Other Isotope-Exchange Reactions

We suggest that the exchange proceeds by heterolytic dissociative adsorption like other isotopic exchange reactions. Some such intermediate as the following is then involved. 

Radioactive isotopes of the halogens have found use in the study of isotope-exchange reactions and the mechanisms of various other reactions.The properties of some of the most used isotopes are in Table 17.5. Many of these isotopes are available commercially. A fuller treatment with detailed references... 

These two isomeric forms of R2 are physically different because of the pairing of the nuclear spins on the hydrogen nnclei in the molecule. They can be physically separated by chromatography at very low temperatures. Generally to convert one isomer to the other requires a cleavage of the R-R bond. As with the hydrogen-isotope-exchange reactions, no external acceptor or donor is required. 

This cancellation (always marked in exchanges of acid-base type) can be ascribed to the decrease in force constant C of the central atom with increasing coordination number n, and is sometimes not so significant in other types of isotopic exchange reactions. 

Transition metals also catalyze isotopic exchange reactions. Platinum is the most active catalyst for most heterocycles. The mechanism may involve metallation, addition, o--addition and ir-complex formation. a-Hydrogen exchange in pyridine is favored over 3- and 7-positions, particularly by a cobalt catalyst whereas platinum is much less selective. In isoquinoline both the 1- and 3-position protons are exchanged at almost the same rates with very little exchange at any other position. In 3-substituted pyridines exchange is preferred at the 6-position, the more so as the size of the 3-substituent increases (73AHC(15)140). 

The use of radiotracers is an excellent technique for measuring the solubility product constant of sparingly soluble salts or for making other studies of substances present in low concentrations. Another very important and classic example of the use of radiotracers is that of studying the occurrence and properties of isotopic exchange reactions—reactions of the type... 

Comparison of results on hydrogen exchange in liquid ammonia and in other solvents can be used to obtain information on the nature and mechanism of hydrogen isotope exchange reactions. 

The observed overabundance of deuterated species in molecular clouds and outer disks compared to the measured interstellar D/H ratio of 10 5 is well established. A classical isotopic deuterium fractionation is possible at low temperatures of 10-20 K owing to disbalance between forward and reversed reaction efficiencies H+ + HD 5 H2D+ + H2 + 232K (e.g. Millar et al. 1989 Gerlich et al. 2002). The temperature dependency in an isotope exchange reaction is a consequence of the zero-point vibrational energy difference for the isotopically substituted molecules (Bigeleisen Mayer 1947 Urey 1947). This leads to an elevated ratio ol H2t)+/H compared to HD/H2, which is quickly transferred into other molecules by ion-molecule reactions (see e.g. Roberts Millar 2000 Roberts et al. 2003). For example, the dominant reaction pathway to produce DCO+ is via ion-molecule reactions of CO with H2D+. In disks it results in a DCO+ to HCO+ ratio that increases with radius owing to the outward decrease of temperature (Aikawa Herbst 2001 Willacy 2007 Qi et al. 2008). 

In addition, he published other communications significant to chemical kinetics. This work was contemporary with that of Professors H. S. Taylor and E. K. Rideal on this subject. Among his communications, those describing isotopic exchange reactions of gaseous hydrogen with water and/or benzene on metal catalysts with enzymes or on enzymes alone may be the... 

Substitution reactions are well documented for the three Group 6 metal hexacarbonyls. Since they are low-spin d systems, reactivity is normally lower than that of other metal carbonyls. To promote substitution reactions, it is necessary to use elevated temperatures or uv irradiation. The hexacarbonyls usually undergo substitution via a dissociative mechanism, unless direct attack of a nucleophile at coordinated carbonyl groups occurs. Usually MoCCO) is more reactive than its two congeners. This is shown, inter alia, by kinetic studies on the isotopic exchange reaction between M( CO)6 and The higher kinetic lability of Mo(CO)6 may effect some of the reductions and oxidations, when the electron transfer process is accompanied by loss of carbonyl groups, e.g., equations (a), (e)-(g), and (1). 

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. 

Relative volume changes in isotopic exchange reactions, on the other hand, are very small except for hydrogen isotopes and therefore there is a minimal pressure effect. Clayton (1981) showed that at pressures of less than 20 kb the effect of pressure on oxygen isotope fractionation is less than 0.1 %. and lies within the... 

The third important concept introduced by Taylor was the use of model reactions, "yard sticks" to determine the mode of activation of molecules by surfaces. For hydrogen activation, Taylor(15) proposed the conversion of ortho to para hydrogen as a measure of the catalytic activity of a surface. This turned out to be more complicated than was first realized. A physical magnetic effect was also operative as was shown among others by Diamond and Taylor(27) for the case of rare earths and by Turkevich and Selwood.(25) Later Laroche and Turkevlch(29) used magnetic resonance to quantify the catalytic effect of charcoal and to differentiate it from dissociative process. The discovery of deuterium opened up the use of isotope exchange reactions as delicate "model reactions" for elucidation of the activation of molecules. Immediately after H. Urey announced the discovery of heavy water in 1932, Taylor(30) realized its potential as a tool in catalytic research and engaged in a massive production in Princeton of heavy water. 

---