical reaction isotope exchange

Our own calculations [25], which predict that the H20 molecule is a thermodynamically stable species up to at least 4000 K, are in agreement with these subsequent results. We, therefore, believe that it is possible to employ the methods described in this paper to make fully converged quantum-mechanical predictions of equilibrium constants for chemical and isotope exchange reactions within a sunspot. 

Isotopic exchange is the best example of the power of radiotracer techniques in chemical research. Isotopic exchange reactions have no inactive analogies. A system with two states of an element in equilibrium apart from isotopic distribution can be studied ... 

Most of the chemical properties of tritium are common to those of the other hydrogen isotopes. However, notable deviations in chemical behavior result from isotope effects and from enhanced reaction kinetics induced by the ( -emission in tritium systems. Isotope exchange between tritium and other hydrogen isotopes is an interesting manifestation of the special chemical properties of tritium. 

Slightly removed from this in rigor is the use of a substituent to make a pure exchange into a net chemical reaction. No isotopic label is then needed. For example, the first reliable estimate of the rate constant for the exchange of ferrocenium ions and ferrocene was made on the basis of kinetic data for processes such as... 

Is the paramagnetic adduct between CO and Cluster A a kinetically intermediate in acetyl-CoA synthesis Questions have been raised about whether this adduct is a catalytic intermediate in the pathway of acetyl-CoA synthesis 187, 188) (as shown in Fig. 13), or is formed in a side reaction that is not on the main catalytic pathway for acetyl-CoA synthesis 189). A variety of biochemical studies have provided strong support for the intermediacy of the [Ni-X-Fe4S4l-CO species as the precursor of the carbonyl group of acetyl-CoA during acetyl-CoA synthesis 133, 183, 185, 190). These studies have included rapid ffeeze-quench EPR, stopped flow, rapid chemical quench, and isotope exchange. 

One further approach, which has not properly been explored, is based upon the axiom of Harbottle s (29) that if an isotopic difference is found, there must have been little reaction subsequent to the initial hot stage. That is, these subsequent reactions are expected to be normal chemical reactions with essentially no isotopic preference, such that any such reaction would tend to wash out possible isotope effects. This problem is worth pursuing further, since some isotopic effects have been observed where subsequent exchange is to be expected. 

Even when forward reactions proceed rapidly at laboratory conditions, as is observed with Se(IV) and Cr(VI) reduction, evidence exists that chemical and isotopic equilibrium are not approached rapidly. Altman and King (1961) studied the kinetics of equilibration between Cr(III) and Cr(Vt) at pH = 2.0 to 2.5 and 94.8°C. Radioactive Cr was used to determine exchange rates, and Cr concentrations were greater than 1 mmol/L. Time scales for equilibration were found to be days to weeks. The mechanism of the reaction was inferred to involve unstable, ephemeral Cr(V) and Cr(IV) intermediates. Altman and King (1961) stated that the slowness of the equilibration was expected because the overall Cr(VI)-Cr(III) transformation involves transfer of three electrons and a change in cooordination (tetrahedral to octahedral). Se redox reactions also involve multiple electron transfers and changes in coordination. 

Isotope exchange includes processes with very different physicochemical mechanisms. Here, the term isotope exchange is used for all situations, in which there is no net reaction, but in which the isotope distribution changes between different chemical substances, between different phases, or between individual molecules. 

Isotope exchange reactions are a special case of general chemical equilibrium and can be written... 

For isotope exchange reactions in geochemistry, the equilibrium constant K is often replaced by the fractionation factor 6c. The fractionation factor is defined as the ratio of the numbers of any two isotopes in one chemical compound A divided by the corresponding ratio for another chemical compound B ... 

The change in concentration of a molecular entity (being transformed) per unit time and usually symbolized by (f). This change in concentration (/.c., dc/dt) occurs in one direction only and applies to the progress of a reaction step (or sequence of steps) in a complex scheme that may even involve a set of parallel reactions. The term chemical flux can also refer to the progress of a chemical reaction(s) in one direction while that system is at equilibrium (Le., via isotope exchange at equilibrium). 

In previous sections we have shown clearly that intramolecular dihydrogen bonds X-H- H-Y, with X and Y representing various chemical elements, can exist in both the solid state and in solution. In addition, the bonds can be a critical factor in the control of molecular conformational states or effects on rapid and reversible hydride-proton exchanges related to the process shown in Scheme 5.1, or the well-known H-D isotope exchanges in similar subsystems [23]. Such bonds could also play an important role in the stabilization of transition states, appearing as a reaction coordinate in many transformations. This is particularly... 

Comment After you have solved the problem, you should find that ACr° is much smaller for isotopic exchange reactions than for "normal" chemical reactions. Sometimes AC/ for a reaction is called the driving force for the reaction, and the reaction rate is assumed to be proportional to AC/. Because isotopic reactions are not any slower than chemical reactions, you can see that the driving force concept defined this way is not very helpful.)... 

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