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Elucidation of Reaction Mechanisms - Isotopic Labeling

Isotopic Labeling - Introduction

In an isotopically labelled compound, a single atom is deliberately replaced by one of its isotopes. If such a compound is used as a starting product in a chemical reaction, this isotope can be detected and localized in the products and possibly even in the intermediate products with specific analytic methods. In this way, the position of an individual atom in a molecule, its displacement inside the molecule, and its transfer between different moelcules can be observed.

In organic chemistry and biochemistry, the isotopes 2H (deuterium), 3H (tritium), 13C, 14C, 15N, and 18O are frequently used for isotopic labeling experiments.

View of the inside of a scintillation counter.

The isotopes 3H and 14C are β emitter. That is, they are radioactive and emanate electrons with a half-life of 12.26 years (3H) or 5600 years (14C), respectively. In the products, or intermediates, these isotopes can thus be detected by the scintillation method. This method is extremely sensitive. However, it does not yield any structural information. Therefore, the products must be separated and individually analysed by the scintillation method in order to determine which product contains the radioactive isotope. If the exact position of the radioactive isotope in the product molecule is desired, usually lengthy, unequivocally proceeding degradation reactions and subsequent analysis of the individual degradation product are required.

Picture of a NMR spectrometer.

The isotopes 2H, 13C, 15N, and 18O are not radioactive, but stable. 13C and 15N are directly detected by NMR spectroscopy. 2H is not NMR-active, but can, however, indirectly be identified by the lack of a corresponding signal in the 1H NMR spectrum. 18O is also not NMR-active. However, it can be indirectly located by characteristic shifts of the 13C signals of the 18O-bonding carbon atoms in the 13C NMR spectrum. Nevertheless, mass spectrometry is more often used than NMR spectroscopy in identifying the isotope 18O.

These methods, especially the NMR spectroscopy, are not as sensitive as the scintillation method. However, above all the NMR spectroscopy has an enormous advantage in that it also yields extensive structural information. As a result, no lengthy degradation reactions are usually required in determining the position of the isotope in the product molecule.

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