Isotopic self-exchange

Isotopic self-exchange in B2H6 B2De mixtures again readily occurs with a reaction order of 1.5 (221). Shapiro and Keilin, by the use of B-enriched dibor-ane, also found that boron exchanges equally easily (288). Reliance on the step... 

In the homogeneous case, the derivation of an activation-driving force relationship and of an expression of the intrinsic barrier is less straightforward. It is interesting in this connection to relate the kinetics of crossexchange reactions (31) to those of the two self-exchange reactions (also called identity or isotopic reactions) [(32) and (33)]. 

Lind J, Shen X, Merenyi G, Jonsson B (1989) Determination of the rate constant of self-exchange of theCVCV- couple in water by 180/160 isotope marking. J Am Chem Soc 111 7654-7655 Liu Y, Pimentel AS, Antoku Y, Barker JR (2002) Temperature-dependent rate and equilibrium constants for Br (aq) + Br(aq) <=> Br2- (aq). J Phys Chem A 106 11075-11082 Marcus RA (1993) Elektronentransferreaktionen in der Chemie - Theorie und Experiment (Nobel Vortrag). Angew Chem 105 1161-1172... 

The rate of self-exchange reactions can generally be measured by isotopic tracer methods, but in several cases other techniques (optical rotation, nmr, epr) are more useful. Reactions like (18),... 

With new developments in technology, increasingly more rapid reactions can be investigated and many chemical, electrochemical and biological systems were studied. Consequently, the ET field developed in many directions as depicted in Fig. 1.4 [2]. Isotopic exchange reactions or, as they are now more generally termed, self-exchange reactions (since non-... 

In comparison, the level of detail in the understanding of radical ion reaction mechanisms is much lower for a number of reasons. Due to the inherently complex nature of the electron transfer-chemical reaction-electron transfer (ECE) mechanism, measurement of substituent, solvent and isotope effects will usually provide a combination of effects on all the steps involved. Introducing a donor substituent on a substrate will, for example, not only change the relative stability of the transition structures and intermediates with localized charges, but will also affect the rate constant of electron transfer and self-exchange between two substrates as well as the rate of back electron transfer. 

The rate of proton transfer has been measured for a number of metal hydride/organic amine combinations. The rates appear to follow Marcus behavior see Marcus Treatment), in which the rate goes up with driving force (equation 21, where ab is the rate of proton transfer between AH and B , and Kxr is the equilibrium constant for the proton transfer). Proton transfer appears to be the slow step in the process, rather than slow electron transfer followed by fast H atom transfer, because the rates show an isotope effect. For example, in the self-exchange of [CpM(H,D)(CO)3]/[CpM(CO)3] , kn/ko is 3.6, 3.7, and 3.7 for Cr, Mo, and W. There seems to be a good relation between thermodynamic acidity and kinetic... 

The result of isotopic tagging can be obscured by fast self-exchange of the tag and the regular isotope between positions on the molecule or between different molecules, say, between reactant and solvent. Such "scrambling," if it occurs, makes the technique useless. 

So far the important results of the isotopic-exchange experiments may be summarized as follow s S3) (a) Exchanges occur in which BHi remains an entity, as in the B2H6 self-exchange the rate of deuterium exchange is three times the rate of boron exchange, (b) Exchanges occur... 

The study of electron transfer reactions began in earnest when radioactive isotopes, produced for nuclear research and the atom bomb program during World War II, became accessible. Glen Seaborg, in a 1940 review of artificial radioactivity, noted the first attempt to measure the self-exchange reaction between aqueous iron(III) and iron(II), equation (1.9).1"... 

Equation (1.9) is an example of a self-exchange electron transfer reaction for which AG° = 0. One cannot tell the difference between reactant and product by color alone, hence the need for isotopic labeling indicated by the asterisk. 

Nuclear tunneling is potentially a significant consideration in outer-sphere radical electron transfer reactions. The case of reduction of NO2 to NO2 is notable in that nuclear tunneling is predicted to increase the self-exchange rate constant by a factor of 79 relative to the classical value.75 Kinetic isotope effect measurements could provide experimental evidence for nuclear tunneling. 180/160 KIE measurements have indeed provided evidence for nuclear tunneling in reactions involving the O2/O2 redox couple.76... 

Another example involves the Ru3+aq/Ru2+aq system, where the self-exchange process may be distinguished through having different oxygen isotopes present in the two aquated... 

The simplest reactions in solution chemistry are electron self-exchange reactions (Equation 6.10), in which the reactants and products are the same (the asterisk is used to identify a specific isotope). The only... 

Particle Controlled Isotopic Ion Exchange Kinetics (Self-Diffusion). 

Until recently, particle-controlled isotopic ion exchange or self-diffusion has not been studied in zeolites A and X because the rates are too high in the 1-10-/ crystals that are available commercially. One solution to this problem has been to develop experimental techniques to measure fast reaction rates. The other solution has been to grow large batches of well-formed single crystals as large as 100/ and thereby increase the half-times of the reactions by as much as 104 times. 

H. S. Sherry We have thus far only considered isotopic ion exchange reactions. Our equations are written to describe this process and not ion exchange. Thus, our diffusion coefficients are for self-diffusion and are constant. 

For the Na/BaA samples, the x-ray results indicate that the thermal stability of the A lattice is reduced considerably when there are >2 Ba p.u.c. That lattice collapse occurs on calcination of samples containing this Ba concentration is demonstrated also by a sudden decrease in the ability to recover 22Na by self-exchange and the appearance of an endo-therm on the DTA curve. However, the x-ray results show that up to a concentration of at least 4.5 Ba p.u.c. some structure is retained after calcination. The isotope experiments also demonstrate that above concentrations of about 3.7 Ba p.u.c. there is a relative increase in the amount of 22Na recoverable from calcined samples. These 2 points, taken in conjunction with the observed maximum in Figure 2 and the inability to prepare samples in the concentration range 2.7-3.7 Ba p.u.c., may indicate that the presence of 4 Ba ions p.u.c. is one of relative stability. Presumably, the first 2 divalent ions entering the crystal replace the 4 monovalent ions which are not located by x-ray structural analysis (3) and... 

Electron transfers are perhaps the simplest of all chemical reactions, at least when no chemical bonds are broken or formed. The availability of many radioactive isotopes due to nuclear developments in World War II made possible the early experiments in the electron-transfer field. Artificial radioactivity made it possible to measure the rates of a large number of isotopic-exchange (self-exchange) electron-transfer reactions in aqueous solution. Prior to about 1950, it was commonly believed that a self-exchange reaction such as [where the asterisk marks a radioactive isotope]... 

An electron transfer (ET) reaction is defined here as an oxidation-reduction reaction that occurs between two coordination compounds. The compounds may be the same species, but where the metals have different oxidation states (a selfexchan reaction), or are completely different species (a cross-reaction). An example of a self-exchange reaction is given in Equation (17.41), where Co is an isotopically labeled cobalt a cross-reaction is given by Equation (17.42). 

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