Isotopic-Kinetic Experiments

Mazda has developed and commercialized a catalyst with Pt supported on a mixed oxide of Ce02 with Zr or Pr for light-duty diesel engine cars. Isotopic kinetic experiments using 02 revealed that formation of the mixed oxide increases the mobility of lattice oxygen and thereby soot oxidation. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

A reaction that has been much investigated is the hydrolysis of esters, e.g. (164), by aqueous base, i.e. eOH. It is found to be kinetically second order, and lsO isotopic labelling experiments on (164) have... 

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... 

Reduction of aldonolactones and their derivatives with isotopically modified reducing agents leads to sugars labeled at the anomeric center. Glycosides substituted with deuterium or labeled with tritium are widely employed for kinetic isotope-effect measurements, mechanistic studies, isotope-tracing experiments, and so on. 

The rearrangements of b-(acyloxy), (i-(phosphatoxy)alkyl, and related systems have been reviewed [51,52] and representative kinetic data are given in Table 2 above. As revealed by isotopic labeling experiments, the acyloxy... 

In a variation of this method, Tencer and Stein (1978), mixed the isotopic quasi-racemate to near, but not exactly, zero rotation so that at a certain time, tz, the observed optical rotation of the reaction mixture was zero. The equations for this type of kinetic experiment enable one to calculate the difference between the individual isotopic rate constants from tz and the ratio of rate constants (the KIE) from te and tz provided that the ratio of the initial rotations for the two isotopic substrates is known. Usually it is preferable to... 

A hydrogen isotope with a nucleus consisting of one proton and two neutrons (the nucleus is referred to as the triton). Tritium, a radioisotope symbolized by iH or T, decays by negative beta emission (0.01860 MeV) with a half-life of 12.32 years. The atomic weight of tritium is 3.01605 amu. It is frequently used in metabolic and kinetic experiments. The following decay data indicate the time followed by the fraction of original amount at the specified time 0, 1.000 1 month, 0.995 2, 0.991 3,... 

The activation of (P-P)Pd" promoters in MeOH proceeds via formation of Pd"-OMe (Eq. (1)) that can straightforwardly initiate the catalysis cycle or generate Pd"-H via P-H elimination, yielding formaldehyde (Eq. (2)) [16]. The fast kinetics under real copolymerisation conditions do not allow for the spectroscopic detection of Pd-H initiators. However, their formation has been unambiguously assessed by end-group analysis, isotopic labelling experiments and model reactions [Ij. 

Kinetic experiments with the acids HX and DX, performed under pseudo-first-order conditions, have revealed inverse kinetic isotope effects, shown in... 

This situation is illustrated in Eq. 7 where B represents the conjugate base of the solvent, BH, used in the kinetic experiment. After the base, B, removes the deu-teron from the acid donor, A, (Eq. 7a) it is still complexed to it. At this point, the deuterated base, DB, may diffuse away and be replaced by a proton-bearing analogue, HB (Eq. 7b), or it can return the deuteron to the conjugate base of the acid. Under the typical conditions, the step in Eq. 7b is irreversible (the concentration of HB is always much greater than DB) and the rate of isotope exchange can be expressed in terms of the following elementary rate constants. 

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... 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

The mechanism of the catalysis of the CO reduction of NO via (113) using this rhodium system has been investigated by kinetic studies (187) and by an isotope labeling experiment (234). Scheme I presents the mechanism as currently viewed. From initial rate studies it was established that the reaction is first order in total rhodium concentration, indicating that N—N bond formation occurs intramolecularly. The rate also appears to be first order in the partial pressure of CO and zeroth order in the partial pressure of NO over the range 200-400 Torr. We conclude from these results that the rate determining step is attack of CO on the catalytically active species (42) (187). 

In this model the unimolecular constants are relative to the turnover number and the bimolecular constants are chosen to yield equilibrium constants in units of millimolar. The model is primarily based on dead-end inhibition by CrATP, the Michaelis constant for ATP in the ATPase reaction, the isotope partitioning experiments of Rose et al. (65), and various binding and kinetic constants found in the literature. The final model was based on a computer simulation study attempting to discover what combination of rate constants would lit the isotope partition data and the observed kinetic and binding constants. 

Unlike the Rh-based hydrogenation of a-(acylamino)acrylates, the corresponding Ru chemistry has not been studied extensively. Ru complexes of (S)-BINAP and (S,S)-CHIRAPHOS catalyze the hydrogenation of (Z)-a-(acylamino)cinnamates to give the protected ( -phenylalanine with 92% ee [74] and 97% ee [75], respectively. It is interesting that the Rh and Ru complexes with the same chiral diphosphines exhibit an opposite sense of asymmetric induction (Scheme 1.6) [13,15,56,74,75]. This condition is due primarily to the difference in the mechanisms the Rh-catalyzed hydrogenation proceeds via Rh dihydride species [76], whereas the Ru-catalyzed reaction takes place via Ru monohydride intermediate [77]. The Rh-catalyzed reaction has been studied in more detail by kinetic measurement [78], isotope tracer experiments [79], NMR studies [80], and MO calculations [81]. The stereochemical outcome is understandable by considering the thermodynamic stability and reactivity of the catalyst-enamide complexes. 

The alternative mechanisms shown to the right and to the left of 1 in Scheme 1.2 both account for the kinetics results and the initial product analysis, and both have literature analogies. However, isotope labelling experiments (the asterisk indicates the site of 170 or lsO incorporation) allowed a distinction between the two. In the path to the left, protonation of the hydroxyl of 1 with loss of labelled water as nucleofuge would lead to the evolution of unlabelled N2O, and the residual adamantyl cation would be intercepted either by the liberated labelled water molecule or by an unlabelled solvent water molecule. In this event,... 

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