Isotope effect hydrogen-deuterium

Limbach H H 1991 Dynamic NMR spectroscopy in the presence of kinetic hydrogen/deuterium isotope effects NMR Basic Principles and Progress vol 23, ed P Diehl, E Fluck, H Gunther, R Kosfeld and J Seelig (Berlin ... 

Both of these reported kinetic hydrogen-deuterium isotope effects are disturbingly small, yet they are probably too large to be considered secondary isotope effects. These results lend support to the intermediate complex hypothesis, but they can be accommodated equally well by all three of the mechanisms that have been considered. These results, therefore, afford no basis for discrimination among the possible mechanisms. 

In view of the large hydrogen-deuterium isotope effect of 5.26, Baneijee and coworkers proposed that the proton transfer mechanism (Scheme 41) is also operating. In this mechanism, pyridine behaves as a base and abstracts a proton in the rate-determining step. 

Kopple K, Meyerstein D, Meisel D (1980) Mechanism of the catalytic hydrogen production by gold sols. Hydrogen/deuterium isotope effect studies. J Phys Chem 84 870-875... 

Nacson, S. Harrison, A.G. Dependence of Secondary Hydrogen/Deuterium Isotope Effects on Internal Energy. Org. Mass Spectrom. 1985,20,429-430. 

Next reviews were dedicated to problems of hydrogen-bonded systems. Hydrogen/deuterium isotope effects on NMR parameters in liquids and solids have been reviewed by Limbach et al.11 Review covers period to 2004 and illustrates the correlation of intermolecular hydrogen-bonded systems geometry and H/D isotope shifts and coupling constants, particularly measured in the solid state and in liquids at low temperature. Several reviews concern the isotope effects on intramolecular hydrogen-bonded systems.12-17 Since that time several new papers dedicated to hydrogen-bonded systems were published, mostly on intramolecular systems.18-24... 

Distribution Functions and Hydrogen-Deuterium Isotope Effects in Nonthermal Activation Systems. In Sec. II-D, hydrogen-deuterium isotopic rate ratios for monoenergetic systems were discussed. In practice, the measured effects are ratios modified by the energy distribution functions and should be compared to kan/kaD rather than to k,n/ktn. A s appropriate for the system under investigation, one of eqs. (19)-(22) is written for each of the isotopic species and a ratio, kttn/kaD, is thus constructed for comparison of isotope effects. These need not be listed in detail. It should be noted that the distribution function for the normal and isotopically substituted systems will usually be somewhat different (Fig. SB). 

Distribution Functions and Hydrogen-Deuterium Isotope Effects in Thermal Activation Systems. The isotope effects in thermal activation systems are determined to a large degree by the energy distribution function and kt and /(e) must be simultaneously considered. The equilibrium high-pressure effect is considerably different from the non-equilibrium low-pressure case, and they are discussed separately. 

Song, Z. Beak, P. Investigation of the mechanisms of ene reactions of carbonyl enophiles by intermolecular and intramolecular hydrogen-deuterium isotope effects Partitioning of reaction intermediates, J. Am. Chem. Soc. 1990,112, 8126-8134. 

The growing evidence (for example. References 36, 117, 118) that the hydrogenation of CO on various metals proceeds via the carbon formed by dissociative adsorption of CO led to the use of the hydrogen/deuterium isotope effect as one attempt to better define that the synthesis follows this reaction pathway. Another reason was to modify the mass of the hydrocarbon peaks so that interference from gases such as H2O, CO2, N2, etc., would not prevent analysis of products from transient isotope studies by mass spectrometry. However, the evidence from the early studies with H2/D2 led to conflicting viewpoints. Sakharoff and Dokukina obtained kjj/ku = 0.77 for a Co catalyst, whereas McKee reported a value of 2.2. 

The amount of azine in the product increases when the initial concentration of diazodiphenylmethane is increased. This occurs because the added diazodiphenylmethane increases the probability of the coupling reaction. Finally, it is probable that the diazodiphenylmethane rather than the diazodiphenylmethane radical anion is the reactant in the formation of the azine because the hydrogen-deuterium isotope effect indicates the primary reaction of the radical anion is with a hydrogen from the solvent. 

The equal amounts of ethylene and ethylene-<72 formed from the reactions of compounds 29 and 30 show that the formation of the intermediate in the C—H activation step is not rate-determining. If this step were rate-determining, much less ethylene-<72 would have been formed from compound 30 because there would be a primary hydrogen-deuterium isotope effect associated with the insertion reaction. 

Primary kinetic hydrogen-deuterium isotope effects are usually calculated from the ratio of the rates of elimination of isotopically labelled substrates. An uncertainty in rate coefficients of 2% arises from a titrimetric determina-tion23.26,4o whereas a spectrophotometric approach, usually the monitoring of the olefin concentration under conditions of first-order kinetics, gives an improved precision During attainment of the transition state, a... 

More often, one sees simplifications of the equations designed to apply to particular models. The maximum hydrogen/deuterium isotope effect for a proton transfer,... 

The first relates to the alkoxybenzoic acids. These materials have been studied where the acid hydrogen has been replaced by deuterium. Hydrogen/deuterium isotope effects are generally large and so it could confidently be expected... 

Limbach, H.-H. Dynamic NMR Spectroscopy in the Presence of Kinetic Hydrogen/ Deuterium Isotop Effects. Vol. 23, pp. 63-164... 

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