Hydrogen-deuterium exchange theory

A number of theoretical works have been devoted to the study of the hydrogen-deuterium exchange reaction. Hauffe (25) examined this reaction from the standpoint of the boundary layer theory of chemisorption. Dowden and co-workers (26) undertook a theoretical investigation of the hydrogen-deuterium exchange reaction from the viewpoint of the theory of crystalline fields. 

We shall consider the hydrogen-deuterium exchange reaction from the viewpoint of the electronic theory of chemisorption and catalysis (27),... 

Evidently, all the regularities indicated above must be explained by the theory of the hydrogen-deuterium exchange. 

We see that the correlation between the electrical conductivity of a specimen and its catalytic activity established by the electronic theory (1) must show up distinctly and in fact reveals itself in the case of the hydrogen-deuterium exchange reaction. 

Let us now turn to a comparison of theory with experiment. Comparing (95), (84), and (68), we find that the dependence of the photocatalytic effect K on the position of the Fermi level at the surface s and in the bulk cv of an unexcited sample for the oxidation of water is the same as for the oxidation of CO or for the hydrogen-deuterium exchange reaction. For this reason, such factors as the introduction of impurities into a specimen, the adsorption of gases on the surface of the specimen, and the preliminary treatment of the specimen will exert the same influence on the photocatalytic effect in all the three reactions indicated above. The dependence of K on the intensity I of the exciting light must also be the same in all the three cases. 

The electronic theory furnishes, as we have already seen, a general recipe for the consideration of heterogeneous photocatalytic reactions. This recipe was obtained in Section I of the present article. In Sections III, IV, and V it was applied to the reactions of hydrogen-deuterium exchange, oxidation of CO, and synthesis of hydrogen peroxide, respectively. These are the most thoroughly studied photocatalytic reactions. 

Because zinc oxide is a relatively well-understood oxide semiconductor, we shall first review its properties as a hydrogenation catalyst in the catalytic hydrogen-deuterium exchange reaction. Since the latter essentially measures the rate of reversible chemisorption of hydrogen at equilibrium, data on the hydrogen chemisorption will be included in this survey. Any theory of hydrogen chemisorption on zinc oxide must explain all the following well-established facts. 

These results seemed to be inconsistent with any simple electronic theory of hydrogenation catalysis they were, however, relevant to the general concept that dehydration of oxide catalysts should leave the surface in a strained, catalytically active condition ( , S). A systematic study was therefore undertaken of the activation of pure y-alumina for ethylene hydrogenation and hydrogen-deuterium exchange the effects of pretreatment, drying conditions, and rehydration were investigated. 

Theory of Gas-Phase Hydrogen/Deuterium (H/D) Exchange Experiments... 

Similar results have been obtained for methane 12) and for ethane 19). The values quoted in Table II also illustrate the point that the distribution of deuterium between hydrogen and propane differs from the value expected for a random distribution. With the ratio of pressures used, the expected percentage for the mean deuterium content of the hydrocarbon would be 33.3, which is substantially less than the experimental value of 40.9 %. This type of deviation is also found with other hydrocarbons, but it does not affect the validity of using classical theory for the calculation of the interconversion equilibrium constants in studies of mechanism of exchange reactions. More accurate values for these equilibrium constants are necessary, however, if one is interested in the separation of isotopes by chemical processes. 

Furthermore, the use of isotope effect on chemical shifts has a very nice feature, that is, they are easy to measure. Isotope effects on chemical shifts are typically measured in compounds in which an XH proton is exchanged with deuterium, but deuterium may, of course, be also introduced at carbon although this is more labor intensive. Other pairs of isotopes can also be used (see later). To describe the methods, the theory behind is essential. As in tautomeric systems we typically are dealing with light atoms such as hydrogen, quantum effects have to be taken into account. One assumption usually made in isotope-effect studies is the Bom-Oppenheimer approximation, whereas others treat the effects in the nonadiabatic way. The former is usually the case in the examples given, whereas... 

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