Isotope exchange experiments

The stoichiometry of the compound was investigated by isotope exchange experiments along the lines described in the preceding section. The pure... 

The benzilic acid rearrangement, though catalyzed by hydroxide ion, probably belongs in the same category as carbonium ion rearrangements. Kinetic and isotopic exchange experiments show that there is a rapid reversible addition of hydroxide ion.841 242... 

Fig. 7. Temperature dependence of transverse relaxation rates of bound H2O in [A1(H20)6] with various concentrations of A1(C104)3, Mn(C104)2, and HCIO4 and exchange rates obtained from isotope exchange experiments ( ).

An isotope effect technique applied in enzyme-catalyzed isotope exchange experiments measuring reaction rate as a function of the [D20]/[H20] ratio. Such studies often provide information concerning the effect of the environment of the transition state on exchangeable protons. 

The question raised by Anderson (1970,1971) and Anderson et al (1973) as to whether anion point defects are eliminated completely by the creation of extended CS plane defects, is a very important one. This is because anion point defects can be hardly eliminated totally because apart from statistical thermodynamics considerations they must be involved in diffusion process. Oxygen isotope exchange experiments indeed suggest that oxygen diffuses readily by vacancy mechanism. In many oxides it is difficult to compare small anion deficiency with the extent of extended defects and in doped complex oxides there is a very real discrepancy between the area of CS plane present which defines the number of oxygen sites eliminated and the oxygen deficit in the sample (Anderson 1970, Anderson et al 1973). We attempt to address these issues and elucidate the role of anion point defects in oxides in oxidation catalysis (chapter 3). 

The formation of acyclic enediols is, apparently, the initial reaction that leads to dehydration products. Sugar enediols are transitory compounds that have never been isolated. However, because, when treated with either acid or base, an aldose gives rise to its 2-epimer, as well as to its 2-keto isomer, a persuasive argument is provided for the 1,2-enediol as the intermediate common to each of the products. The evidence in favor of these intermediates is based primarily on isotope-exchange experiments, on reactions that involve isomeriza-tions of O-methyl sugars, and on kinetic measurements.10... 

Fig. 1. Amounts of 238U released from solid samples vs. the percentages of 233U remaining in solution in U isotope exchange experiments performed by Del Nero et al. (1999a) using weathered pelites (6.2 and 6.8) and an Fe-crust (10.3) from Bangombe.

Hydrogen-Tritium Exchange Studies. Isotope exchange experiments were conducted as described previously for the 7-phase hydrides (except TiMo2, that does not form the 7-phase). The results of the isotope separation determinations are shown in Table VI, where the parameter a is defined as ... 

Isotope exchange experiments with purified F reveal a remarkable fact about the enzyme s catalytic mechanism on the enzyme surface, the reaction ADP + P, ATP + H20 is readily reversible—the free-energy change for ATP synthesis is close to zero When ATP is hydrolyzed by Fi in the presence of 180-labeled water, the Pj released contains an 180 atom. Careful measurement of the 180 content of P, formed in vitro by Fx-catalyzed hydrolysis of ATP reveals that the P, has not one, but three or four 180 atoms (Fig. 19-21). This indicates that the terminal pyrophosphate bond in ATP is cleaved and re-formed repeatedly before P, leaves the enzyme surface. With P, free to tumble in its binding site, each hydrolysis inserts 180 randomly at one of the... 

Positional isotope exchange experiments conducted on creatine kinase,45 hex-okinase,46 and pyruvate kinase47 have provided no evidence in favor of a dissociative mechanism. In each case, the data are consistent with a straightforward nucleophilic attack on phosphorus to generate a trigonal bipyramidal intermediate.47... 

Capillary-in-capillary mixers were used for electrospray ionization mass spectrometry (ESI-MS), which allows one to perform on-line kinetic studies for a wide range of applications in chemistry, bioorganic chemistry, isotope exchange experiments and enzymology, just to name a few [133], ESI-MS is a method alternative to the traditionally employed quench-flow techniques with off-line analysis. 

As was explained in Sect. 1.5.2, any given isotopic exchange experiment will be kinetically of first order. For systems where the rates of adsorption and desorption are rapid, it follows from the two-phase model of catalysis and eqn. (23) that the observed first-order rate constant is given by... 

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