Isotope exchange process

Studies of kinetic energy release distributions have implications for the reverse reactions. Notice that on a Type II surface, the association reaction of ground state MB+ and C to form MA+ cannot occur. In contrast, on a Type I potential energy surface the reverse reaction can occur to give the adduct MA+. Unless another exothermic pathway is available to this species, the reaction will be nonproductive. However, it is possible in certain cases to determine that adduct formation did occur by observation of isotopic exchange processes or collisional stabilization at high pressures. 

Basically, whenever isotopic exchanges occur between different phases (i.e., heterogeneous equilibria), isotopic fractionations are more appropriately described in terms of differential reaction rates. Simple diffusion laws are nevertheless appropriate in discussions of compositional gradients within a single phase— induced, for instance, by vacancy migration mechanisms, such as those treated in section 4.10—or whenever the isotopic exchange process does not affect the extrinsic stability of the phase. 

The first attempts to quantify isotope exchange processes between water and rocks were made by Taylor (1974). By using a simple closed-system material balance equation these authors were able to calculate cumulative fluid/rock ratios. 

The imprint of local conditions can also be seen at other coastal and continental stations. The examples in Table 3.1 demonstrate that varying influences of different sources of vapor with different isotope characteristics, different air mass trajectories, or evaporation and isotope exchange processes below the cloud base, may often lead to much more complex relationships at the local level between 8D and 8 0 than suggested for the regional or continental scale by the global Meteoric Water Line equation. 

These difficulties created an incentive to focus theoretical investigations on polynuclear ions, which were structurally well characterized, and, though innovative experimental studies using NMR to follow the dynamics of oxygen-isotope exchange processes were beginning to yield unprecedented details of elementary reactions in aqueous solutions (36). 

Equilibrium isotope-exchange processes involve the redistribution of isotopes of an element among various species or compounds. At equilibrium, the forward and backward reaction rates of any particular isotope are identical. Isotopic equilibrium between two compounds does not mean that their... 

The work of Jones and Lu has embraced a wide range of reactions including hydrogenations, borohydride reductions, aromatic dehalogenations, decarboxylations and hydrogen isotope exchange processes, (Scheme 9.3, Eqs. 1-3). In addition to the accelerated rates of reaction, new environmentally friendly routes have been developed, particularly solventless reactions that minimise waste production and facilitate containment107-110. 

As indicated previously, NMR may be used simply as an analytical technique for monitoring the decomposition of a reactant or formation of a product. In addition, NMR and ESR merit a special mention due to their importance in studying the dynamics of systems at equilibrium these so-called equilibrium methods do not alter the dynamic equilibrium of the chemical process under study. They have been used to study, for example, -transfer reactions, valence isomerisations, conformational interconversions, heteronuclear isotopic exchange processes (NMR) and electron-transfer reactions (ESR). These techniques can be applied to the study of fast or very fast reactions by analysis of spectral line broadening [16,39],... 

The fact that different isotopes of an element do not have the same physical-chemical properties means that kinetic and isotope exchange processes can lead to variations in isotopic composition. This phenomenon is usually referred to as isotope fractionation. Isotope fractionation, in most cases, leads to only small differences in isotopic composition. Consequently, isotope ratios are generally reported in terms of parts per thousand (per mil, o/00) differences. While it is possible to report isotope compositions in absolute terms, it has been found most convenient to report them relative to a standard with a composition typical of common natural materials. These two considerations result in the commonly used "8" notation ... 

For TiC>2, this difference is more than the unit of pK. That is because in the case of metal oxides, that contains some anionic or cationic contaminants, as was previously mentioned, beside adsorption reactions, that form the electric charge at the interface, some ion or isotope exchange processes take place. The contribution of these processes is visible in Fig. 9 that presents the adsorption as a function of pH dependence, as an increase of cation adsorption below pHpzc and/or anion above pHpzc. This method may be applied to the determination of complexion constants only of the very pure metal oxides. 

Retention A situation in which the relative geometry of the chiral centre under investigation is the same at the end of the sequence of reaction steps as it was at the beginning. The opposite is inversion. If the geometry is randomised, then there is racemisation. Where racemisation occurs by proton exchange, and the racemisation takes place faster than the related isotopic exchange process, then the process is called isoracemisation. 

Comparison between the composition profiles versus ramp temperature observed over pre-oxidised 0.5% RhOx/Ce02 when in contact with N 0 + C 0 (cf. fig. 2a) rather than with N 0 only (cf fig la) reveals the absence of detectable 0 - N 0 in the former. Furthermore, the profiles in fig. 2c confirm the lack of any evidence for the oxygen isotope exchange process over the 0.5% Rh/Ce02 sample when in the HTR condition in contact with the equimolar mixture, thereby doubly emphasising the inhibitory role of C 0 co-reactant upon the O.i.x process observed over the same material equivalently pre-treated but in contact only with Blockage or removal by CO of the sites or species active for o.i.x. on... 

The activation energy for the M( CO)g- CO isotopic exchange process is lower for molybdenum than for chromium and tungsten " ". ... 

Similar intermediates seem to be involved in the net trans addition in hydrogenation, in racemization during exchange, and in transfer of the isotopic exchange process from one side of the cyclopentane ring to the other. 

Because of space limitations, we have limited ourselves to reactions of positive ions and have omitted processes like collisional dissociation which, so far as multiple collisions are concerned, are strongly related to vibrational excitation and deexcitation, and we have also omitted isotopic exchange processes, which show many similarities to proton-transfer reactions, isomerization processes, and switching reactions. 

As an example of an industrial isotope exchange process, let us consider the production of heavy water by the chemical reaction... 

Isotopic Exchange Reactions. - In the previous section the kinetics of isotope exchange processes were discussed. The emphasis of this section will be on mechanisms of exchange processes. Exchange reactions can provide valuable information about the breaking and making of bonds at the catalyst surface, the effects of chemical environment on the reactivity of bonds and the turnover of molecules between the gas phase and adsorbed state. Surface adsorbed intermediates, thus identified, may also participate in other, more complicated, reactions on the same catalyst. 

The rapid protonation-deprotonation process can be monitored by using either a deuterated alkane or a deuterated superacid. When the reaction is carried out in the presence of carbon monoxide, it is also possible to compare the isotope exchange process with the protolysis reaction which produces a smaller alkane and a smaller carbenium ion which is trapped by CO, forming stable oxocarbenium ions that are easily converted into esters for analysis (e.g.. Scheme 2). 

Experimental data indicated that the isotope exchange process is described by first order equations, and in most cases, e.g., on supported C0M0/AI2O3 catalysts by a superposition of two curves representing two types of sulfur mobility, the more and the less mobile i.e., rapidly and slowly exchangeable sulfur. In general, the H2S molar radioactivity a (in percent of the initial molar radioactivity of catalyst sulfide sulfur), as a function of the produced H2S-[X(cm )], is given (Fig. 7) as a superposition of curves... 

Isotope-exchange process for USA, Atomic Energy concentrating deuterium... 

Chemical reactions occur in many commonly practiced separation processes. By chemical reactions, we mean those molecular interactions in which a new species results (Prausnitz et al, 1986). In a few processes, there will he hardly any separation without a chemical reaction (e.g. isotope exchange processes). In some other processes, chemical reactions enhance the extent of separation considerably (e.g. scrubbing of acid gases with alkaline absorbent solutions, solvent extraction with complexing agents). In still others, chemical reactions happen whether intended or unintended estimation of the extent of separation requires consideration of the reaction. For example, in solvent extraction of organic acids, the extent of acid dissociation in the aqueous phase at a given pH should be taken into account (Treybal, 1963, pp. 38-41). Chemical equilibrium has a secondary role here, yet sometimes it is crucial to separation. 

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