Isotopic scrambling experiments

In oxygen isotope scrambling experiments, a mixture of 1602/160180/1802 ( 1/2/1 ratio) is reacted with iron porphyrin. Such a mixture can be obtained by electrical discharge of an equimolar mixture of 1602 and 1802. 

One of the greatest open questions in the field of fullerenes is the understanding of how such compact, closed structures form so efficiently from carbon vapor. Under appropriate conditions, as much as 20% of the carbon vaporized in an arc or a laser desorption source is incorporated into fullerenes however, no laboratory chemical synthesis has succeeded It is known from an isotope-scrambling experiment that the fullerenes are formed by the condensation of carbon atoms in the hot carbon vapor, as opposed, for example, to small sheets of graphite torn from the electrodes, and subsequently curling up.[Me90b]... 

Hydrogen dissociation by Ti-catalyzed NaAlH4 was proven by static hydrogen isotope scrambling experiments [170]. 

In fact, molecular dynamics calculations have suggested that the endocyclic pathway may be operative in enzyme-catalyzed processes. In order to delineate the extent to which this alternative pathway occurs in solution, an isotopic scrambling experiment was performed. 

Interestingly, such interaction can lead to complete dissociation of the carbon monoxide molecule, as shown by isotope scrambling experiments on Rh(lll)... 

Fig. 30. Carbon monoxide dissociation on a Rh(l 11) surface as a function of potassium coverage as determined by thermal desorption isotope scrambling experiments with C 0 and C ...

The rate of isotopic exchange has been found to correlate with the catalytic activity for a series of catalysts [531] or even to be the same as the rate of ammonia synthesis [521]. Model calculations show that the coverage by N is high during NH3 synthesis [396]. This is not necessarily the case during a N2 isotopic scrambling experiment. This may explain why the rate of NH3 synthesis does not always correlate perfectly with the rate of N2 isotopic... 

After approximately 3 min on stream in the isotopic mixture of CH4/02/C 02/He, a steady-state value in the rate of formation is obtained (Fig. 3). This value is used to estimate the relative contribution of the CO2-reforming route to the overall production of CO (direct + indirect routes). Proper analysis of this result, taking into consideration scrambling of isotopic oxygen Figure 3. Transient isotopic labelling experiment atoms between the CO2 and O2 molecules... 

In the standard experiments on the MIF there is extensive isotopic exchange ("isotopic scrambling"), such as Q -I- OO QOO QO -i- O. However, there is also a body of experimental data on ozone formation obtained under very special experimental conditions [18] in which isotopic exchange is minor. In these experiments ratios of recombination rate constants, such as the -F... 

Although the TPD spectra indicate that CO2 is the only desorbing species, the fact that the isotopic mixing experiments show some scrambling is not consistent with the adsorption of isolated CO2 molecules. Therefore a process represented by ... 

Based on the data obtained (Figure 8) from the control experiments A and B, which only showed the unlabeled (m/z 118) or the fully labeled 2-oxopropanol (m/z 121), the two isotopomers obtained from mixture C clearly indicated that no isotope scrambling had occurred (the fragments at m/z 119 and m/z 120 are caused by M+1 or M-I fragments of m/z 118 or m/z 121, respectively). Thus, the data confirmed that the 2-oxopropanol present in the reaction system is exclusively formed as a carbon-3 modul from either glucose or the carbon-13 labeled glucose, respectively. 

In the following, the effects of these exchange processes and isotope scrambling on the level populations and line shapes of the PH IP experiment are shown. Details of the numerical calculations are found in the original paper [62]. 

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