Isotopic labels scrambling

Seburg and McMahon have performed matrix isolation experiments on both 3-methyldiazirine and diazoethane in an attempt to observe ethyfidene. They were unsuccessful in detecting the desired carbene but did observe isomerization of diazoethylene to 3-methyldiazirine. When matrix-isolated diazoethylene was photolyzed at 8K, ethene and 3-methyldiazirine were observed as the major products in a ratio of 10 1. Additionally, unlike Moore and Pimentel, Seburg and McMahon did not observe isotopic label scrambling when the photolyses were conducted in an Nj matrix. 

Problem 30.8 Propose a mechanism to account for the fact that heating 1-deuterioindene scrambles the isotope label to all three positions on the five-membered ring. 

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... 

Thus, upon photolyzing a mixture of two nitrites, one of which contains an isotopically labeled nitrogen, the products should show the original isotopic distribution if mechanism (a) is operative, the original isotopic distribution in unreacted nitrite but scrambling in the nitroso product if (c) is operative, and complete isotopic scrambling if (b) is operative. 

Compounds labeled with multiple isotopes of the same atom provide the opportunity to observe virtual isomerization reactions that go undetected in conventional studies.There have been relatively few modern studies of these virtual isomerization reactions," " because the reactions themselves are not common and because of difficulties in drawing general conclusions from the observation of the scrambling of isotopic label during solvolysis (Scheme 8). 

Scheme 16.5. The FVP of isotopically labeled phthalic anhydride (7) indicates a scrambling of carbon atoms. Yet, it is not clear whether a reversible ring contraction of 4 is responsible for this exchange or if it takes place already at the stage of ketenecarbene (12). According to recent calculations, the barrier for the rearrangement 4 9 is only 32 kcal/...

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

Isotopic labeling is a powerful tool being used to understand the nature of transition states as well as the mechanisms of surface-catalyzed reactions. By in situ MAS NMR spectroscopy, the scrambling of labels at room temperature was investigated upon adsorption of p C-1]- 1-octene on calcined zeolite HZSM-5 (wsi/... 

The nitrone (54a) was the sole product when nitrite ester (53a) was photolysed in toluene. When (53a) was labelled with 1SN in the nitrite ester group and photolysed in the presence of unlabelled (53b) the isotopic label was scrambled equally between the product nitrones (54a) and (54b) recovered nitrite ester (53b) remained unlabelled.57 Nitrone (54a) was only a minor product when (53a) was... 

Internal Ion Pair Return—collapse of a contact ion pair to covalent material, sometimes referred to as hidden return if the covalent material is formed without scrambling of isotopic label or without partial racemization ... 

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