Isotope effect

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Isotope effect (also isotope effect ) describes the differences in the chemical and physical properties of substances that result from the fact that the element in question or, in a chemical compound , one of the elements is present in the form of one or the other isotope .

Kinetic isotope effect

Isotope effects are generally very small because the different isotopes of an element have very similar masses. The striking exception occurs with the lightest element hydrogen : Since a deuterium is twice as heavy as a protium , chemical bonds with deuterons have a significantly lower zero-point oscillation energy , so they behave more traditionally . This is accompanied by a significantly increased activation energy for reactions that break this bond, with the result that such reactions are significantly slower at the same temperature than for bonds with hydrogen atoms: a C — H bond breaks around seven times faster than one at room temperature C-D bond.

For this reason, many natural products show a characteristic isotope distribution of deuterium, depending on their synthesis route, which can be easily detected with the help of NMR spectroscopy . For example, the isotope ratio of D to H in the ethanol can be used to determine whether the wine was fermented from grape sugar , i.e. from grapes, or whether the ethanol originates from illegally added sucrose , i.e. beet sugar. Many coveted natural substances could now be produced artificially relatively cheaply. To reproduce the nature-identical isotope distribution would be chemically very demanding and therefore more expensive than the natural product.

The kinetic isotope effect is also used in the investigation of chemical reaction mechanisms . To do this, a hydrogen atom whose bond is broken during the reaction (primary isotope effect) or a neighboring one (secondary isotope effect) is replaced by deuterium. Conclusions about the reaction mechanism can then be drawn from the distribution of the isotopes in the reaction product, which can be easily determined by means of NMR spectroscopy, and possibly also from changes in the reaction rate.

Dynamic isotope effect

The mass of a molecule also influences the dynamic properties, such as molecular rotation and translational movement (rotational diffusion and translational diffusion ) in molecular liquids. As with the kinetic isotope effect, noticeable effects occur especially when replacing hydrogen with deuterium. At 25 ° C, the self-diffusion coefficient of H 2 O is 23% greater than that of D 2 O. A similar effect also occurs with the rotational diffusion of water. Because of the inverse behavior of diffusion and viscosity, the viscosity of water H 2 O at 25 ° C is 23% lower than that of heavy water D 2 O. For other simple, molecular liquids, the dynamic isotope effect on viscosity and self-diffusion is 25 ° C, if you replace all hydrogen atoms with deuterium, still in the range of approx. 10%. In the case of methanol, for example, the effect is 14%, dimethyl sulfoxide 12%, ethanol 8% and benzene 6%.

Spectroscopy

In many types of spectroscopy , different isotopes or isotope-labeled compounds show slightly different spectral bands. In the case of heavier elements, the isotopes of which 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 obtained by distillation (with great effort) . It has a boiling point 1.42 ° C higher than light water. In natural water there is a low concentration of D 2 O, which - also as a result of its slightly increased boiling point - initially accumulates in the sump during distillation. Since water molecules exchange their protons with one another very quickly ( autoprotolysis ), D 2 O is formed in the sump . When heavy water is obtained by electrolysis, the kinetic isotope effect (see above) is used; heavy water decomposes more slowly and is therefore enriched. However, this process can only be profitable due to the high market value of the (light) hydrogen produced as a by-product.

Individual evidence

  1. ^ Atkins, Physical Chemistry, 2nd Edition, VCH 1996
  2. ↑ Hermann Weingärtner: Chapter 3. NMR studies of self-diffusion in liquids . In: Annual Reports Section 'C' (Physical Chemistry) . tape 91 , 1994, p. 37-69 , doi : 10.1039 / PC9949100037 .
  3. ↑ Manfred Holz, Xi-an Mao, Dieter Seiferling: Experimental study of dynamic isotope effects in molecular liquids: Detection of translation-rotation coupling. In: Journal of Chemical Physics Volume 104, 1996, pp. 669-679, doi: 10.1063 / 1.470863 .

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