Isotopes kinetic effects, 276 applications

OXYGEN KINETIC ISOTOPE EFFECTS APPLICATIONS TO MECHANISTIC STUDIES 437... 

The use of solvent isotope effects in studies of reaction mechanism and the theoretical interpretation of the kinetic effect of replacing H2 O by D20 have been thoroughly described [122, 123, 204, 211], Results for reactions involving proton transfer to and from carbon [122, 123, 204] have played a major role in the development of the fractionation factor theory for explaining solvent isotope effects, but other reactions [211(b), 211(c)], for example, nucleophilic substitution at saturated carbon, have also been well studied. In this section it will be shown how detailed information about a proton transfer transition state can be obtained by studying the solvent isotope effect for a reaction with known mechanism. Reactions with the A—SE2 mechanism will be discussed since this probably represents the most widely studied example of the application of solvent isotope effects in proton transfer to and from carbon [42, 47, 122,123, 204, 211(a), 212],... 

Kinetic effects have been of great interest in cosmochemistry since the mid-1990s and we now review the generally accepted theory of evaporation and condensation in some detail. This discussion focuses on isotope fractionations, but is equally applicable to elemental fractionations. 

The Westheimer effect and its implications form the main subject of this chapter, and no attempt is made to consider for example the more traditional application of primary isotope effects to the study of reaction mechanisms. However, a further point that is emphasized is that interpretations of isotope effects may be appreciated without resort to calculations, and before discussing kinetic effects some time is spent in considering, from a qualitative standpoint, the origins of hydrogen isotope effects and the isotopic properties of stable molecules and equilibria. In this preliminary review previous accounts of hydrogen isotope effects are extensively used [2-9] and among these... 

Vanicek, J. and Miller, W.H. (2007) Efficient estimators for quantum instanton evaluation of the kinetic isotope effects application to the intramolecular hydrogen transfer in pentadiene. /. Chem. Phys., 127, 114309. 

A special type of substituent effect which has proved veiy valuable in the study of reaction mechanisms is the replacement of an atom by one of its isotopes. Isotopic substitution most often involves replacing protium by deuterium (or tritium) but is applicable to nuclei other than hydrogen. The quantitative differences are largest, however, for hydrogen, because its isotopes have the largest relative mass differences. Isotopic substitution usually has no effect on the qualitative chemical reactivity of the substrate, but often has an easily measured effect on the rate at which reaction occurs. Let us consider how this modification of the rate arises. Initially, the discussion will concern primary kinetic isotope effects, those in which a bond to the isotopically substituted atom is broken in the rate-determining step. We will use C—H bonds as the specific topic of discussion, but the same concepts apply for other elements. 

Most of the chemical reactions presented in this book have been studied in homogeneous solutions. This chapter presents a conceptual and theoretical framework for these processes. Some of the matters involve principles, such as diffusion-controlled rates and applications of TST to questions of solvent effects on reactivity. Others have practical components as well, especially those dealing with salt effects and kinetic isotope effects. 

Abstract The theory of molecular vibrations of molecular systems, particularly in the harmonic approximation, is outlined. Application to the calculation of isotope effects on equilibrium and kinetics is discussed. 

The chapter starts with a brief review of thermodynamic principles as they apply to the concept of the chemical equilibrium. That section is followed by a short review of the use of statistical thermodynamics for the numerical calculation of thermodynamic equilibrium constants in terms of the chemical potential (often designated as (i). Lastly, this statistical mechanical development is applied to the calculation of isotope effects on equilibrium constants, and then extended to treat kinetic isotope effects using the transition state model. These applications will concentrate on equilibrium constants in the ideal gas phase with the molecules considered in the rigid rotor, harmonic oscillator approximation. 

The use of reduced isotopic partition function ratios to study kinetic isotope effects was first undertaken by Bigeleisen this work was corrected and elaborated by Bigeleisen and Wolfsberg. References are cited at the end of this chapter. Application of the equations developed above to specific chemical reactions will be found in Chapter 10, where other theoretical approaches will also be presented. 

Abstract The theoretical framework needed for interpretation of kinetic isotope effects on unimolecular reactions is reviewed. Application to the satisfactory rationalization of the theoretically puzzling mass independent isotope effect observed for oxygen isotope fractionation in extraterrestrial samples is described. 

Equations (8) and (10) are applicable to stable isotope systems where isotopic fractionation occurs through mass-dependent processes which comprise the majority of cases described in this volume. These relations may also be used to identify mass-independent fractionation processes, as discussed in Chapter 2 (Birck 2004). Mass-dependent fractionation laws other than those given above distinguish equilibrium from kinetic fractionation effects, and these are discussed in detail in Chapters 3 and 6 (Schauble 2004 Yormg and Galy 2004). Note that distinction between different mass-dependent fractionation laws will generally require very... 

As logical as this diagnostic method is, one needs to realize its lack of absolute applicability. The observed magnitude of the kinetic isotopic effect is not great, and the aforementioned statement of independence of the electron affinity from the increase in molecular mass of the substrate is not obvious. This postulate should be proved in each case. Benzophenone, taken as an isotopic mixture of C=0 and C=0 gives a mixture of anion-radicals with a decreased proportion of C=0 isotomer when reduced with potassium in HMPA (Stevenson et al. 1987b). In effect, this means that for the heavier isotopomer of benzophenone, the electron affinity is smaller. 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

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