The Reaction of Hydrogen-Deuterium Exchange

The hydrogen-deuterium exchange H2 + D2 — 2HD is the simplest heterogeneous reaction taking place on the surface of semiconductors. This reaction has been thoroughly studied experimentally. It has been shown that under the influence of illumination (other conditions being constant) the rate of the reaction is considerably changed. 

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), 

We shall first consider the reaction occurring in the dark. Let us enumerate the basic regularities established experimentally and awaiting an explanation. 

For example, Heckelsberg and his associates (S3) have discovered that the introduction of a donor impurity (Al) into ZnO increases the reaction rate, while the addition of an acceptor impurity (Li) retards the reaction. 

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The catalytic activity in relation to a given reaction occurring on the surface is characterized by the rate g of this reaction, i.e., by the amount of reaction products formed under the given external conditions per unit time on unit surface area. An expression for g has different forms for different reactions. For the reactions of hydrogen-deuterium exchange, oxidation of CO, and synthesis of H2O2, this expression will be derived in Sections III, IV, and V, respectively. 

Thus, Kohn and Taylor (40) point out that the y irradiation of zinc oxide which speeds up the reaction of hydrogen-deuterium exchange lowers the magnitude of the effect when a donor impurity is introduced into the specimen. 

Lunsford and Leland (42) studied the reaction of hydrogen-deuterium exchange on crystals of MgO containing V-centers. As known, a Y-center in an ionic crystal, being a cationic vacancy with a hole localized near it, plays the role of an acceptor. These authors have found that the photocatalytic effect is intensified as the concentration of V-centers in a crystal increases, which is in accord with the experiments carried out by Kohn and Taylor. 

Freund (44) studied the influence of ultraviolet light on the catalytic activity of zinc oxide in relation to the reaction of hydrogen-deuterium exchange. The author noted that the photocatalytic effect was positive and that it decreased with rising temperature. 

Two types of hydrogen replacement are discussed here (1) the base-induced hydrogen-deuterium exchange reactions and (2) the hydrogen-metal exchange reactions. 

Isotopic Exchange Reactions. Exchange reactions between the isotopes of hydrogen are well known and well substantiated. The equihbrium constants for exchange between the various hydrogen molecular species have been documented (18). Kinetics of the radiation-induced exchange reactions of hydrogen, deuterium, and tritium have been critically and authoritatively reviewed (31). The reaction T2 + H2 — 2HT equiUbrates at room temperature even without a catalyst (30). 

J. W. Pyper and C. K. Briggs, Kinetics of the Radiation-induced Exchange Reactions of Hydrogen, Deuterium, and Tritium, Eawrence Eipermore Eaboratory Report... 

Fig. 8. Reaction rate of hydrogen-deuterium exchange as a function of the position of the Fermi level at the surface of a crystal.

The unconverted alcohol recovered from the reaction product was examined closely for evidence of hydrogen-deuterium exchange. Infrared... 

In addition to the fact that hydrogen/deuterium exchange reactions can be helpful to probe ion structures as will be shown later, they can also reveal mechanistic details such as the site of reaction within ions. For example, the pentadienyl anion exchanges four protons rapidly, demonstrating, as shown in (12), that proton addition occurs more rapidly at the ends of the conjugated system than in the middle (Stewart et al., 1977 DePuy et al., 1978a). 

J. Turkevich I was in error in the statement that hydrogen-deuterium exchange reaction is the only reaction that takes place with the help of acid sites, for we have found that butene-1 to butene-2 transformation is catalyzed by both Bronsted and Lewis acid sites. The Bronsted sites, however, give a marked stereospecificity in producing nonequilibrium mixtures of cis and trans, while the Lewis acid sites give an equilibrium mixture of the geometric isomers. 

Proton magnetic resonance techniques have been used for the measurement of rates of hydrogen-deuterium exchange of pyrazine (in CHsOD-CHsONa at 164.6") (591) for a study of protonation of pyrazine (1472) for analysis of the reaction mixture from quatemization of 2-substituted pyrazines with methyl iodide (666) for elucidation or confirmation of the structures of alkylpyrazines obtained by alkylation of pyrazines with aldehydes and ketones in the presence of a solution of an alkali or alkaline earth metal in liquid ammonia, or a suspension of these metals in other solvents (614) for a study of changes in chemical shifts produced on ionization of 2-methyl and 2-amino derivatives of pyrazine in liquid ammonia (665) for characterization of methoxymethylpyrazines (686) for the determination of the position of the A -oxide function in monosubstituted pyrazine V-oxides and the analysis of V-oxidation reactions (838) for a study of the structure of the cations of fV-oxides of monosubstituted pyrazines (1136) and for the determination of the structure of the products of peroxyacetic and peroxysulfuric acid iV-oxidation of phenyl- and chlorophenylpyrazines (733b). 

Run 247 exhibits a higher fraction of exchange into methyl than the other runs and its distribution pattern approaches that of run 10. The catalyst of run 247 was not activated in the usual way. In the course of hydrogen-deuterium equilibration reactions described in the next section, the catalyst had been held in the vicinity of 273° for 15 hours. 

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