Labelled atoms

The Morgan Algorithm classifies all the congeneric atoms of a compound and selects invariant-labeled atoms (see Section 2.5.3.1). The classification uses the concept of considering the number of neighbors of an atom (connectivity), and does so in an iterative manner (extended connectivity, EC). On the basis of certain rules. 

It is recommended that the reader become familiar with the point-group symmetry tools developed in Appendix E before proceeding with this section. In particular, it is important to know how to label atomic orbitals as well as the various hybrids that can be formed from them according to the irreducible representations of the molecule s point group and how to construct symmetry adapted combinations of atomic, hybrid, and molecular orbitals using projection operator methods. If additional material on group theory is needed. Cotton s book on this subject is very good and provides many excellent chemical applications. 

As a general rule, in both equilibrium and kinetic studies, the process that forms the stronger bond is favored more for the heavier isotope. If the labeled atom is more firmly attached in the transition state, as it was for Eq. (9-100), then one can even find an inverse kie. One rule to remember Substitution with a heavier atom favors the stronger bond in any equilibrium, including the quasi-equilibrium of TST. 

Scheme 28.—The seven specifically labeled AIRs prepared. The sites of labeling are indicated with the labeled atoms in bold type.

Scheme 29.—Carbon atom C-2 of AIRs is the precursor of the methyl of pyramine. The labeled atoms are printed in bold type.

The UHF solution appears when the hopping integral t becomes small and leads to a spin density wave. The localization of the MOs leads to a and b atom centered orbitals, localized around odd and even labelled atoms respectively. 

Scheme 4.9 Synthesis plan for triclosan showing labelled atoms Involved in target bond forming steps.

Figure 4.17 Summary of oxidation number changes by reaction stage for labelled atoms in Scheme 4.9 for triclosan synthesis.

The isotope dilution method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultratrace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decay during the period of measurement, all of the labelled atoms are measured. 

The photodegradation of para-aramid in an 0 atmosphere allows the differentiation between the accelerated experimental photooxidative conditions from its usual daylight exposure effects. This study illustrated an estimation of the rates of photooxidation of a commercial para-aramid product (i.e., DuPont s Kevlar-29 woven fabric) based on the oxygen-18-labelled carbon dioxide ( CC and CC ) decarboxylated from the sample. The oxygen-18-labelled atoms, which are inserted in the macromolecules, were analyzed for the photodegradation processes. This technique also allows the radial l O-distribution measurement from the fiber surface toward the fiber center. 

The extra mass associated with these labeled atoms may cause compounds containing deuterium or tritium atoms to react more slowly than compounds with ordinary hydrogen atoms. 

In the second class of reactions, stannanes are added to carbon-carbon it bonds. This enables either (i) the direct addition of a labelled tin atom to an organic substrate when the tin atom is isotopically labelled, or (ii) the introduction of an unlabelled tin group that is subsequently replaced by another isotopically labelled atom such as 36C1, to give a labelled compound. These two types of reactions are discussed separately in the following paragraphs. 

Another synthetic route to a labelled compound involves adding an unlabelled tin hydride to a carbon-carbon it bond and then replacing the tin moiety with an isotopically labelled atom, such as 125I112 (equation 86). 

Figure 4 Heterocycles for which the tautomeric equilibrium in aqueous solution has been studied using free energy calculations including explicit solvent. Tautomerism occurs by exchanging a proton between the labeled atoms.

Let us assume that we perform a CLE We supply a cell culture with a medium containing labeled carbon substrate and harvest the cells after a while. Now, the labeled material contains not only the information about how much label has been taken up but also which metabolites are labeled, in which position with which probability, and even which combinations of labeled atoms within one molecule occur with which probability. 

To understand the evaluation of a CLE, we need to introduce some terms The word isotopomer is a combination of the terms isotope, and isomer. An isotopomer is one of the different labeling states in which a particular metabolite can be encountered [248] that is, a molecule with n carbon atoms has 2" isotopomers. These are usually either depicted using outlined and filled circles for unlabeled and labeled atoms, respectively (see Fig. 14), or are described in text format for example, C 010 would be the isotopomer of a three-carbon molecule labeled at the second position. An isotopomer fraction is the percentage of molecules in this specific labeling state. The positional enrichment is the sum of all isotopomer fractions in which a specific carbon atom in a specific metabolite is labeled [248]. Consequently, the usage of isotopomers enables to account for more information While a molecule with n carbon atoms will yield n positional enrichments, there are 2 — 1 isotopomer fractions (the 2"th measurement is redundant as, by definition, isotopomer fractions must sum up to unity) [260],... 

Ideally, molecules should be labelled by introducing a radioactive isotope in place of a normal atom, e.g. carbon-14 replacing a carbon-12 in a carbohydrate. This method of labelling involves the synthesis of the molecule either in vivo or in vitro and the use of enzymes often permits the isotope to be introduced in a particular position in the molecule. The position of the labelled atom should be indicated wherever possible as for example in glucose- 1-14C. 

Oxidation is a cyclical pathway which removes a C2 (acetyl) unit form the fatty acyl-CoA on each cycle. The designation [3 derives from the traditional system of labelling atoms within fatty acid molecules where the carbon attached to the carboxyl group is a and the methyl carbon is always CO (omega) ... 

To detain an nnpaired electron and facilitate the azocoupling, the o-dinitrobenzene anion-radical was tested in the reaction (Todres et al. 1988). Such an anion-radical yielded an azo-coupled prodnct according to Scheme 1.2 (the nitrogen oxide evolved was detected). The reaction led to a para-snbstitnted prodnct, entirely in accordance with the calculated distribution of spin density in the anion-radical of o-dinitrobenzene (Todres 1990). It was established, by means of labeled-atom experiments and analysis of the gas prodnced, that azo-coupling is accompanied by the conversion of one of the nitro gronps into the hydroxy gronp and the liberation of nitric monoxide. In other... 

The size of heavy atom KIEs is usually considered to be normal (larger than unity) when the total bonding at the labeled atom is decreasing in the TS and inverse (less than unity) when the bonding is increasing. This is true of course, but an equally or even more important factor that determines the size of a primary KIE is the dynamic character of the TS the KIE is larger if the labeled atom is involved to a greater extent in the reaction-coordinate motion. The importance of the reaction-coordinate contribution has been well... 

Fig. 1 Comparison of molecular structures of [(tBu)2(Me)C-OAlH2]2 with [(tBu)(Me) (H)C-0A1H2]4 [7]. As in the following figures the smaller non-labeled atoms represent the hydride positions, the bigger ones stand for carbon atoms. The hydrogen atoms attached to carbon are not drawn...

The idea of the evidence is rather simple and can be elucidated by means of the following experiment. Let us consider, for example, a molecule of 2-methylpentane labeled in a branched position by 13C 2-methyl- 13C(2)-pentane. If the consecutive reactions in the adsorbed state are with a given metal of low extent, and this is certainly true for Pt or Pd, then the appearance, among the product, of 3-methyl-l3C(3)-pentane is very strong evidence of the operation of the 5C (cyclic) intermediates. Only via a ring closure at one place and an opening at another place of the molecule can a label move simultaneously with the branch. On the other hand, when the branch and labeled atom become separated by isomerization, this is evidence of the operation of the 3Cay complexes (see Fig. 5). 

It should have more than two labeled atoms, to avoid isotopic contribution of the analyte. 

The labeled atoms must not undergo exchange at any point during the analysis. 

In a simple displacement of benzyl isonitrile by K14CN from II, all the labeled atoms should appear in the isonitrile complex and none in the benzyl isonitrile. But the label was present in all of the products isolated and in each to a different extent. For example, the benzyl isonitrile was more radioactive than the N-benzyl formamide which must have been formed by addition of water to benzyl isonitrile. The results indicate that the internal isomerization (III V, Equation 7), the generation of benzyl isonitrile and N-benzyl formamide, and the polymerization of benzyl isonitrile are concomitant reactions. 

Fig. 19. Partial pressure of Te2(g) along the CdTe(s) three-phase curve, upper left, and along the HgTe(s) three-phase curve plotted on a log scale versus 1000/T Uppermost line labeled P° is the vapor pressure of Te(l). Short line segments are for liquids with labeled atom fraction . Experimental points for CdTe are from Brebrick (1971b), those for HgTe are from Su et al. (1981) and Brebrick and Strauss (1965).

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