Isotope effects hydration

The accepted abundances of hydrogen and deuterium are 99.9844 /o and 0.0156 /o. Because of the largest relative mass differences, the hydrogen isotopes show the largest variations in the abundance ratio. The major reasons for variations are differences in vapor pressures, equilibrium and kinetic isotope effects, hydration and ultrafiltration. 

The second-order rate constants for hydration and the kinetic solvent isotope effect for hydration of several 2-substituted 1,3-butadienes ate given below. Discuss the information these data provide about the hydration mechanism. 

A direct irreversible proton transfer in limiting stage of 1-ethoxybut- l-en-3-yne hydration is confirmed by the value of kinetic isotopic effect k ilk = 2.9. For fast reversible proton transitions this value is less than 1. 

The solvent deuterium isotope effect for hydration of 4a and CH3C=C-0CH=CHCH3 were A Hjo/ DjO =2.13 and h o/ DjO" 1-90, respectively (8, 6). No deuterium was incorporated at the acetylenic position in 4a when this compound was reisolated after partial hydration in D2 O. 

The general acid catalysis, the deuterium solvent isotope effects, and the lack of deuterium incorporation upon partial hydration in D2 0 are particularly convincing evidence for a rate-determining protonation and the discrete intermediacy of a vinyl cation such as 6. 

Noyce and Schiavelli (21) also measured the rate of hydration of the above phenylacetylenes in deuteriosulfuric acid solution. Values of the observed solvent isotope effects, kn jo/l D,o. ar summarized in Table III. 

Solvent Deuterium Isotope Effects for the Hydration of Phenylacetylenes at 25° ... 

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. 

The rates of hydration of substituted phenylpropiolic acids give a rho of —4.77 when plotted against a, comparable to Ihe acid-catalyzed isomerization of czs-cinnamic acid, with a rho value of —4.3. The solvent deuterium isotope effects are 3.7-S.2 for the isomerization of cinnamic acids at... 

KINETIC ISOTOPE EFFECTS AND ARRHENIUS PARAMETERS FOR THE OXIDATION OF FLUORAL HYDRATE BY Mn(VH) AND Mn(VI)... 

Typical non-enolising aldehydes are formaldehyde and benzaldehyde, which are oxidised by Co(III) Ce(IV) perchlorate and sulphate , and Mn(III) . The main kinetic features and the primary kinetic isotope effects are the same as for the analogous cyclohexanol oxidations (section 4.3.5) and it is highly probable that the same general mechanism operates. kif olko20 for Co(III) oxidation of formaldehyde is 1.81 (ref. 141), a value in agreement with the observed acid-retardation, i.e. not in accordance with abstraction of a hydroxylic hydrogen atom from H2C(OH)2-The V(V) perchlorate oxidations of formaldehyde and chloral hydrate display an unusual rate expression, viz. 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

O Neil JR (1986) Theoretical and experimental aspects of isotopic fractionation. Rev Mineral 16 1-40 Oi T (2000) Calculations of reduced partition function ratios of monomeric and dimeric boric acids and borates by the ab initio molecular orbital theory. J Nuclear Sci Tech 37 166-172 Oi T, Nomura M, Musashi M, Ossaka T, Okamoto M, Kakihana H (1989) Boron isotopic composition of some boron minerals. Geochim Cosmochim Acta 53 3189-3195 Oi T, Yanase S (2001) Calculations of reduced partition function ratios of hydrated monoborate anion by the ab initio molecular orbital theory. J Nuclear Sci Tech 38 429-432 Paneth P (2003) Chlorine kinetic isotope effects on enzymatic dehalogenations. Accounts Chem Res 36 120-126... 

The only kinetic isotope effects so far reported for these reactions are those given by Pocker (1960), without experimental detail. He reports closely similar values for the rates of solvent-catalysed hydration of the species CHg. CHO, CD3. CHO, CH3. CDO and CD3. CDO in water at 0° C the replacement of CH3 by OD3 increases the velocity by about 7%. The same effect is reported for solutions in deuterium oxide at 0° C, presumably super-cooled. A comparison was also made of rates of hydration in HjO and DgO at 0°C, giving the following values for k(H.z0)lk(T>20) in presence of different catalysts H+/D+, 1 -3 AcOH/AcOD, 2 5 AcO , 2-3 H2O/D2O, 3-6. Almost exactly the same ratios were obtained by measuring rates of dehydration at 25° C in dioxan containing 10% of H2O or D2O and various catalysts. The presence of a considerable solvent isotope effect is consistent with the mechanism given in Section IV,B, and it would not be expected that substitution of deuterium on carbon would have an appreciable effect on the rate. 

Ab initio MO calculations have been carried out for two carbocation-generating reactions the 6 nI reaction of protonated 1-phenylethanol (H2O leaving group) and the acid-catalysed hydration of styrene. Optimizations were done at the MP2/6-31G level. The 6 nI transition state lies half way between the reactant and the product with respect to the bond lengths, charge distribution, and secondary deuterium isotope effects. 

Fig. 5A The dependence on pH of the deuterium isotope effect in the hammerhead ri-bozyme-catalyzed reaction. Black circles show rate constants in H2O gray circles show rate constants in D2O. Solid curves are experimentally determined curves. The apparent plateau of cleavage rates above pH 8 is due to disruptive effects on the deprotonation of uridine and guanosine residues. Dotted lines are theoretical lines calculated from pKa values of hydrated Mg ions of 11.4 in H2O and 12.0 in D2O and on the assmnption that there is no intrinsic isotope effect (a=kH2o/kD2o=l is the coefficient of the intrinsic isotope effect). The following equation was used to plot the graph of pL vs log(rate) log kobs=log(kmax)-log(l+10

To explain this different fractionation behavior, Taube (1954) postulated different isotope effects between the isotopic properties of water in the hydration sphere of the cation and the remaining bulk water. The hydration sphere is highly ordered, whereas the outer layer is poorly ordered. The relative sizes of the two layers are dependent upon the magnitude of the electric field around the dissolved ions. The strength of the interaction between the dissolved ion and water molecules is also dependent upon the atomic mass of the atom to which the ion is bonded. 

Dehydration of the hydrate gives uracil depleted in tritium, as is to be expected, but the small amount of the depletion indicates a large isotope effect in the dehydration. Further indication of large isotope effects is the observation80 that the tritium content of uracil irradiated in tri-tiated water contains only 27% of the expected amount of tritium. Dehydration of tritiated hydrate in tritiated water also gives uracil depleted in tritium but less so than the product obtained in ordinary... 

I am net able to offer an entirely convincing explanation here. Wre are tempted to propose the hydrated electron suggested by Dorfman to account for the lack of kinetic isotope effect. 

For isoenzymes I and II, the CO2 hydration rates are independent of buffer at high buffer concentrations, indicating thereby that a reaction step other than the buffer-dependent step becomes rate limiting. Studies of both hydration and dehydration reactions at high concentrations of buffers in H20 and DoO indicated that the kinetic parameter, kCSLt, for isoenzyme II has large isotope effect (k jkV) 3-4) (45b). This is consistent with involvement of H+ transfer in the rate-limiting step. The H+ transfer half-reaction is composed of at least two steps,... 

Similar isotope effects for human isoenzyme I (157c) on kc t and Km for C02 hydration are 1.7. Silverman and Tu (161) report an isotope effect of 2.5 for H2180 release and suggest that the intrinsic isotope effect of intramolecular H+ transfer might be significantly smaller in isoenzyme I than in isoenzyme II. Hence, the H20-splitting step might also limit the rate of C02 hydration in isoenzyme I. Human isoenzyme I has three titratable active-site histidines with pKa values... 

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