Water isotope effect

Of particular interest in connection with halide ion relaxation rates, is the observation that there are marked upfield shifts of the ions resonances when D2O is substituted for H2O (see Section 6.5). According to the simple method by which Deverell [245 246] accounts for electronic distortion effects, one would then expect to observe larger field gradients with H2O as solvent than with D2O. Since the absolute shielding is not accurately known (see Section 6.1) it is not possible to predict quantitatively the relative effect on the quadrupole relaxation rate. Estimates give a change of the order of 5 %. 

Unfortunately, no systematic studies of the water isotope effect on halide ion quadrupole relaxation have been performed. Wennerstrom 

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N. Nandi, S. Roy and B. Bagchi, Ultrafast solvation dynamics in water isotope effects and comparison with experimental results, J. Chem. Phys., 102 (1995) 1390-7. 

Table 6.3. Water Isotope Effect on the Shielding of Alkali and Halide Ions...

NaClO. and LiClO.. No water isotope effect on shielding was detected. 

The finding by Craighead and Bryant [49S] of the same Cl relaxation rate for H2O and D2O solutions of NaClO is surprising in relation to possible models of the relaxation mechanism. We have reexamined this question and find, for a number of concentrations, an appreciably higher relaxation rate in D2O than in H2O (Table 9.1). The water isotope effect on relaxation is found to be 1.17 0.08 which is close to that found for halide ions and suggests, with the same... 

By protodetritiation of the thiazolium salt (152) and of 2 tritiothiamine (153) Kemp and O Brien (432) measured a kinetic isotope effect, of 2.7 for (152). They evaluated the rate of protonation of the corresponding yiides and found that the enzyme-mediated reaction of thiamine with pyruvate is at least 10 times faster than the maximum rate possible with 152. The scale of this rate ratio establishes the presence within the enzyme of a higher concentration of thiamine ylide than can be realized in water. Thus a major role of the enzyme might be to change the relative thermodynamic stabilities of thiamine and its ylide (432). 

Kinetic isotope effects are an important factor in the biology of deuterium. Isotopic fractionation of hydrogen and deuterium in plants occurs in photosynthesis. The lighter isotope is preferentially incorporated from water into carbohydrates and tipids formed by photosynthesis. Hydrogen isotopic fractionation has thus become a valuable tool in the elucidation of plant biosynthetic pathways (42,43). 

For this type of reaction the value of the solvent deuterium isotope effect is often a conclusive argument for the proposed mechanism 16). Rate measurements of 1 in acetic acid-acetate buffers in light and heavy water resulted in an isotope effect ktiiO lkozo of 2.5, and A oac/ doac of 9. A ratedetermining proton transfer to the /3-carbon atom of the enamine has been proposed and accounts for the experimental results I6-18 Eq. (5). 

In addition to the 4 stable isotopes sulfur has at least 9 radioactive isotopes, the one with the longest half-life being which decays by activity (Kmax 0.167 MeV, 87.5 d). can be prepared by Cl(n,p), S(n,> ) or S(d,p) and is commercially available as SeicmcQt H2S, SOCb and KSCN. The radiation has a similar energy to that of C ( mav 0.155 MeV) and similar counting techniques can be used (p. 276). The maximum range is 300 min in air and 0.28 mm in water, and effective shielding is provided by a perspex screen 3-10 mm thick. The preparation of many - S-containii compounds has been... 

However, measurements of substituent effects supported the hypothesis that the aryl cation is a key intermediate in dediazoniations, provided that they were interpreted in an appropriate way (Zollinger, 1973a Ehrenson et al., 1973 Swain et al., 1975 a). We will first consider the activation energy and then discuss the influence of substituents, as well as additional data concerning the aryl cation as a metastable intermediate (kinetic isotope effects, influence of water acitivity in hydroxy-de-di-azoniations). Finally, the cases of dediazoniation in which the rate of reaction is first-order with regard to the concentration of the nucleophile will be critically evaluated. 

Kinetic studies of mercuration have also been used as a test for hyperconjugation. Toluene and toluene-aaa-mercuric acetate (0.5 M) in acetic acid containing water (0.25 M) and perchloric acid (0.050 M) and an isotope effect, ArH/ArD = 1.00 0.03, obtained. This insignificant effect was considered as evidence against the participation of hyperconjugation in electron supply by a methyl group449. 

If one limits the consideration to only that limited number of reactions which clearly belong to the category of nucleophilic aromatic substitutions presently under discussion, only a few experimental observations are pertinent. Bunnett and Bernasconi30 and Hart and Bourns40 have studied the deuterium solvent isotope effect and its dependence on hydroxide ion concentration for the reaction of 2,4-dinitrophenyl phenyl ether with piperidine in dioxan-water. In both studies it was found that the solvent isotope effect decreased with increasing concentration of hydroxide ion, and Hart and Bourns were able to estimate that fc 1/ for conversion of intermediate to product was approximately 1.8. Also, Pietra and Vitali41 have reported that in the reaction of piperidine with cyclohexyl 2,4-dinitrophenyl ether in benzene, the reaction becomes 1.5 times slower on substitution of the N-deuteriated amine at the highest amine concentration studied. 

Leffek, Llewellyn and Robertson (1960a, b) made careful conductometric determinations of deuterium kinetic isotope effects on the solvolysis rates (in water) of some ethyl, isopropyl and n-propyl sulphonates and halides. In the case of the n-propyl compoimds,... 

The experiments of Bott (17) and Noyce (19-21) show that a vinyl cation best represents the intermediate in the hydration of phenylacetylenes. In particular, the large solvent Isotope effects observed indicate a rate-limiting protonation and formation of a vinyl cation, for these values are not in agreement with solvent isotope effects observed for compounds which react by other possible mechanisms, such as one involving equilibrium formation of the vinyl cation followed by the slow attack by water. 

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