Kinetic isotope effects limiting

We now carry the argument over to transition state theory. Suppose that in the transition state the bond has been completely broken then the foregoing argument applies. No real transition state will exist with the bond completely broken—this does not occur until the product state—so we are considering a limiting case. With this realization of the very approximate nature of the argument, we make estimates of the maximum kinetic isotope effect. We write the Arrhenius equation for the R-H and R-D reactions... 

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. 

Apart from a few studies (ref. 7), the use of deuterium kinetic isotope effects (kie s) appears to have had limited use in mechanistic studies of electrophilic bromination of olefins. Secondary alpha D-kie s have been reported for two cases, trans-stilbene fi and p-substituted a-d-styrenes 2, these giving relatively small inverse kie s of... 

The deuterium kinetic isotope effect between BH3-THF and BD3-THF was obtained by measuring the reaction rate constants of the two reactions with the unsaturated sulfoxide (Sj-40 independently via React-IR. The k(BH3)/k(BD3) is 1.4, consistent with hydrogen transfer not being the rate-limiting step [15, 16]. 

Hydrogen motion, H+, H or H, is often involved in the rate-limiting step of many enzyme catalysed reactions. Here, QM tunnelling can be important and is reflected in the values of the measured kinetic isotope effects (KIEs) [75], Enzyme motion... 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

In some cases the coupling reaction is found to be base-catalysed, and this is found to be accompanied by a kinetic isotope effect, i.e. /c, > k2> and the breaking of the C—H bond in (35) is now involved in the rate-limiting step of the reaction. 

The factors that influence elimination v. substitution are discussed subsequently (p. 260). Evidence for the involvement of C—H bond fission in the rate-limiting step—as a concerted pathway requires— is provided by the observation of a primary kinetic isotope effect (cf. p. 46) when H is replaced by D on the ft-carbon. 

Jacobs, G., Khalid, S., Patterson, P.M., Sparks, D.E., and Davis, B.H. 2004. Water-gas shift catalysis Kinetic isotope effect identifies surface formates in rate limiting step for Pt/ceria catalysts. Appl. Catal. A Gen. 268 255-66. 

The k pathway is three times faster in D+/D20 than in H+/H20 for la. The reverse kinetic isotope effect suggests that the rate-limiting event for the k pathway could involve protonation of an amido-nitrogen or an N-Fe bond, forming the stronger N-H bond as the weaker N-Fe bond is cleaved. The k 3 pathway is rationalized as involving pre-equilibrium peripheral protonations of the TAML macrocycle (Scheme 1). The dependence of obs on [H + ] is then given by Eq. (4), which corresponds... 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

Proton transfer to carbenes is indicated by the following kinetic data (i) the rates of X-H insertion reactions increase with increasing acidity of the proton donor HX (ii) normal deuterium kinetic isotope effects are observed, kHx > DX (hi) alcohols react faster than ethers. However, mechanistic conclusions cannot be drawn from rates that are close to the diffusion limit. Thus,... 

The reactions, with rate coefficients well below the diffusion-limited values, are thought to occur by direct proton transfer from the donor acid into the molecular cavity. The kinetic isotope effect for proton transfer was observed to vary as a function of the pX-value of HA and to pass through a maximum value kHA/kDA 4.0, the maximum occurring for a reaction with ApA" = pA (HA) — pA ([2.1.1]H22+) = ca + 1. A similar large kinetic isotope effect kHA/kDA = 3.9 was observed for protonation of the cryptand by H20 and D20 in the isotopically different solvents (Kjaer et al., 1979). 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

---