Water deuterium exchange

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. 

Taniguchi, T., Harada, H. and Nakato, K. (1978). Detemination of water adsorption sites in wood by a hydrogen-deuterium exchange. Nature, 212, 230-231. 

Labile protons can always be positively identified by in situ exchange with D2O. In practice, a normal H NMR spectmm is recorded then deuterium exchange of labile protons is achieved by simply adding a drop of deuterated water (D2O) to the NMR sample. Labile protons in -OH, -COOH, -NH2 and -SH groups exchange rapidly for deuterons in D2O and the H NMR is recorded again. Since deuterium is invisible in the iH NMR spectmm, labile protons disappear from the NMR spectmm and can be readily identified by comparison of the spectra before and after D2O is addition. 

Analogous parahydrogen conversion and deuterium exchange reactions, catalyzed by NH2, have been observed in liquid ammonia (Wilmarth and Dayton, 61). The kinetics are of the same form as those of the OH -cat-alyzed reaction in water and the mechanism is open to similar interpretations. The NH2 -catalyzed reaction is much faster, its rate constant at —50° being 10 times that of the OH -catalyzed reaction at 100°. The assumption of equal frequency factors for the two reactions leads to a calculated activation energy for the NH2 -catalyzed reaction of about 10 kcal. This low value has been attributed to the much greater base strength of NH2 relative to OH . The results provide some support for the hydride ion mechanism. 

Evidence that the presence of water is important in these reactions has been obtained by Hansford (82). Pretreatment of the catalyst with a stream of predried air at 500°C resulted in a marked decrease in the rate of cracking. Further, if deuterium oxide was substituted for the water in the catalyst a large percentage of the deuterium exchanged with hydrogen atoms contained in the hydrocarbon undergoing reaction. Hansford also pointed out that effective catalysts for cracking reactions are always prepared from one or more hydrous oxides. 

It has been concluded from deuterium exchange experiments, using ethylene and heavy water, that the addition of an adsorbed proton to adsorbed ethylene is the actual rate-determining step. It can be seen that the two schemes differ, mainly in that the latter includes dissociative adsorption of water on the surface of the catalyst and does not specify the adsorption of ethylene, but they are consistent in that they assume the formation of a carbonium ion as the rate-determining step. 

Metal-free corrin is stable only in the protonated form and reversibly deprotonates at C-8 and C-l 3 in neutral or basic media (pX mcs 8.6 in dimethyicellosolve- water, 1 l).250 Therefore, deutera-tion under basic conditions (Na0D-KCN/Bul0D-D2O (1 1), r.t., M = H2, CoHI(CN)2, Ni2+) occurs at those positions. The electrophilic proton-deuterium exchange (TFA/Bu 0D-D20 (1 1), r.t., M = H2, Coih(CN)2, Ni2+) is more facile at C-5 and C-15 than at C-10 as predicted from MO calculations.239,250... 

Consider the reaction in which deuterium exchanges with ordinary hydrogen in water ... 

Several authors have reported that they did not succeed in isolating a number of pseudoazulenes when solutions in dilute mineral or glacial acetic acids were diluted with water or other bases.51-71,77-86 It was only possible to recover polymer-like material. In the subsequent decomposition the nucleophile attacks the conjugated acid of the pseudoazulenes at a position adjacent to the heteroatom, probably resulting in a fission. The hydrogen-deuterium exchange catalyzed by acids has been reported for systems 29,86 31,86 and 49.135... 

Deuterio-ammonia was synthesized by means of the reaction between heavy water and magnesium nitride, with subsequent drying by potassium amide (Landsberg et al., 1956). Compounds containing deuterium in a specific position were obtained either by deuterium exchange or by organic synthesis (Shatenshtein and Izrailevich, 1957). 

Stabilized ylides react with aldehydes in water to give Wittig products, sometimes with remarkable acceleration.260 For example, pentafluorobenzaldehyde reacts with ester-stabilized ylide, Ph3P=CHC02Me, at 20 °C in 5 min in 86% yield, with 99 1 E Z-selectivity. Water s ability to stabilize the polar transition state of the reaction, and its participation in the reaction (as determined by deuterium exchange), are discussed. 

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