Isotope effect determination

The intramolecular kinetic isotope effect determined in reaction of BTNO with p-MeO-C6H4CH(D)0H gave a h/ d ratio of 5.6 in MeCN , consistent with a rate-determining H-abstraction step. Additional determinations gave a h/ d of 7 with PhCH(D)OH, and 12 for the intermolecular competition of fluorene vs. 9,9-dideuteriofluorene. The latter value supports the contribution of tunnelling already commented on for reaction of PINO with various C—H donors ( h/ d values in the 11-27 range) . ... 

A complete set of 13C kinetic isotope effects determined (by a natural abundance CMR method) for addition of lithium dibutylcuprate to cyclohexenone, in THF at —78°C, have been shown to be consistent with those calculated theoretically for ratedetermining reductive elimination from an intermediate square-planar copper complex.120 Thus, die KIE (12k/13k) = 1.020-1.026 at C(3) is indicative of substantial bonding change, and partial alkyl transfer can explain the significant low KIE = 1.011-1.016 for Ca of die butyl group. 

The RT (at pH 9) equilibrium isotope effect determinations are presented in the upper, lower and right-hand side of equation 185. They have been used to calculate the isotope effect for the equilibrium between p-nitrophenolate and PNPA in water since the PNPA undergoes hydrolysis during the equilibrium. The 18 feq isotope effect for deprotonation was found to be 1.01533. The earlier reported value376 was 1.018 0.002. 

Reaction (19) causes larger isotope effect determined for product than effect for substrate due to additional fractionation of analysed carbon atom in the side reaction. For mechanistic analysis 13C KIE based on the substrate analysis was used. DFT calculations of isotope effects for each step of the reaction led to conclusion that the rate-determining step involves breaking of the P-C bond in the tetragonal pyramidal intermediate. 

The mean values of 13C isotope effects determined in three independent experiments for unreacted substrate (81-92% completion) and product (10% completion) are summarized in Figure 12. 

The resulting isotope effects determined relative to the para carbon are summarized in Figure 13. Independently of used catalyst similar 13C isotope effects were observed for olefinic carbons. This fact suggests that in the presence of the bulky chiral catalyst as Rh2(S-DOSP)4 the geometry of the cyclopropana-tion transition state was not change. 

Small isotope-effects can be detected by double-labeling techniques, in which the carbon skeleton is labeled with 14C, and the ratio of 14C to tritium is measured both in the substrate and the product. Care must be taken in the observation and interpretation of isotope effects determined from the hydrogen-isotope content of the product. Just as in non-enzymic reactions (see p. 154), discrimination against the substrate containing deuterium or tritium leads to an increase in the isotopic content of the substrate, and this decreases the apparent isotope-effect towards the end of the reaction. 

These equations are not easily related to equations such as (2), since one proposes a mechanism (force constant asymmetry) for the fall of isotope effect, and the others ignore totally the isotope effect source, and merely assume that it exists, that is that Aq > Ah or that Cd > ch It would of course be possible to calculate the term "iH 4> associated with any particular maximum isotope effect determined by ehx — dx from the isotope effect calculated with Eq. 6 (or Eq. 8), but the force constants could not be so calculated, since there is no unique set of force constants giving a particular vih... 

Hermes JD, Cleland WW. Evidence from multiple isotope effect determinations for coupled hydrogen motion and tunneling in the reaction catalyzed by glucose-6-phosphate dehydrogenase. J. Am. Chem. Soc. 1984 106 7263-7264. 

WW Cleland. Isotope effects determination of enzyme transition state structure. Methods Enzymol 249 341—373, 1995. 

The exponents described by Saunders, sometimes called mixed isotopic exponents , are shown in Eq. (11.17). The exponent describes the relationship between the H/T isotope effect from substitution at site one (determined when protium is at site two), and the site-one D/T isotope effect (determined when deuterium is at site two). If the two sites are distinguished as giving primary and secondary isotope effects, the first exponent in Eq. (11.17) resembles the single-site Swain-Schaad exponent Eq. (11.9) for a primary isotope effect, and the second exponent in Eq. (11.17) resembles a single-site secondary Swain-Schaad exponent. However, the mixed isotopic exponents necessarily involve isotopic substitution at two sites and should not be confused with single-site Swain-Schaad exponents. 

C-2 was observed in the residual alanine, it can be inferred that once the carhanion is formed, it reacts readily with the thioester. The deuterium kinetic isotope effect determined with [ H]Ala was weak 1.3),... 

Many examples of the use of the cumulati-ve methods of competing reactions for isotope effect determinations exist, and Ropp (1952) has summarized much of the early work in which these methods were applied to determinations of isotope effects of carbon, nitrogen and oxygen isotopes by the use of equations (31), (33) or their equivalents. 

Hydrogen and carbon isotope effects are the most sensitive means for studying the elementary acts in hydrocarbon reactions. Unfortunately, precise determinations of isotopic compositions deal mainly with hydrocarbons produced in natural conditions (Section IV.C). No systematic isotope effect studies for light hydrocarbon production or decomposition were carried out in controlled laboratory conditions, except some deuterium isotope effect determinations carried out on rearrangements of cycloalkanes, reviewed in this section. 

Thus the secondary and primary deuterium isotope effects determined in this study also indicate that entropy and zero-point energy factors associated with breaking of the C—D bond and migration in the activated complexes are important for the structural isomerization to yield butene-1 and butene-2, and reaction path B (equation 197) must be included in the mechanistic considerations concerning the cyclopropane isomerization. But the higher activation energy for isobutane formation (g = 64.3 kcal mol" ) than that for butene-2 or butene-1 (Q = 62.0 0.6 kcal mol" ) indicates also that the rupture of... 

The efficiency of oxidation of open-chain alkyl, cycloalkyl, and unsaturated alcohols in acetonitrile by 9-phenylxanthylium ion (PhXn+) was dependent on the alcohol stmc-tures. Structure-reactivity relationship was discussed with relation to formation of a carbocationic transition state (C +-OH). Kinetic isotope effects determined at a-D, p-D3, and OD positions for the reaction of 1-phenylethanol suggested a hydride-proton sequential transfer mechanism that involved a rate-limiting formation of the a-hydroxy carbocation intermediate. Unhindered secondary alkyl alcohols were selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C(7)-C(ll) cycloalkyl alcohols reacted faster than cyclohexyl alcohol, whereas the strained C(5) and C(12) alcohols reacted slower. Aromatic alcohols were oxidized efficiently and selectively in the presence of aliphatic alcohols of comparable steric requirements. ... 

Bronsted exponents for the ionization of a series of nitroalkanes and ketones, measured mainly by Bell and his collaborators [48, 55], are shown in Table 6. Although a common set of bases was not used in all cases, the bases were confined to carboxylate anions. The values are compared with isotope effects determined for ionization of the substrates with H2O as the base [48,55], and it is apparent that kff/ko increases steadily with )8, with only ethyl acetoacetate and acetylcyclohexanone out of line, and that the correlation with p is better than that with either the reactivity or pK of the substrate. The isotope effects are rather small because water is a weak base and in most cases the transition states should be strongly asymmetric in structure. Moreover because )8 refers to the much stronger carboxylate anions the absence of an isotope maximum at a = 0.5 is not surprising [48]. As would be expected, available measurements for carboxylate anions give large values of k k, but the results are too fragmentary to permit any further conclusion. 

In cases where exchange cannot be avoided, experiments can be done with varying ratios of heavy and light isotopes. A number of experiments are performed at different degrees of deuteration (22%, 40%, 50%, 75%, and 100% D) and chemical shifts plotted versus the percentage of the heavy isotope. The plot can be extrapolated to 0% heavy isotope and the isotope effect determined. An example is shown in Figure 6.5. This situation is always found if the solvent is water or alcohols such as CD3OD. 

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