Tritium isotope effect

Other tritium isotope effects of significant magnitude have been observed in ion exchange (qv) (27) and gas chromatography (qv) (28,29). Many other examples have been described (9). 

Additionally, it has been noted that Tetralin operates via hydride transfer, at least in its reduction of quinones. Thus it has been shown that Tetralin readily donates hydrogen to electron-poor systems, such as quinones at 50°-160°C. The reaction is accelerated by electron-withdrawing substituents on the H-acceptor and polar solvents, and is unaffected by free radical initiators (6). These observations are consistent with hydride transfer, as is the more recent finding of a tritium isotope effect for the reaction (7). 

In sulfonation on the other hand, a tritium isotope effect is observed.287 Sulfonation is a reversible reaction and the fact that it is less exothermic is compatible with a slow, rate-determining dissociation of the intermediate. The transition state for the slow second step has a less covalent carbon-hydrogen bond than the ground state and hence the reaction is faster for deprotonation than for detritonation. 

At room temperature the chemical and radiochemical yields of 155 were different. The chemical yields were in the 30-40% range, while the radiochemical, not very reproducible yields were in the 6-15% range. Cattel and coworkers122 assigned these differences to tritium isotope effect in the Wittig reaction. No correlation between the specific activity of 155 and the degree of chemical conversion of 157 into 155 has been presented. The temperature dependence of the observed secondary tritium isotope effect has also not been... 

A tritium isotope effect in high-performance liquid chromatography of 11 eicosanoids has been observed. Multi-tritium-labelled eicosanoids were eluted earlier than the corresponding unlabelled eicosanoid. Variations in retention time are 3-7%, depending on the separation conditions as well as on the number and position of the tritium substituents238. 

In addition, Kozuka and Lewis measured the tritium isotope effect for the reaction between the n -hexyl, the 2-hexyl and the 2-methy 1-2-pentyl radicals with triphenyltin hydride and triphenyltin hydride-t see the last three entries in Table 11. The isotope effect of 2.55 found for the triphenyltin hydride-w-hexyl radical reaction was slightly smaller... 

The primary tritium KIE (k /l ) and the secondary tritium KIE ( h/ h) were determined in two different experiments carried out in the following way. If Ra is the radioactivity of the original reactant and / ROh is the radioactivity of ROH (ROT) determined at low (<5%) extents of reaction, the primary tritium KIE is given by equation (52). If the radioactivity of the styrene isolated at low extents of reaction, Rs, is also measured, the secondary tritium isotope effect can be calculated from (53). 

These reactions proceed through symmetrical transition states [H H H] and with rate constants kn,HH and kH,DH, respectively. The ratio of rate constants, kH,HH/kH,DH> defines a primary hydrogen kinetic isotope effect. More precisely it should be regarded as a primary deuterium kinetic isotope effect because for hydrogen there is also the possibility of a tritium isotope effect. The term primary indicates that bonds at the site of isotopic substitution the isotopic atom are being made or broken in the course of reaction. Within the limits of TST such isotope effects are typically in the range of 4 to 8 (i.e. 4 < kH,HH/kH,DH < 8). 

Relative Values for Deuterium and Tritium Isotope Effects... 

The observation of a primary tritium isotope effect (H/T) that is substantially larger than the value predicted on the basis of the semiclassical Swain-Schaad relation (Chart 3) from a heavy-hydrogen (DAT) isotope effect. The same information can be expressed in terms of a Swain-Schaad exponent required to relate the two isotope effects that is substantially larger than the semiclassical value of 3.26. 

Kaldor, S.B., Eredenburg, M.E. and Saunders, W.H. (1980). Mechanisms of elimination reactions 32. Tritium isotope effects and tunnel effects in the reaction of 2,2-diphenylethyl-2-t derivatives with various bases. J. Am. Chem. Soc. 102, 6296-6299... 

SECONDARY ISOTOPE EFFECTS. Changes in reaction may also result from isotopic substitutions at positions that are immediately adjacent to the reaction center (/.e., the bond broken/made in the chemical reaction under investigation). We deal here only with so-called secondary ce-isotope effects, and we will limit the scope further by considering only a deuterium and a tritium isotope effects on carbon. Isotopic substitution by heavier nuclides will also give rise to a isotope effects, but they are quite small. The magnitudes of the a isotope effects for C—compared to C— as well as for C—compared to C—are also relatively small, frequently necessitating the use of special techniques. 

A procedure using both deuterium and tritium isotope effects on to obtain intrinsic isotope effects for... 

Although the pure titanium-hydrogen system exhibits the normal isotope effect, many titanium alloys show the inverse effect. The exchange of pro-tium-tritium mixture with the hydrided phase of these alloys has demonstrated an inverse protium-tritium isotope effect in Ti-V, Ti-Mo, Ti-Cr, Ti-Mn, and the ternary alloy TiCrMn (1). On the other hand, Ti-Co, Ti-Fe, and Ti-Ni systems exhibit the normal isotope effect. Clearly much can be learned from a study of these systems. 

Table 6. Tritium isotope effects for hydroxylic quench agents...

The two preceding methods have been combined to determine the tritium isotope effect. In a tritium-labeled substrate in D20, the change in infrared radiation absorption arises almost entirely from release of protons because the concentration of the tritium species in the reactant is small. Thus, the rate constant kH (determined by the change in the DOH absorption) represents release of protons. The constant kT for release of tritium to the solvent is determined from radioactivity measurements of water from the same reaction mixture. In the enolization of... 

In experiments of major importance, first published in 1950, Melander found that in the nitration and bromination of a number of benzene derivatives the tritium isotope effect (kHlkT) is not 10-20 as is to be expected if carbon-hydrogen bond breaking occurs in the rate-determining step, but rather is less than 1.3. The direct displacement mechanism was thus ruled out, and the two-step mechanism of Equation 7.70 with the first step rate-determining was implicated.157... 

Deuterium and tritium isotope effects in the base-catalysed C -hydron transfer from 2-(1-hydroxylbenzyl)oxythiamin (HBOT 458)... 

Deuterium and tritium isotope effect study of the methyl-methylene elimination in the enzyme catalyzed biosynthesis of (R)- and (S)- limonenes (506)... 

However, under somewhat different conditions (aqueous acetonitrile at 85°), no evidence for the free carbonium ion could be found (Bethell et al., 1965). Moreover, the invariance of the product proportions when water is replaced by deuterium oxide, coupled with the observation of a large tritium isotope effect on the formation of diphenylmethanol, is consistent only with the ylid mechanism (equation 21) (Bethell et al., 1969). For reaction of diarylmethylenes with alcohols, substantial hydrogen-isotope effects are observed, consistent with both equations 21 and 22. 

Fig. 8.7 Partial reaction profile of alanine racemization by DadB enzyme. Dotted lines indicate kinetically insignificant steps, and the dot-dash line indicates kinetic tritium isotope effect. (Reproduced with permission from Faraci and Walsh, Biochemistry, 27, 3275 (1988)).

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