Kinetic isotope effect

Isotope Effect (including isotope ) refers to the differences in the chemical and physical properties of substances resulting from the fact that the element or in a chemical compound of the elements in the form of one or the other isotope is present.

Kinetic isotope effect

Isotope effects are generally very small, since the different isotopes of an element have very similar masses. The striking exception occurs when the lightest element hydrogen to: Since a deuterium is twice as heavy as a Protium, chemical bonds with deuterons have a zero-point vibrational energy significantly deeper, so they behave in a quasi-classical. This is accompanied by a significantly higher activation energy for reactions that break this bond, with the result that such reactions at the same temperature run significantly slower than bonds with H atoms: a C -H bond breaking around seven times faster at room temperature as a C-D bond.

Therefore, show many natural substances, depending on their synthesis route in nature, a characteristic isotopic distribution of deuterium, which may be in NMR spectroscopy readily detected. For example, it can be demonstrated to the isotopic ratio of D to H in ethanol, whether the wine from glucose, was consequently fermented from grapes or ethanol from illegally added sucrose, so comes beet sugar. Many coveted natural products could be relatively low artificially produced today. The nature-identical isotopic distribution recreate would, however, chemically very demanding and therefore more expensive than the natural product.

Also in the study of chemical reaction mechanisms is the kinetic isotope effect application. For this purpose, a hydrogen atom, the bond is broken during the reaction (the primary kinetic isotope effect ), or an adjacent (secondary isotope effect ), are replaced by deuterium. From the easily identifiable by NMR spectroscopy distribution of isotopes in the reaction product, and possibly also from changes in the rate of reaction then conclusions can be drawn on the reaction mechanism.

Dynamic isotope effect

The mass of a molecule also affects the dynamic properties such as the molecular rotation and translational movement ( rotational and translational diffusion diffusion) in molecular liquids. As with the kinetic isotope effect occur mainly in the replacement of hydrogen by deuterium noticeable effects. Then at 25 ° C, the self-diffusion coefficient of H2O to 23 % greater than that of D2O. A similar effect also occurs in the rotational diffusion of the water. Because of the inverse behavior of the diffusion and the viscosity, the viscosity of the water at 25 ° C. H 2 O then 23% lower than that of heavy water D2O. In other simple molecular liquids, the dynamic viscosity and isotope effect is self-diffusion at 25 ° C when all hydrogen atoms are replaced by deuterium, nevertheless still within the range of about 10%. Thus, the effect in the case of methanol at 14%, dimethylsulfoxide 12%, ethanol 8% and 6% of benzene.

Spectroscopy

In many types of spectroscopy different isotopes or isotopically labeled compounds show slightly different spectral bands. For heavier elements whose isotopes differ by only a few percent by mass, these differences often disappear completely in the natural line width of the spectrum.

Applications

Heavy water ( D 2 O ) can be ( at great expense ) obtained by distillation. It has a higher boiling point than 1.42 ° C light water. In natural water is present in low concentration D2O, what - also due to its slightly higher boiling point - initially accumulates during distillation in the swamp. Because water molecules exchange their protons very rapidly interconnected ( autoionization ), formed in the bottom D2O. In the recovery of heavy water by electrolysis of the kinetic isotope effect (see above), use is made of heavy water decomposes more slowly and accumulates thereby. Profitable but this method can only be due to the high market value of the resulting as a by- product ( light ) hydrogen.