Equilibration isotope effect

Isotope effects, magnetic, magnetic field effects and, on the products of organic reactions, 20, 1 Isotope effects, on nmr spectra of equilibrating systems, 23,63 Isotope effects, steric, experiments on the nature of, 10,1... 

The isotope effects have been interpreted in terms of a mechanism involving two equilibrating zwitterionic intermediates (equation 110). In this instance, the k step is partially reversible and both the intermolecular and intramolecular isotope effects are a composite of the isotope effects in several steps (Schemes 26 and 27). 

If the reaction rates of a specific carbene with various quenchers are studied in the same solvent, and with small concentrations of Q, K will be constant. Relative reactivities for the singlet state of a spin-equilibrated carbene can thus be derived. However, few researchers have varied the acidity of ROH, estimated kinetic isotope effects, and compared alcohols with ethers (Table 4). The data indicate proton transfer to diarylcarbenes (139d, 139k, 205, 206)112-117 and diadamantylcarbene (207).118... 

Prakash et al. (1985) used the deuterium isotope effect on the l3C NMR spectrum of [47] to provide further evidence for the symmetrical, homoaromatic nature of this ion. They prepared the specifically deuterated trishomocyclopropenyl cation [57] by superacid treatment of the corresponding alcohol [58]. The 13C NMR spectrum of [57] displayed a triplet for the deuterated methine only 0.2 ppm to higher field than the undeuterated methine, indicating only an isotopic perturbation of resonance and not a rapidly equilibrating classical ion system (see Siehl, 1987). 

The understanding of isotope effects on chemical equilibria, condensed phase equilibria, isotope separation, rates of reaction, and geochemical and meteorological phenomena, share a common foundation, which is the statistical thermodynamic treatment of isotopic differences on the properties of equilibrating species. For that reason the theory of isotope effects on equilibrium constants will be explored in considerable detail in this chapter. The results will carry over to later chapters which treat kinetic isotope effects, condensed phase phenomena, isotope separation, geochemical and biological fractionation, etc. 

A ratio defining the isotopic distribution of two isotopes equilibrated between two different chemical species. If X, followed by a subscript, represents the mole fraction of an isotope (denoted by that same subscript), then the fractionation factor, often symbolized by , with respect to chemical species A and B is (Xi/X2)a/(-X i/-X 2)b- Fractionation factors can also refer to different sites, A and B, within the same chemical species. As an example, the deuterium solvent fractionation factor, used in studying solvent isotope effects, is = (AD/A"H)soiute/(- o/... 

Repeated deprotonation of 278 removed due to a high H/D kinetic isotope effect the 1-proton, forming the dideuterio compound 279 with low diastereoselectivity . It is quite likely that a dynamic thermodynamic resolution is the origin. Intermediate 277 is configurationally labile, enabling an equilibration of the diastereomeric ion pairs 277 and epi-211. Similar studies were undertaken with 1-phenyl-l-pyrid-2-ylethane (280) and l-(4-chlorophenyl)-l-(pyrid-2-yl)-3-(dimethylamino)propane (281) (50% eef. An improvement of the achieved enantiomeric excesses resulted when external chiral proton sources, such as 282 or 283, were applied (84% ee for 280 with 283 and 75% ee for 281). 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

The overall mechanistic picture of these reactions is poorly understood, and it is conceivable that more than one pathway may be involved. It is generally considered that cycloheptatrienes are generated from an initially formed norcaradiene, as shown in Scheme 30. Equilibration between the cycloheptatriene and norcaradiene is quite facile and under acidic conditions the cycloheptatriene may readily rearrange to give a substitution product, presumably via a norcaradiene intermediate (Schemes 32 and 34). When alkylated products are directly formed from the intermolecular reaction of carbenoids with benzenes (Scheme 33 and equation 36) a norcaradiene considered as an intermediate alternatively, a mechanism may be related to an electrophilic substitution may be involved leading to a zwitterionic intermediate. A similar intermediate has been proposed143 in the intramolecular reactions of carbenoids with benzenes, which result in substitution products (equations 37-40). It has been reported,144 however, that a considerable kinetic deuterium isotope effect was observed in some of these systems. Unless the electrophilic attack is reversible, this would indicate that a C—H insertion mechanism is involved in the rate-determining step. 

Thermolysis of l,l-difluoro-2,3-diphenylcyclopropane in supercritical CO2 has allowed the rate of geometrical isomerization [i.e. cis-( 109) to /ra/M-(109)] and racemization [i.e. (/< )-( 109) to (S)-( 109)] to be determined from O2 dependence of the trapping rate of the postulated intermediate 1,3-biradical.246 Above 150 °C, the formation of 2,2-difluoroindane and its decomposition products is reported. A similar thermally induced equilibrating series of stereomutations has been observed with the analogous non-fluorinated cyclopropane in which rate constants and deuterium exchange isotope effects are reported.247 Theoretical studies of this isomerization have focused on classical248 and quasi-classical trajectories.249... 

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