Exchange effects hydrogen-deuterium

The hydrogen-deuterium exchange rates for 1,2-dimethylpyrazolium cation (protons 3 and 5 exchange faster than proton 4 Section 4.04.2.1.7(iii)) have been examined theoretically within the framework of the CNDO/2 approximation (73T3469). The final conclusion is that the relative reactivities of isomeric positions in the pyrazolium series are determined essentially by inductive and hybridization effects. 

The first term was found to correspond to the rate of enolisation (measured by an NMR study of hydrogen-deuterium exchange at the methylene group). The second term predominates at [Cu(II)] > 10 M and is characterised by a primary kinetic isotope effect of 7.4 (25 °C) and a p value of 1.24. Addition of 2,2 -bipridyl (bipy) caused an increase in 2 up to a bipy Cu(II) ratio of 1 1 but at ratios greater than this 2 fell gradually until the enolisation term dominated. The oxidation of a-methoxyacetophenone is much slower but gives a similar rate... 

A large number of works have been devoted to the effect of impurities on the rate of the hydrogen-deuterium exchange reaction. 

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. 

The same authors (41) working with specimens of silica gel observed a positive photocatalytic effect in the course of the hydrogen-deuterium exchange reaction. In this case the introduction of an acceptor impurity into a catalyst enhanced the action of irradiation. 

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. 

Kohn and Taylor (40) also studied the influence of illumination on the hydrogen-deuterium exchange reaction using specimens of barium, calcium, lithium, and sodium hydrides. If the specimens were annealed in the hydrogen atmosphere, the photocatalytic effect on these specimens was positive. And if the specimens of the same hydrides were preliminarily calcined in vacuum, the irradiation of these specimens retarded the reaction. 

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. 

Fig. 9. Sign and magnitude of the photocatalytic effect of the hydrogen-deuterium exchange.

The introduction of an impurity into a specimen (accompanied by a change in tv and es ) will transfer us from one point to another in Fig. 9. Suppose that when a donor impurity is introduced into the specimen (decrease in v and e8 ), we are transferred from the point A to the point B. This involves a decrease in K, as can be seen from Fig. 9. Such a decrease in the photocatalytic effect caused by the addition of donor impurities has been observed by Kohn and Taylor (40) who studied the photoreaction of hydrogen-deuterium exchange on zinc oxide exposed to y radiation. 

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. 

A. A. Kossiakoff, M. Ultsch, S. White, C. Eigenbrot, Neutron Structure of Subtilisin BPN Effects of Chemical Environment on Hydrogen-Bonding Geometries and the Pattern of Hydrogen-Deuterium Exchange in Secondary Structure Elements , Biochemistry 1991, 30, 1211-1221. 

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