Isotope Effect Data

How can such an unusual effect be explained The currently accepted answer is that there are two kinds of ion pairs contact ion pairs and solvent-separated ion pairs. A solvent-separated ion pair includes one (or perhaps several) solvent molecule between the cation and anion. It is distinct from a completely dissociated ion pair 

A very non-nucleophilic anion, such as perchlorate, can replace the leaving group at the solvent-separated ion pair stage. Upon reversion to a contact ion pair but with perchlorate, the perchlorate cannot form a stable compound with the carbenium ion. Hence, this salt becomes involved in the mechanism at a stage that blocks internal return, converting the carbenium ion to a more reactive form. This increases the rate of product formation more than an increase in the dielectric constant of the medium due to the polarity of the salt. 

Winstein, S., and Robinson, G. C. Salt Effects and Ion Pairs in Solvolysis and Related Reactions. IX. The f/7rt o-3- j-anisyl-2-butyl system. /. Am Chcm. Soc., 80,169 (1958). Winstein, S., Klinedinst, P. R., Jr., and Robinson, G. C. Salt Effects and Ion Pairs in Solvolysis and Related Reactions. XVII. Induced Common Ion Rate Depression and the Mechanism of the Special Salt Effect. /. Am. Chcm. Soc., 83,885 (1961). 

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Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Also, since ratios of isotopic ratios are commonly used to report isotope effect data (see Equations 7.17 and 7.18 for example), the actual numerical value of the abundance ratio of the standard is not important since it drops out from the final equation expressing the isotope effect ... 

There are two mechanistic possibilities left, either hydride transfer precedes decarboxylation, or vice versa. These two possibilities can be distinguished using Equations 11.51 and 11.53. Within experimental error only Equation 11.51 is consistent with the isotope effect data collected in Table 11.1, thus confirming that the reaction proceeds via a stepwise mechanism with hydride transfer to triphosphate nucleotide (NADP+) and intermediate formation of oxalacetate preceding decarboxylation ... 

Examination of one real-life case may benefit the reader s understanding. Strittmatter studied the primary kinetic isotope effects arising in the NADH-dependent cytochrome bs reductase (EC 1.6.2.2). The oxidation of NADH and subsequent reduction of cytochrome bs is facilitated by the enzyme-bound FAD group, and the kinetics of the direct transfer of a hydrogen from the A-face (or pro-R) of NADH to the flavin can be monitored by the loss of the 340 run absorbance of the NADH s dihydropyridine ring. Using deuterated isotopic isomers of NADH and several related compounds, Strittmatter obtained the primary kinetic isotope effect data compiled in the table below. 

In what is now a classical study in enzyme kinetics, W. J. Albery and J. R Knowles developed a strategy for establishing a reaction coordinate diagram (shown in Fig. 2) for triose-phosphate isomerase catalysis using solvent exchange and kinetic isotope effect data. 

Williams and co-workers provided solvent isotope effect data that supported the mechanism of Scheme 4P The rearrangement of N-phenyl-hydroxylamine (pAl = 1.9) in aqueous H2SO4 exhibits an inverse solvent isotope effect at pH > 2 and a normal solvent isotope effect of 1.5 in the plateau region at pH < 1.0. This is expected for the mechanism of Scheme 4... 

The enzyme-product complexes of the yeast enzyme dissociate rapidly so that the chemical steps are rate-determining.31 This permits the measurement of kinetic isotope effects on the chemical steps of this reaction from the steady state kinetics. It is found that the oxidation of deuterated alcohols RCD2OH and the reduction of benzaldehydes by deuterated NADH (i.e., NADD) are significantly slower than the reactions with the normal isotope (kn/kD = 3 to 5).21,31 This shows that hydride (or deuteride) transfer occurs in the rate-determining step of the reaction. The rate constants of the hydride transfer steps for the horse liver enzyme have been measured from pre-steady state kinetics and found to give the same isotope effects.32,33 Kinetic and kinetic isotope effect data are reviewed in reference 34 and the effects of quantum mechanical tunneling in reference 35. 

Table II. Summary of Isotope Effect Data for TDPG-Oxidoreductase and UDP Galactose-4-Epimerase (40)°...

The first directly-measured evidence that C—H bond rupture in the methyl group is the rate-determining step in the thermal de compn of TNT has been obtained with isothermal differential scanning calorimetric (DSC) analysis using deuterium isotope effects (Ref 96), DSC analysis has also been used to determine the kinetic parameters of thermal decompn (Ref 92). Others (Ref 101) have also studied the mechanism of the reaction using the deuterium isotope effect. Data are available on heat generation in... 

A distinction between mechanisms A1 and A2 on the basis of solvent isotope effect data is probably feasible if model calculations [102] of isotope effects are carried out with special consideration of the particularities of the reaction under study. In a similar way, it may be possible to distinguish between rate-determining and pre-equilibrium proton transfer with the aid of model calculations for reactions with kH/kD values in the region around 0.7 to 0.9. 

The increase of the rate coefficient with increasing acidity of the solution is probably caused by an additional reaction between ArCOOH and H30+, according to rate eqn. (68) (involving [H30+]) as observed also in the decarboxylation of aromatic amino-acids. This assumption is confirmed by comparisons of isotope effect data. According to Bourns results [247] of carbon isotope effects (Table 22), carbon—carbon bond cleavage is still relatively unimportant in the transition state of the reaction in 1 M HC104. On the other hand, the solvent isotope effect is decreased from ca. 6 in 0.02 M HC1 to 2.0 in 1.0 M HC1 (Table 24) [259]. If the reaction sequence... 

Several papers in this volume deal with transport in various kinds of liquids others examine critically the fundamental statistical-mechanical theory determining isotope effects for both equilibrium and kinetic processes in condensed as well as gaseous systems. These studies are of interest not only because they serve as a framework for comparing the merits of different isotope separation processes, but they provide powerful tools for using isotope effect data to obtain an understanding of inter-molecular forces in condensed and adsorbed phases and changes in intramolecular forces in isolated molecules. The title of this volume has accordingly been broadened from that of the symposium to reflect the wider scope of its contents. 

The steps shown by rate coefficients are fast compared to the others. This is evident from the effect of solvent basicity upon the rate of rearrangement. Similar rate coefficients were obtained in the presence of diphenyl ether, dimethyl aniline, acetic acid or with neat solutions . The kinetic isotope effect data given in Table 24 provides additional support for the fact that k is high compared to the other rate coefficients. 

The dotted lines are meant to convey a minimal degree of covalent bonding kinetic isotope effect data for hydrolysis of a-glucosyl fluoride were fitted to a transition state structure with bond orders to leaving group and nucleophile of only 0.001. 

I 70 Nuclear Tunneling in the Condensed Phase Hydrogen Transfer in Enzyme Reactions Table 10.2. Kinetic isotope effect data for various ADH enzymes ). 

The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of water is greatly increased by ERG substituents. The reaction of vinyl ethers with water in acidic solution is an example that has been carefully studied. With these reactants, the initial addition products are unstable hemiacetals that decompose to a ketone and alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results pertain to this step. The mechanistic features are similar to those for hydration of simple alkenes. Proton transfer is rate determining, as demonstrated by general acid catalysis and solvent isotope effect data. ... 

Transition-state structures computed from multiple kinetic isotope-effect, data have provided important information on the catalytic mechanisms of glycosylases. This is exemplified by the refined transition-state structures derived by Schramm and his associates and used to probe the hydrolytic reactions catalyzed by nucleosidases and the ADPR (adenosine diphosphori-bosyl) transferase toxins of Vibrio cholerae and Corynebacterium diphthe-... 

The palladium chloride-coppeifll) chloride couple (28, 29) used industrially in the Wacker process oxidizes olefins to carbonyl compounds. Experimental kinetic and isotope effect data (30) seem to indicate that a TT-olefin complex is initially formed in a series of preequilibrium steps. The rate-determining step is postulated to be a rearrangement of the TT-olefin complex to a cr-complex followed by the final breakdown of the cr-complex to products. Figure 13 depicts the widely accepted Henry mechanism (31). 

In their discussion, the authors used a mathematical analysis to address the C5 covalent mechanism using the available isotope effect data. The secondary hydrogen isotope effect was an effect on the kinetic constant V/K, in-... 

The way this has been expected to Influence an isotopic rate constant ratio has been discussed frequently in the literature (14), and Is illustrated In Fig. 5. lUnetlc-isotope-effect data can be expressed in the form... 

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