Kinetic isotope effects dependence

Intermolecular kinetic isotope effects depend strongly upon reactant properties. Consider the extreme case of a primary deuterium isotope effect in a molecule with only one hydrogen replaced by deuterium, e.g. the rate coefficient for (CDCl3)f - (CC13)+ + D compared with that for... 

Because kinetic isotope effects depend on the hydrocarbon reaction mechanism, one would expect different isotopic fractionations involving water-organic interaction. Some preliminary results (Xiao 1998) on the carbon isotopic fractionations associated with hydrolysis reaction indicate that they are much smaller than those associated with HC thermal cracking. This suggests that a different isotopic model is needed for oil and gas generation in many environments with high water concentration. 

For intramolecular isotope effects, as one starts from a unique precursor, the two processes correspond to the same internal energy distribution, so that useful information can be inferred even from ionabun-dances measured without internal energy selection (like metastable dissociations or field ionization kinetics). This is not the case for intermolecular effects, where there is no warranty that the two internal energy distributions are the same. Within the RRKM framework, the intramolecular kinetic isotope effect depends only on the transition state properties (critical energy, rotational constants and vibrational frequencies) and not on the reactant properties. 

The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. 

In confirmation, when acetone-dg was studied, a very substantial kinetic isotope effect was observed, being about 7 to 12 depending upon the base catalyst. 

Despite its apparent simplicity, the PK pyrrole synthesis has retained its mystique since being discovered. Several investigations into the PK mechanism have been reported, including a gas phase study. Current evidence (intermediate isolation, kinetics, isotope effects) suggests the following (abbreviated) mechanism for the formation of pyrrole 13. However, the specific PK mechanism is often dependent on pH, solvent, and amine and dicarbonyl structure, especially with regard to the ringclosing step. 

Kinetic isotope effects also show a dependence upon the reactivity of the electrophile. Thus some reactions, e.g. positive chlorination, show no isotope effect whereas others, e.g. sulphonation, do show an isotope effect. There are two ways of visualising the reasons for this and they are closely related. Very... 

It is apparent from equation (16) that if k x becomes much larger than k 2, the rate will depend upon k 2 and so a kinetic isotope effect will be observed. Now kL j will become large if there is steric hindrance to formation of the intermediate, and a number of examples are now known where an electrophile which normally gives no isotope effect, does so if formation of the intermediate is hindered. 

A recent paper by Leffek and Matheson (1971) nicely complements this work, as it describes the results of a careful investigation of the temperature dependence of the kinetic isotope effect in the reaction studied by Kaplan and Thornton (1967). It is found that AAH = 134 + 30 cal mol and dd/S = 0-15 + 0-09 cal mol deg , demonstrating that the isotope effect is primarily due to an enthalpy difference, and providing support for the steric interpretation suggested by Kaplan and Thornton (1967). 

The electronic, rotational and translational properties of the H, D and T atoms are identical. However, by virtue of the larger mass of T compared with D and H, the vibrational energy of C-H> C-D > C-T. In the transition state, one vibrational degree of freedom is lost, which leads to differences between isotopes in activation energy. This leads in turn to an isotope-dependent difference in rate - the lower the mass of the isotope, the lower the activation energy and thus the faster the rate. The kinetic isotope effects therefore have different values depending on the isotopes being compared - (rate of H-transfer) (rate of D-transfer) = 7 1 (rate of H-transfer) (rate of T-transfer) 15 1 at 25 °C. 

Kinetic data exist for all these oxidants and some are given in Table 12. The important features are (i) Ce(IV) perchlorate forms 1 1 complexes with ketones with spectroscopically determined formation constants in good agreement with kinetic values (ii) only Co(III) fails to give an appreciable primary kinetic isotope effect (Ir(IV) has yet to be examined in this respect) (/ ) the acidity dependence for Co(III) oxidation is characteristic of the oxidant and iv) in some cases [Co(III) Ce(IV) perchlorate , Mn(III) sulphate ] the rate of disappearance of ketone considerably exceeds the corresponding rate of enolisation however, with Mn(ril) pyrophosphate and Ir(IV) the rates of the two processes are identical and with Ce(IV) sulphate and V(V) the rate of enolisation of ketone exceeds its rate of oxidation. (The opposite has been stated for Ce(IV) sulphate , but this was based on an erroneous value for k(enolisation) for cyclohexanone The oxidation of acetophenone by Mn(III) acetate in acetic acid is a crucial step in the Mn(II)-catalysed autoxidation of this substrate. The rate of autoxidation equals that of enolisation, determined by isotopic exchange , under these conditions, and evidently Mn(III) attacks the enolic form. 

Relatively detailed study has been done for the reaction pathways over Au/Ti02 catalysts mainly because of simplicity in catalytic material components. The rate of PO formation at temperatures around 323 K does not depend on the partial pressure of C3H6 up to 20vol% and then decreases with an increase, while it increases monotonously with the partial pressure of O2 and H2 [57]. A kinetic isotope effect of H2 and D2 was also observed [63]. These rate dependencies indicate that active oxygen species are formed by the reaction of O2 and H2 and that this reaction is rate-determining [57,63,64]. 

Based on C-H versus C-D zero point vibrational differences, the authors estimated maximum classical kinetic isotope effects of 17, 53, and 260 for h/ d at -30, -100, and -150°C, respectively. In contrast, ratios of 80,1400, and 13,000 were measured experimentally at those temperatures. Based on the temperature dependence of the atom transfers, the difference in activation energies for H- versus D-abstraction was found to be significantly greater than the theoretical difference of 1.3kcal/mol. These results clearly reflected the smaller tunneling probability of the heavier deuterium atom. 

Interestingly, in a comparison of the CD3 and CHj carbenes, an unusual temperature dependence of the kinetic isotope effect was observed. In contrast to typical reactions, the ratio of rates of H versus D shift, k /ko, actually increased as temperature was raised. In fact, k was measured to be larger than k at 248 K. It was suggested that these results required a normal temperature dependence of the isotope effect for the classical component of the reaction, but an unusual diminished isotope effect for the QMT reaction. 

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. 

German ED, Kuznetsov AM (1981) Dependence of the hydrogen kinetic isotope effect on the reaction free energy. J Chem Soc, Faraday Trans 1 77 397—412... 

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... 

San Filippo and coworkers163 have determined the temperature dependence of the primary hydrogen-deuterium kinetic isotope effects for the hydrogen transfer reactions between several organic radicals and tributyltin hydride (deuteride) see equation 116. 

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